Segfaults are our friends and teachers
kamalmarhubi.com
kamalmarhubi.com
> The Rust program got a segmentation fault because it attempted to write to inaccessible memory, but only through the stack pointer. None of the undefined behaviors disallow this, which I think is why it’s ok for this Rust program to segfault.
What I got out of that is that Rust does not work as advertised if there are still situations where a program could segfault. The entire premise of Rust, as I understood it at least, is that it does things in a safe manner and the programmer does not have to worry about it. Now I learned that there are undefined behaviors. In my view, for a language that bills itself as safe, there should not exist such things as undefined behaviors. As far as I am concerned, then, based on the advertising of Rust, this is false advertising.
(Basically, stack probes are only implemented on Windows, and we need them on the other platforms, but it hasn't been implemented yet, for various reasons.)
https://www.reddit.com/r/rust/comments/4ghy5h/blog_by_kamal_...
If I understood right, the stack probes are like this: detect stack allocations that are larger than a page so that they might go past guard pages. In those occasions, do a tight loop of memory accesses with one-page intervals that would trigger the guard pages, to prevent the access going past the guard pages going unnoticed. The probes are done only on large allocations, so the costs are minimal compared to the size of the allocation.
If the stacks grows only by small increments, like on normal recursion, guard pages are going to catch it.
On the same grounds you could label Java as a language "not ready for prime time yet".
If I had it my way, Java would be a criminal offense carrying a minimum prison sentence of at least 20 years, and if I ever have my own company and I catch someone using Java, I will fire them on the spot with so much gusto! It will be so awesome.
Furthermore, this is a known bug, and not too high priority probably since it's pretty rare to stack allocate something huge and then use it in a way that could cause a vulnerability (instead of crashing). The fix would involve replacing the segfault with a stack overflow, which also causes a crash.
I thought Rust was a language billing itself as safe. GCC and Clang are compilers, they have not mounted a campaign to bill themselves as safe, nor are they trying to bill themselves as the best thing since sliced bread.
This particular type of segfault differs to typical segfaults in other languages because it is guaranteed to happen and guaranteed to be a segfault/kill the program. Segfaults in C code often hint at more serious bugs (e.g. use after free) that can be exploited for remote code execution etc.
The bug is that the stack overflow detection isn't perfect: if writes to particularly large stack frames happen in just the wrong way, then a program can write beyond the guard page instead of being killed. This is obviously unfortunate, and the fix is ensuring LLVM has support for stack problems on all platforms (it currently has them on Windows).
(It can be fixed in rustc itself by inserting stack probes on large stack allocations, but it won't be able to catch cases where LLVM moves things around and in the process creates a new large stack allocation; i.e. after tons of inlining)
Ultimately, it's very hard to weaponize a segfault caused by a stack overflow (and causing a segfault through a guard-page-skipping overflow itself is a rare thing), so as far as practical aspects are concerned, this doesn't really matter.
Like everyone has said so far, it's a bug, a bug which can and will be fixed.
Does the presence of miscompilations in GCC mean that GCC is not ready for prime time?
We can't fix it fully in rustc because we don't know the stack size, which can grow with aggressive inlining, for example.
I suppose we could summarily probe allocas we know are larger than the page size, which would solve this particular situation (one large variable), but it's not a panacea.
The key thing from the reference [0]:
> "Type checking provides the guarantee that these issues are never caused by safe code."
It's subtle, but I think the situation is that "some segfaults are caused by undefined behaviour, and some undefined behaviour causes segfaults". Neither fully contains the other. One thing I was trying to get across is that a segmentation fault has a very specific meaning, and that meaning is not "bad thing was done with pointers".
[0]: http://doc.rust-lang.org/reference.html#behavior-considered-...
I know that Rust in particular tries to be very open about the caveats to its claims, but this is the kind of commentary that causes any ambitious project to try to minimize and hide its weaknesses instead of openly and honestly discussing them. I opened this discussion knowing that there would be a comment just like yours, and it made my heart feel heavy.
Something as core as proper stack handling is to me basic functionality. If the aim is to be a safe language, and there are undefined behaviors, or in this case such bugs, then this is at odds with the Rust public relations effort.
And, any project which resents such technical / conceptual criticism leaves one with much to ponder.
rustc handles it just as much as other compilers do. It should be safer than that, but it's currently not due to a compiler bug, and features making it safer are not "basic stack handling".
This is not undefined behavior, as has been said multiple times in this thread; please stop repeating this lie. This is a deterministic crash that can't be exploited easily.
