6 years and 360 patches to clean all instances of strnpy out of the Linux kernel | Hacker News Reader

6 years and 360 patches to clean all instances of strnpy out of the Linux kernel

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This article is a bit misleading. Linux kernel programming uses C, but not the C standard library and never has. The string functions discussed here are "helper" functions included in the kernel and are not part of the standard library.

The C standard library doesn't have strscpy or the others; it still has strncpy.

Should be using strcpy_s or strncpy_s.

> Should be using strcpy_s or strncpy_s

These functions are optional in the C standard and not always present. AFAIK, they are not included in glibc as of 2025.

Annex K (which is what that is) is sufficiently unpopular as to have been under discussion to be removed from the standard. Few implementations exist, and even fewer conform to the standard (e.g. Microsoft's doesn't).

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1967.htm

Furthermore, since the kernel uses no C library, it's the kernel developer's choices what to implement and use.

Isn't Microsoft the same group that still won't allow you to write "import <cstdint>" in C++ ?

VC++ has the only actually usable header units implementation across C++ compilers.

Unpopular because it came from Microsoft. But it works, and with my safeclib even usable in the kernel.

The cited document is outdated. This is a better overview: https://rurban.github.io/safeclib/doc/safec-3.9/d6/d31/md_do...

> Back in 1972, the goal was to be as fast and efficient as possible. Hackers weren’t a concern since computers weren’t connected.

On the contrary, one of the reasons Multics got a higher security score than UNIX when analysed by DoD was caring about security with PL/I.

Also remember Sun's "The network is the computer", and the Morris worm in 1985 as UNIX started to be widespread across university campus.

Ah, and the failure to add fat pointers to C, as Dennis Ritchie proposal wasn't taken by WG14, nor improved upon.

C authors followed their own path with Alef, Limbo, and finally Go.

BTW: I was experimenting a bit more with string and string view (strv) types in my e toy library: https://codeberg.org/uecker/noplate/

The tests are maybe better examples than those on godbolt. https://codeberg.org/uecker/noplate/src/branch/main/tests/st...

How does strv2cstr work? I assume it doesn't allocate, so not sure how it can add a null terminator.

It will not add one if there isn't one already. (I should probably rename this function, work in progress..). But the type is an array of the correct length.

Edit: renamed to strv2array

Ah, okay. In that case I feel like the only sane way to approach this is to completely abolish null-terminated strings, and reimplement everything (including stuff like printf's format and arguments) in terms of strv. Otherwise if there has to be a 2cstr function, it should be an allocating one.

Reimplementing everything is not an option in C. But one should be able to pass string views directly to printf.

The allocating version is what string_dup does. strv2cstr is certainly more dangerous. But as since the size is encoded in the return type, compilers can add bounds checking (and partially already do so).

strncpy was originally used to write into fixed length buffers[1]. This becomes obvious when considering the padding behavior, as described in the C standard[2]: "If the array pointed to by s2 is a string that is shorter than n characters, null characters are appended to the copy in the array pointed to by s1, until n characters in all have been written."

strlcpy, often touted as a replacement, does not elicit the padding behavior but has another flaw: It is supposed to return the length of the string it tried to create, for example, so the user can call realloc without calling strlen again.[3] However, this final "strlen-tail" in strlcpy isn't bounded by the size parameter which describes dest, not src.

While strscpy is a marked improvement, there is still something to be careful about: It can read past the end of the src-buffer, when sizeof src < sizeof dest and src is not nul-terminated.[4] (Set the count argument to something like min(sizeof dest, sizeof src) to avoid that).

[1] - https://softwareengineering.stackexchange.com/a/438090

[2] - https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf, 7.26.2.5 p. 3

[3] - https://manpages.debian.org/jessie/libbsd-dev/strlcpy.3.en.h...

[4] - https://manpages.debian.org/testing/linux-manual-4.8/strscpy...

> It can read past the end of the src-buffer, when sizeof src < sizeof dest and src is not nul-terminated.

So basically, don't invoke a function "strscpy — Copy a C-string into a sized buffer" on something that is not a C-string. Its description specifically states that it copies a string into a buffer. Compare with the wordings in standard of strcpy ("The strcpy function copies the string pointed to by s2 (including the terminating null character) into the array pointed to by s1") and strncpy ("The strncpy function copies not more than n characters (characters that follow a null character are not copied) from the array pointed to by s2 to the array pointed to by s1... If the array pointed to by s2 is a string that is shorter than n characters...").

It's not a function to copy an array into an array (there is memcpy/memmov for that); it's a function to copy a string into an array which, after the function is finished, will certainly be a string (unless it's zero-sized, sigh).

Previously: https://news.ycombinator.com/item?id=48612943

As I expected, this is about removing strncpy() from the Linux Kernel, not glib.

Removing strncpy() from glib would be awful for applications :)

Edit. Meant glibc, thanks mattst88

Confusion comes from Linux being used as a replacement term for any distro

I suspect you mean glibc.

GP may have been commenting that they don't find this development particularly glib.

When a wordpress site has more ads than arstechnica.

Arstechnica has ads?

Someone here browses without an ad blocker?

This made me chuckle. Great movie.

The HN title says `strnpy` instead of `strncpy` .....

Not going with something already widely deployed (like strlcpy), which could also be used in userland (strscpy can't, it's return value in case of failure is out of scope) is exactly what I would expect from Linux. You do you!

It would have been so much funnier if they let Claude Mythos do it, commenting it would have a much harder job from now on.

str | cp it's back. easy.

No mention of strlcpy (which was the safe replacement for strncpy in the stdlib about 15 years ago when I first heard of the former).

Apparently strscpy handles un-terminated input strings a bit better than strlcpy, but not scanning past the given length.

Starting with "The C string library" made me instantly tune out. C has a standard library, which has some string-related functions. There is no "C string library".

string.h

It's a header file not a library.

Define library. Surely a library is a collection of functions, not a collection of files, so you could have a single file library. I don't know at what size a collection of functions becomes a library, but I don't think anyone does.

Ultimately this file is supposed to be the one stop shop for all string related needs, so in what sense isn't that a library.

I think they mean it's part of the standard library, instead of being a separate library.

It's not a header file, it's just a header, strictly speaking: the standard explicitly allows it to not be an actual file, and indeed there are implementations that don't have standard headers available as actual on-disk files. And, also strictly speaking, it should be <string.h>. You know, if we're being pedantic.

My point isn't about that, though. My point is if you start a blog post with "The C string library", my confidence in your ability to discuss the topic is shot before I even finish reading the sentence.

And my point is that if you start nitpicking a perfectly understandable (if slightly imprecise) and widely-used terminology, you better not open yourself to the self-same criticism lest people would justifiably ignore your opinion.

After all, what C has is "the C library". It's what standard literally calls it (it doesn't even explicitly provide for existence of other kinds of libraries) but everyone calls it "the standard library". Is it correct? Arguably, no.

You're still falling back on "what the standard says". It's not a correctness question.

So you merely discard others' opinions merely on the fact of them using speech patterns that are not even incorrect, you just personally dislike them? Okay, that's fine, I guess, although you initially explicitly framed it as a correctness problem: "There is no "C string library".

It is not only not mportant at all, but also not real concern

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