Stop Memsetting Structures
anmolsarma.in
anmolsarma.in
EDIT: Actually I have to apologize, because I’m not certain what the C standard says about whether the padding bytes can change when changing members of the structure (6.2.6.1):
6 When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values. 51)
If that means that the padding can actually also change when just assigning individual members (which I think it does, but footnote 51 only calls out copying the whole struct by assignment, and I’m not 100% sure that “including a member object” means what I think in this context), then that means that copying an unpacked structure across information boundaries is technically always unsafe, because any change of a value in the struct might leak information by changing the padding to undefined values. It’s safe with explicitly packed structs, though (which should be the majority of the cases, as you’d have a representation independent of the architecture), but then so is what the author describes. I’d appreciate if someone could clear this up! memset’ing can still help you getting a structure quickly into a well-defined state, though.
C is not a very high-level language. Apart from where undefined behavior is called out, you can and should assume how data is represented on a low level. It was made for writing operating systems and their kernels after all, where interfacing in a bit-accurate way is common.
There are platforms on which NULL, integer 0 and floating point 0 is not the same. Hence memsetting won't do the right thing. On modern platforms they are, but still, you don't want to get the Standard C Weenies on your back - they're even more annoying than the Rust Evangelism Strike Force. You don't want demons coming out of your nose, do you? :)
They do have a point, though. Suppose you are handling a struct you thought were memsetted but was not (your stupid team mate used those damn initializers!), then when serializing it you will get undefined bytes in the stream, possibly triggering hash values to mismatch causing mysterious rebuilds and other stuff.
Wrt to field accesses, yes those can overwrite padding bytes. Suppose a is an 8 bit char field, but padded to 4 bytes. Then "foo->a = 22" could be translated to "MOV [RSP+16], EAX" which overwrites the 3 padding bytes with undefined values. The solution, as you mention, is to use packed structs but in those field accesses are much less efficient.
Bottom line: code whichever way you want. :) Me, I like to live in the fast lane from time to time and those memset/cpy/cmp functions are sooo handy. :)
If you said memset(&p, 0, sizeof(p)) then that's not guaranteed to work on all implementations.
> [...] The technicality of the memset example is that it does not set the bits of the pointer by referring to it as a pointer, so the requirement that 0 behave as if it was a null pointer does not apply.
In short, there may exist counter-intuitive but nonetheless standards-compliant platforms where tests such as "assert((uintptr_t)(void * )0 == 0);" or "int x = 0; assert((void * )x == (void * )0);" would fail.
Code doing that may break with a compiler change or a compilation flag change on one end of the connection, even if you can guarantee both ends use the same CPU.
What if you do know the exact layout and it happens to have a gap between fields?
(Though I should mention that macros actually do make this doable, but they're hardly pretty, and I have yet to see many people embrace this approach, if they realize it's possible at all.)
Yes? Of course you do that? Or pick one of a hundred libraries that do that for you - protobuf, cap'n proto, flatbuffers, etc...
Protobuf serialization functions can turn into a lot of generated code too (read: big binaries) and stomp all over your icache, because they're different for each type. More "data-driven" approaches using type-generic functions with some runtime type-specific data (like proto descriptors&reflection) can be a good tradeoff for code that isn't as "hot". Typically more branchy than a memcpy or a type-specific function though.
And by sticking with the serialization libraries with higher performance, like flatbuffers and capnproto, you also lose a little bit of future flexibility, since its typically harder to change the format of these structures compared to higher level libraries.
> None of them are going to compare to something like sending a raw struct on the wire, if you need absolute performance.
I don't know about that.
For simple flat structs containing integers only (no pointers), Cap'n Proto is nearly identical to raw structs.
For complex structures with pointers, sending raw structs doesn't work.
Admittedly you may suffer unwanted code bloat linking in the Cap'n Proto library if all you really want to do is send a flat, raw struct.
> And by sticking with the serialization libraries with higher performance, like flatbuffers and capnproto, you also lose a little bit of future flexibility, since its typically harder to change the format of these structures compared to higher level libraries.
I disagree with this. Cap'n Proto's compatibility story is essentially the same as Protobuf and JSON: You can add new fields and old programs will ignore them.