> And, any project which resents such technical / conceptual criticism leaves one with much to ponder.
No, their comment said "inflammatory/unnuanced/ill-informed criticism". Which yours is.
I wrote: "and there are undefined behaviors, or in this case such bugs". "Whoever can read is clearly at an advantage".
> No, their comment said "inflammatory/unnuanced/ill-informed criticism". Which yours is.
I'll tell you what: if I had deployed Rust in production and I ran into crap like the article described, I'd be really pissed.
This can lead to bad memory reads if you try really hard to write code that leads to it, and that too it will happen a small fraction of the time -- all the other times it will segfault-crash (and would be caught in testing or in deployment without being exploited).
What Rust does promise is that undefined behavior will never happen if you don't use code blocks marked as unsafe (which include the C FFI). In other words, if you are able to trigger undefined behavior without using unsafe code, that is a compiler or library bug.
EDIT: as noted by steveklabnik, this is not undefined behavior. Actually you should get a better error message instead of a segmentation fault (see the link to the issue on GitHub in steveklabnik's comment).
In release mode LLVM seems to be optimizing out the entire program due to dead-code elimination (or maybe sibling-call elimination?), but in debug this is what you get:
<anon>:1:1: 3:2 warning: function cannot return without recurring, #[warn(unconditional_recursion)] on by default
<anon>:1 fn main() {
<anon>:2 main()
<anon>:3 }
<anon>:2:5: 2:11 note: recursive call site
<anon>:2 main()
^~~~~~
<anon>:1:1: 3:2 help: a `loop` may express intention better if this is on purpose
thread '<main>' has overflowed its stack
fatal runtime error: stack overflow
playpen: application terminated abnormally with signal 4 (Illegal instruction)
In any case, the Rust language, when it gets guarantees (i.e. a spec), will at the very least guarantee that stack overflow doesn't result in memory corruption, it may not be too opinionated on how exactly implementations make this guarantee. As you say, the rustc compiler currently handles it by guard pages and aborting but not perfectly (it's not hard to have a large stack frame: `let x = [0; 10000]`).
https://users.rust-lang.org/t/rust-guarantees-no-segfaults-w...
I'd be surprised; as a strong general rule, the stack does not get zeroed [Edit: see end of thread! It's the OS zeroing everything - learn something very day]. I'd expect it to segfault because the pointer value is whatever leftover non-zero value happens to be in that piece of memory, so it points into random memory the user program shouldn't be messing with (sticking in a printf to output the value of the pointer confirms this on at least one system). Wouldn't be surprised if some implementations took security really seriously and zero everything, or if a debug build was zero happy, but under normal circumstances, the stack doesn't get zeroed.
No, the operating system (kernel, actually) does not zero out anything. The runtime linker would be the one initializing static memory declared in the ELF BSS sections at execution time. The rest (including the stack and heap) is setup by the prologue (crti.o, depending on the OS crt1.o, and crts.o).
gcc segfault.c
It's zero only because this occurs at the very beginning of the program. In general it would be undefined.
This is easily testable.
#include <stdio.h>
int main(void) {
char* pointers[20];
int i;
for (i = 0; i < 20; ++i) {
printf("%p\n", pointers[i]);
}
return 0;
}
It just happened to be zero in the author's case.
This is because of gcc padding. Programs have to allocate whole page from OS. So if you want just 1 int, you have to get whole page for it (compilers can optimize it in some conditions). This is result of MMU that works for memory block and not for single bytes (performance issue I think) But as I know by default page size have 4KB.
Another reason may be that compiler tries to allocate 2^n bytes because of performance. and 8KB is close enough I think.
This is true only for first thread, other threads have fixed stack size specified on thread creation (on Linux it's 8MB by default), but usually even thread's stack pages are really allocated only on first access.
On UNIX if you really want to bump stack by arbitrary amounts the most portable way is to preallocate your own stack of sufficient size and then use that (either by abusing sigaltstack() or via makecontext()/setcontext() or possibly by creating new thread). But generally, having large local variables is not exactly good idea.
push %rbp
mov %rsp, %rbp
sub $something, %rsp
... actual code ...
mov %rbp, %rsp
ret
Too bad memory is not better segmented then. For instance, when linking against a library, that library's memory ends up in the same "segment" as the program itself. Therefore, right now, you can totally screw up a library's internal data structures without even causing a segfault directly.
...which are caught by the OS and used to either truly kill the process when the stack overflows, or to dynamically allocate more memory as the stack grows downwards. That's how it works on Windows, at least; I'm not as clear about Linux.