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Message-ID: <202310201111.595F790@keescook> Date: Fri, 20 Oct 2023 11:22:27 -0700 From: Kees Cook <keescook@...omium.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Justin Stitt <justinstitt@...gle.com>, Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>, Sagi Grimberg <sagi@...mberg.me>, linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, ksummit@...ts.linux.dev, Azeem Shaikh <azeemshaikh38@...il.com> Subject: Re: the nul-terminated string helper desk chair rearrangement On Fri, Oct 20, 2023 at 10:56:31AM -0700, Linus Torvalds wrote: > On Fri, 20 Oct 2023 at 10:40, Justin Stitt <justinstitt@...gle.com> wrote: > > > > There's some docs at [1]. Perhaps there could be more? > > > > [1]: https://elixir.bootlin.com/linux/v6.6-rc6/source/include/linux/fortify-string.h#L292 > > Note that we have so few 'strlcpy()' calls that we really should > remove that horrid horrid interface. It's a buggy piece of sh*t. Yup, that's on-going. There's just a few left; Azeem has been chipping away at strlcpy. > It does mean that if you used to have > > dst[4]; > strlcpy(dst, "abc", 8); > > then that *used* to work (because it would copy four bytes: "abc\0" > and that fits in 'dst[]'). But > > dst[4]; > strscpy(dst, "abc", 8); > > will overflow dst[], because it will do a word-copy and you told > 'strscpy()' that you had a 8-byte buffer, and it will try to write > "abc\0\0\0\0\0" into the destination. Luckily, we already have checks for these mismatched sizes at compile time (i.e. CONFIG_FORTIFY_SOURCE will already check for pathological cases like above where 8 > sizeof(dst)). > The above is insane code, but it's an example of why a blind > strlcpy->strscpy conversion might change semantics. Totally agreed. All of the recent string conversions have been paying close attention to the behavioral differences. -- Kees Cook
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