(I suspect Flatbuffers is also similar but I haven't looked at it in a long time.)
So...you're not wrong, but it's basically the same reason people use C over something with more ergonomics.
Edit: the last time I wrote a networking library, I also just cast to raw structures, mainly because it was less work (didn't want to integrate a serialization library on both sides of the link). This was fine for a while (and very fast), but also hid some bugs in the protocol, and made sending arrays of data very ugly. Eventually we put protobuf on top, and things ended up much nicer.
If you're doing a large project on supported platforms it's probably worth using a pre-existing serialisation library like protobuffers or something.
You can't be sure you can send such structures to different OSes or CPUs however.
You cannot do this unless the compilers have the same ABI, which is not part of the standard.
Not sure what's the appropriate mitigation. Having to memset is a drag, and it's easy to forget to do it in some place or another. I'm tempted to say that you should only use packed structs for shared memory communication across trust boundaries.
Edit: Also, lots of simulation codes I am aware of tend to send flat arrays anyway. So I'd say that your concerns are mostly theoretical in nature.
Not sending the structure over the network is insufficient, and rigorous memset is a clear solution to this problem.
Also, C structure layout for a specific ABI is strictly determined; if that weren’t the case you couldn’t compile two C compilation units with different flags/compilers and expect them to work.
And there exists the very few really secure memset_s variants with a real memory barrier which guarantees memset to be executed in load and store order, flushing the cache, so that Spectre attacks are mitigated. security-relevant crypto libraries, like openssl and variants don't care about that, because memory barriers are "too slow". only the Linux kernel does it properly. It's still a mess.
That's true, but when sending stuff over the network you don't know if the receiver uses the same ABI.
//GCC: -O0
//MSVC: /Od
#include <memory.h>
#include <stdio.h>
struct S { unsigned char x; int y; };
int f(int f) { S a; memset(&a, f, sizeof(a)); unsigned char y[2]; memcpy(&y, &a, sizeof(y)); return y[1]; }
int g() { S a = { 1, 2 }; unsigned char y[2]; memcpy(&y, &a, sizeof(y)); return y[1]; }
int main(int argc, char *argv[]) { f(0xDD); printf("%#x\n", g()); f(0xFF); printf("%#x\n", g()); }
The compiler could also determine that &a.x)[1] is undefined behavior in C (reading past the end of an object), and take advantage of that.
The way to check that is by disassembling the code, and even that doesn’t say what a C compiler should do, only what these particular compilers do.
But yeah, I should have mentioned the caveats too. I will update the post once I'm back at my computer.
_6 When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values. 51)_
So sure, a memcpy after a memset aught to preserve all the bytes, but if they are updated through a struct which contains padding then the values in the padding bytes will be left unspecified.
Even if memset in a vacuum is guaranteed, look at the quote by anyfoo. "When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values."
If the compiler knows you're going to write to the members right after the memset, it can put the padding back to its previous values. And by that I mean as far as your code can tell it put it back, but in actuality it never zeroed the padding to begin with.
And since there are performance benefits to pretending, now you have a situation where a "helpful" compiler and a malicious compiler have the same effect: data can get leaked.
Also, type-punning is a thing. memset is byte-oriented/memory-oriented. Just because you're using it to zero a struct of a particular kind doesn't mean that's the only way the memory will accessed. Just because reading the padding of some struct is undefined behaviour doesn't mean accessing those bits by some other means is also undefined.
A compiler usually can't eliminate a call to memset() in most practical cases of initialization (where memory leakage is also a concern) because they almost always pass a reference to a routine in another translation unit. Something like memzero_explicit() can be used anyway -- but you're massively overstating the relevance of compilers eliminating dead stores done via memset(). It's much more of an issue for post-destruction memory sanitizing (which is the primary use case for memzero_explicit) than it is for compiler f*ckery when memset() is used for initialization.
You would have to call such a function between the memset and the first time you write to a member. Otherwise the compiler is allowed to say "I put the padding back, and you can't prove otherwise".
> Just because reading the padding of some struct is undefined behaviour doesn't mean accessing those bits by some other means is also undefined.
It's not always undefined, but it says very clearly that the value of padding becomes unspecified.
> they almost always pass a reference to a routine in another translation unit
> you're massively overstating the relevance of compilers eliminating dead stores done via memset
Unless inlining happened, or link-time optimization, or, or...
If the compiler zeroes the memory most of the time, that makes it even scarier. Because all your tests come back clean and safe, then four years later a macro changes and suddenly you're leaking data all over the place.
I don't think I'm overstating the relevance at all. Any security feature that could disappear because of a reasonable, trying-to-help optimization is one that should have a bright red warning label. And this is such a feature. It doesn't require a "sufficiently smart" compiler, and it doesn't require a malicious compiler. This is the kind of thing that can break by accident and ruin everyone's month.
Not in practice. Compilers make use of undefined behaviour to optimize things that are widely applicable and profitable. No real compiler does what you're saying and no future compiler is likely to without explicitly being asked to.
I agree that, by the letter of the spec, you're right, but you're still most certainly overstating the relevance.
I'm not arguing that this isn't a real problem or that people shouldn't use memzero_explicit() (or similar) where security is on the line, as I already said several times in another sub-thread -- I'm just saying that this kind of thing is extreme language-lawyering beyond the realms of probability. It's still not an excuse to be lax, but let's be realistic about the actual likelihood of it happening.
It's not just any, typical kind of dead store elimination. It also would require other optimization passes that I can assure you no mainstream compiler actually does. You can disagree with me if you want, but you're simply wrong.
That step is 90% of the work. Once you can do that, it's straightforward to assess that every field is assigned after a memset, with no intervening reads, and then remove the memset.
I spent several years working on a production grade compiler and I can assure you it's not. But keep just making things up off the top of your head if it makes you feel smart.
Let's look at a basic common case, as a checklist.
1. All fields are definitely assigned.
2. No functions are called except intrisics.
3. Nothing reads from the object on any control path.
4. Memset happens between creation and first assignment.
We agreed that step 1 is a solved problem, right? Are any of 2-4 difficult? Did I miss any prerequisites for the optimization?
Once you can prove 1-4, isn't the optimization pass as simple as looking for memsets applied to structs, checking 1-4, then deleting the call?
So if you're not working on an OS kernel somewhere in the syscall path - really not a common activity even among C programmers - it's not a problem. If you are passing a struct from one part of your program to another part, or to a library you use which is not isolated or sandboxed in some way, it's not a problem. If you pass a struct to a _more_ privileged context like a syscall, it's not a problem. If you are passing structs over a network connection or writing to a file, you'll pack them and be aware of every byte anyway, in order to be compatible with another implementation reading them.
If you _are_ passing a struct from a more privileged context to a less privileged context, then yeah you have to memset().
Simply not true; that context could pass a copy of that structure to a less privileged context other than the original one, without doing anything about the padding.
(Well, you could argue that it's the privileged kernel's fault: it should meticulously floss the structure between the members to clear the padding to zero while preserving the values.)
Someone upthread mentioned "sending over a network": that's an example. Sending over a network (or local socket/pipe) begins with pushing it down to a more privileged context, which then assiduously places all the bytes into a buffer that is blasted out on the wire. In this case, the more privileged context has no idea what the structure even is; it's just a blob of bytes.
All "undefined behaviour" means is that the spec can no longer guarantee anything about the execution of a program once a constraint is violated. It doesn't say anything about the practicality of actually doing it on any given implementation.
FWIW I didn't downvote you, but your reasoning about undefined behaviour seems overly simplistic. It should never be used to explain away security concerns.
You can memset to 0, and on your compiler it might be secure, but it's not enough to keep your security guaranteed against future compilers.
There's a reason functions like SecureZeroMemory exist. In a similar situation, you try to prevent leaking secure secrets by zeroing memory before releasing it. But the compiler sees you never use the variable again, and optimizes away the zeroing.
The message to take away is not "be less paranoid". It's "be more paranoid".
But the problem is that the security concerns and fixes are all undefined here.
So the initial comment is still very right. If you're relying on the padding to be anything in particular, you're in trouble.
No one in this sub-thread has mentioned (or implied) any "fix". You appear to be putting words in my mouth.
> So the initial comment is still very right.
Of course it's right, but it's also misleading when read in the context of the parent comment.
> If you're relying on the padding to be anything in particular, you're in trouble.
No shit.
I'm talking about the comments hermitdev was replying to that were treating memset as a 'fix'.
And 'fix' is shorthand for the opposite of "open[ing] you up to all kinds of info leaks". I don't think that's putting words in anyone's mouth.
To be fair though, memset() usually IS a fix. As mentioned by the kernel memzero_explicit() docs:
> usually using memset is just fine (!)
-- https://www.kernel.org/doc/htmldocs/kernel-api/API-memzero-e...
A conforming C compiler can't just remove memset() as it pleases. The case that most often requires memzero_explicit() is when zeroing an object after destruction, because the compiler thinks it can statically determine that it's a dead store. It very rarely happens that a compiler elides a memset() used for initialization.
I'm not sure why you seem to think that memset() can just be dropped at will for no reason whatsoever or that it's somehow always undefined behaviour.
For the sake of argument, can you show me some example code where it would be conforming to remove a memset() call? Preferably a realistic example and not a Google'd copypasta. Because it's all too easy to just regurgitate things you heard and think you understood, but no so easy to demonstrate it yourself.
Dead Store Elimination (Still) Considered Harmful
struct st s;
bzero(&s, sizeof(s));
// ...
write(fd, &s, sizeof(s));
Having said that, it is possible to get this sort of thing right with C++ and templates (thanks to inlining).
Higher-level languages without compile time code generation (like Java and Python) usually make serialization a train wreck in comparison with “send raw structs” or “make the [C++] compiler generate and typecheck the machine code that does the serialization”
You can get away with directly writing a struct for simple structs, but eventually it will bite you.
The remaining issue is endianness, but you can configure ARM to match x86 at boot so shrug.
Also, if I tell any reasonably experienced developer my protocol sends a packed struct with these fields:
uint8_t version
uint16_t reserved
uint8_t op
int32_t status
uint32_t data_length
// void data[]
I agree that packed structs can be tricky, but they’re much better than the vast majority of serialization libraries I’ve dealt with (including all of the open source ones, and I’ve used all the usual suspects).
The big annoyance is network people all use big endian probably just to spite everyone else.
The people that wrote that spec knew that and went with big endian anyways.
Also I work with people that do network hardware. They can't care less about little or big endian. Makes no difference to them.
ARM in BE mode is pretty rare / nonexistent. Small MIPS chips still float around in mostly low-end networking gear, though, and those do tend to run in BE.
This isn't true of modern processors.
struct S {char data, char pad0, char pad1, char pad2};
int main() {S dataStruct; printf("%s", dataStruct.data); return 0;}
Well, I chose this example specifically to ask whether the resulting memory structure, completely independent of calling a sizeof(), would still be as if I were to call a sizeof() (which then I believe would be 4 bytes large) if not accessing the total size of the struct or any of the padding members.
It makes a difference in the memory footprint, which can become important, if you're programming a device with just 64-bytes total RAM.
Whether you should do this in 2019 is another question, but sometimes it's still the easiest way to implement a binary protocol.
It's easy to write binary protocol code that way but it isn't portable.
enum color {RED, GREEN, BLUE};
char *color_name[] = {
[RED] = "Red",
[GREEN] = "Green",
[BLUE] = "Blue"
};
printf("%s",color_name[RED]);
What you are implementing is a map. Just use std::map.
It really irritates me when people give arguments like this. I seriously don't care what year it is, I care that the code I write works. A lot of projects still use C89 because they want the widest portability. C99 isn't very new, but especially in the embedded and other niche industries, C89 (often with some extensions) is all you get.
That said, "= { 0 };" is almost always usable instead of a memset() and works in C89 too.
And finally, there is absolutely no reason to check if a pointer is NULL just before calling free() on it
This is advice I agree with. The nullcheck is in free() itself (IMHO a good design decision --- especially along error-handling and cleanup paths.)
Embedded and niche industries are forcing their programmers to work with 30 year old tooling? (Instead of 20 year tooling?)
I think at some point, writing yourself a transpiler from C99 to C89 becomes profitable… (The JS community did it for a lot less than that w/ babel…)
I pretty much spit on my keyboard.
I also knew the code base was going to be kinda trashy and it was.
Why does how old it is mean anything at all? I personally choose C89 because of its wider compatibility, and I don't think I'm any worse off because of it. As the saying goes, "it's not about the tool, but how you use it." Incidentally, the best programmers I've worked with have also been the ones with the most conservative choice of tools.
I think at some point, writing yourself a transpiler from C99 to C89 becomes profitable… (The JS community did it for a lot less than that w/ babel…)
If anything, I think the JS community could learn a lot from the "C community" (a very loosely defined term...) --- the huge amount of churn with anything related to JS and web stuff in general turns me off.
If you're actually stuck with C89 you have my sympathy, like all the people stuck still maintaining FORTRAN and COBOL. But that's obviously a niche case with extreme limitations, it doesn't really need to be brought up every time someone mentions C99. Or C++11 for that matter.
struct abc { int a; int b; int c; };
int main() {
struct abc b = {2};
printf("a b c = %d %d %d\n ", b.a, b.b, b.c);
return 0;% c99 test.c
% a.out
a b c = 2 0 0
EDIT: Just noticed your code example uses structs. My example is for arrays, for which these observations are also correct.
> The C and C++ standards guarantee that even if an integer array is located on automatic storage and if there are fewer initializers in a brace-enclosed list then the uninitialized elements must be initialized to 0.
It does work for zero-initialization.
There is history to this practice. And also, foresight: 1) I may want to redefine free(). 2) If I do, my free() does not null-check. 3) Which is why my main code would null-check.
I'm an embedded developer, never do much on application work, and used to do that with C++, so I'm not too familiar with the heap allocation intricacies with C. But I'm quite sure that "delete" in C++ e.g. definitely needs to be null-checked (unless they changed that in some newer C++ revision, I always to try to code version-agnostic, and so far, have always managed).
In practice, I find it better to manually null-check pointers before deleting their contents, it either gets optimized away anyways or is an extra redundancy in code-or platform-migration.
I think redefining standard functions to do something very unexpected is very poor decision, especially when it means you now have to burden all its callsites with an extra check.
But I'm quite sure that "delete" in C++ e.g. definitely needs to be null-checked (unless they changed that in some newer C++ revision
No, it was never needed in C++ either (which given its desire to be backwards-compatible with C, is not surprising.) From the C++98 standard section 5.3.5, paragraph 2: "if the value of the operand of delete is the null pointer the operation has no effect."
No, I wouldn't redefine free(), I'd use my own terminology, but I'd search the code for it and replace it verbatim, and seeing that it apparently is standard practice for both C++ and C to null-check their pointers before deletion, I have all the more reason to do so, because my code assumed that it didn't.
Delete shouldn't null check, it should delete memory at location, no matter what address. It should be up to the implementation to check that pointer is valid.
EDIT: I should maybe add that the next line of code is in my case almost always "pointer = NULL" following "free(pointer)". The null-check is not just a technicality of checking of pointer-validity ("Pointer not null?"), but also code-syntactics ("Has this item not been deleted yet?").
What I am advocating for, is "Before you work on a pointer, check its validity", regardless of what follows after. It's just a general guideline, do it, even if you think it's senseless and redundant. Ever since I've adopted those basics, I've virtually reduced my problems with pointers to zero.
There's danger in accidentally not null-checking, but none in accidentally double-checking. Therefore, the implementation should take care of this (too), or at least consider implications.
I can't really take risks there.
The original author of the article is asking me to "stop" this, which could put a lot of things at stake. No, thank you.
The senior programmer that requested the change came from the application world and used the same argument.
When I told him that the compiler gave lots of warnings about being compatible with C99 through an extension, he asked me to just disable the warnings and keep on going. We did some testing and pushed the update (we had remote devices connected via GSM)
Two weeks later we were pushing a remote downgrade to the previous version because we started to detect anomalous bugs everywhere.
int yes=1;
setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof (yes)) ;
setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &(int) {1}, sizeof(int));
?What are we saying 'yes' to?
The flag SO_REUSEADDR (SO = Socket Option); really, I'd say we're setting the flag / setting the flag to true, but true is a bit of a reserved word, which I'm guessing is why they went with "yes".
The author of that code is trying to make it explicit -- "yes, I want to allow reuse of local addressses". Some will prefer this, some won't.
I personally like the use of the variable, though I think it's not worth getting all bent out of shape about it one way or another. But I guess we all have our pet peeves, and I know I'm guilty of getting all bent out of shape about things that ultimately don't matter much.
Again, I like the use of the variable, because in my opinion it makes the code a bit clearer, but I also don't think it is the end of the world if someone chooses the option without it.
setsockopt(listener, SOL_SOCKET, SO_REUSERADDR, &(int) {true}, sizeof(int));
The content of the padding between members can arbitrarily change during assignments of members, so code relying on the content of it is broken anyways. In the case of an extended struct both methods are equally broken if you don't recompile your module with the updated definition of the struct. Your memset will still have the old size of the struct as the third argument.
sizeof(struct_instance)
> ...if you don't recompile your module with the updated definition of the struct...
And once you recompiled, the C99 version with the pretty initializer is fixed as well, so there is no advantage in using memset regarding this.
Even as a member of the Force (RIIR 1st brigade), I laughed pretty heartily at this. :)
C++ programmers seem 'stressed'
tl;dr: while big, it reduces your cognitive load through strong guarantees. Think of it more as progressive disclosure of complexity.
At some point, I realized that C++ is a language for implementing new languages in a way that is backward compatible with existing programs.
Templates are Turing complete (lazily evaluated, purely functional, and side-effect free), so whatever your language of choice can do can be backported to C++ (granted, painfully, but probably less painfully than throwing out all your legacy code and starting from scratch).
On top of this, C++ supports zero-cost abstraction, so it will get very close to low level C performance (including constant propagation, loop unrolling, type-unsafe transformations, hoisting, inlining, etc, etc) even if you throw piles of lambdas and encapsulated methods at it.
The costs, of course, are that the compiler is slooooowww, and you inherit all the memory safety warts of C by default. (Emphasis on “by default”, because memory safe subsets of C++ are perfectly workable.)
This does not logically follow, since templates are evaluated at compile time and hence have restrictions on what they can do.
> C++ supports zero-cost abstraction, so it will get very close to low level C performance (including constant propagation, loop unrolling, type-unsafe transformations, hoisting, inlining, etc, etc) even if you throw piles of lambdas and encapsulated methods at it.
You do have to be careful with what you're doing, though.
> the compiler is slooooowww
I mean, it's not like the Rust compiler is fast either…
This statement doesn't make any sense. A Lisp program can be fully converted to run at compile time; it'll just be lacking in I/O opportunities.
What app do you use that takes no input? Or produces no output? Let alone both?
For code that only needs to compile as C, I agree. It's one of the best (if not the best addition) to the C language. A nice addition would be default values for struct members in the declaration with compile-time constants.
struct addrinfo hints = {0};
struct addrinfo hints = {};
struct addrinfo hints = ALL_ZEROES;The other (bigger) pain point is that malloc returns void*, and C++ requires a cast.
struct point {
double x, y;
}
#define init_point(x, y) { (x), (y) } /* C90 */
#define init_point(x, y) { .x = (x), .y = (y) } // C99
struct point p = init_point(0.5, 1.5);
Designated initializers eliminate some of the error-prone aspects of C90 style, but don't help enforce the discipline of initializing all members, or a certain subset of members.
Functional abstraction with checked arguments beats fancy new syntax.
struct foo {
void * bar;
} baz;
...
memset(&baz, 0, sizeof(struct foo));
assert(baz.bar == NULL);
The examples listed here are all historical:
If you use memset to initialize structure with pointers with 0 and then test if the pointer are NULL, the compiler could assume that the pointer was not properly initialized and remove the if completely, or actually even remove the whole function.
However, what has bitten me is memsetting structures that I later turn into full-fledged classes in C++. Oops, there went the VMT.
Designated initializers are very nice, as is the ability (in C++) to provide initial-value assignments that run before the constructor.
T foo = {}Technically correct (which is of course the best kind of correct) but you'd be hard pressed to find a system in 2019 where NULL != (void*)0. The most recent machines with non-zero NULL in the C FAQ entry on the matter (http://c-faq.com/null/machexamp.html) date back to the mid '90s.
You'd be hard pressed to find a system at any time where NULL is not equal to a compile-time constant zero. ;-)
This expression is false in any C99 compliant system:
Section 6.3.2.3 [0]:
> An integer constant expression with the value 0, or such an expression cast to type void * , is called a null pointer constant. [1] If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function
> Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.
The technicality of the memset example is that it does not set the bits of the pointer by referring to it as a pointer, so the requirement that 0 behave as if it was a null pointer does not apply.
[0] http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf
[1] In text footnote: The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.17
EDIT: Formatting
NULL == (void*)0.
void *p = 0;
intptr_t i = 0;
Even less intuitively, it is not guaranteed that `(void*)i == p` since `i` is not an integer constant expression, even if the value is known to be 0.
But talking about the actual bit representation. A zero value as a special pointer with the magic property of being invalid is a convention for C on x86, right?
I assume it has to do with it's fast to check the zero-flag in EFLAGS register when doing checks for null pointers.
struct S {size_t a, void *p;};
foo = (struct S) {0};
(You're absolutely correct that the standard does not require NULL be represented with zero bits, though.)
FWIW I'm less strict about standards compliance in the libarchive-derived code, since that frobs lots of unportable bits anyway.
&(int) {1}
int one = 1;
&one
Yes, someone will point out that there are languages that do not have such problems. However, when still using C, one has to be extremely careful...
int yes=1;
setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof (yes)) ;
setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &(int) {1}, sizeof(int));
I'm sorry, no it cannot. Or, rather, not without
1. Switching C90 code to C99; or
2. Switching C++ code to C99.
Good luck with 2.
GCC and Clang support this in C++ as an extension; then you're not writing in standard C++ any longer.
1 is a nonstarter in project that mandates C90.
In other words, there may be valid reasons why the code is that way other than ignorance of compound literals.
That's actually not true. It is the case when initializing arrays, but for structs (and unions) any non-named member has "indeterminate value" (see paragraph 9, section 6.7.8 of ISO/IEC 9899:1999; unchanged in C11 and C18). Well, at least according to the standard - GCC, Clang and MSVC all happen to make it zero, but it's not guaranteed by the C language.
The trick with avoiding a variable when passing an address to a literal value is nice though, I didn't know that. There are some places in my code that could use it ;)
21 If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.
It is worth noting that many C projects go for extreme portability. That's why C is used in the first place, and sometimes C99 is too much. I still sometimes work with compilers that barely support C89. That's for the aerospace industry BTW. There are even some libc that don't accept free(NULL).
If you know the platform you are writing for is not ancient, that's fine but if you are writing portable code, you have to realize these ancient platforms are still alive.
I think ditching K&R C is fine though. I've seen some of it but never encountered a case where it was the only thing the compiler supported.
That’s pretty much the completely opposite of portable, since it violates the C standard.
And if you start a new project, just use the style you prefer and enforce it :)
The author of the article should file a bug report. After all, if they wanted to memset, they'd just do it themselves.
Under the covers, one would hope a good optimizing compiler could choose the same assembly for both, which may very well look "memset like" since fewer, wide writes are generally more efficient.
If there's no explicit initializer I think memset,calloc or ={0} is just fine thanks.
That point is addressed in TFA.
But “stop memsetting” is clearly nonsense if you care about security.
Memset_s is the only way to guarantee that padding is actually zeroed, well memset or bzero_s obviously.
It’s critically important for people to understand that if the C spec says doing X is undefined, then current compiler writers interpret this as allowing them to do anything they want, even when it clearly introduces security vulnerabilities.
The only safe option is to initialize everything.
I say this having spent more than a decade working on software that is always being attacked.
But you should just do it the right way and have a wrapper that deals with endianness differences and such:
struct S my_struct;
// ...
push_int64(&output_stream, &my_struct.value1);
push_string(&output_stream, my_struct.value2);
push_int32(&output_stream, &my_struct.value3);
struct S my_struct;
next_int64(&input_stream, &my_struct.value1);
next_string(&input_stream, my_struct.value2, sizeof(my_struct.value2));
next_int32(&input_stream, &my_struct.value3);The problem is processing any data from the network. You have to assume that data is malicious, and will attack any weakness.
I’m not talking about serialisation at all - I agreed entirely that sending struct padding across the net is less than optimal :)
The point is that the standard cannot help you here because malicious data attacks things outside the scope of the standard.
The original purpose of “undefined behaviour” was to deal with the myriad different platform behaviours of the era (remember at that point in history 1 byte was not necessary 8 bits).
At some point the compiler writers decided to extend the effect of that to “if this behaviour is undefined in the spec” then we are allowed to treat the code as doing anything at all - so we went from this may do different things on different platforms to “the compiler can emit code that does something other than the clear intent or the platform specific behaviour”.
This change in mindset from the compiler devs has been necessary to improve performance in the spec benchmarks, and other micro-benchmarks created primarily to justify the arbitrary definition of “undefined behaviour”.
To give an idea of how capricious the compiler devs are they implemented optimizations that ended up breaking spec (it has undefined behavior), and so rolled those optimizations out. Yet when they literally add security bugs to code that has been secure for a decade, their response has been “well you’re relying on undefined behaviour”.
The whole nonsense can be summed up as a (according to the insane definition of “correct” used by the compiler writers)
Struct {char c;int i;} a,b;
Bzero(&a, size of a)
Bzero(&b, size of b)
If (something) return memcmp(&Zaki, &b, size of a);
Return 3;
Could legitimately be optimized to a function that just returns 3. Because the writes to padding in bzero aren’t uneeefined, as are the reads in memcmp. Therefore the memcmp is undefined, and the compiler can assume undefined behavior cannot occur, therefore that branch can be removed.
Whether the compiler does do that today isn’t relevant: the definition of “correct” used by compiler developers is that that is valid behaviour from them.
The reality is that modern c/c++ developers need to treat the compiler as an adversary, because of a nonsensical definition of “correct code generation” assumed by the compiler writers.
> The original purpose of “undefined behaviour” was to deal with the myriad different platform behaviours of the era (remember at that point in history 1 byte was not necessary 8 bits).
FWIW, CHAR_BIT is implementation defined (not undefined) to be at least 8.
> Therefore the memcmp is undefined, and the compiler can assume undefined behavior cannot occur, therefore that branch can be removed.
I am fairly certain that this memcmp is "implementation-defined" (namely, reading the value of padding bytes is valid but not guaranteed to produce anything useful), which allows the compiler to pick whether it wants to emit that branch (but it must still return some value). If the memcmp was actually undefined the compiler would be "justified" in producing code that wiped your hard drive.
Also, is it that much harder to do this (C++ for example):
struct S {
char c = 0;
int i = 0;
bool operator==(const S &other) const {
return c == other.c && i == other.i;
}
};
S a, b;
if(something) return !(a == b);
return 3;
If you're using memcmp on an object, you should be asserting that std::has_unique_object_representations<T> is true for it (unfortunately this is only available after C++17).
Also, if you are okay with anonymous field names:
std::tuple<char, int> a, b;
if(something) return a != b;
return 3;Things that matter:
* vtable and code pointers: leaks the aslr slide
* data pointers: leak heap location
* type ids: can be used to spoof objects
* length and size fields: can give you information about heap layout
Etc etc.
All of these get attacked.
[1] says that memset_s exists because it is not optimized away if it just clears a variable that will not be used after the call (unlike memset)
On the other end: people who do this for security reasons say not to trust memset_s is 'an optional feature of c11 and not really portable' [2]
[1] https://en.cppreference.com/w/c/string/byte/memset
[2] https://www.cryptologie.net/article/419/zeroing-memory-compi...