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Date: Sat, 8 Oct 2022 21:03:28 -0700 From: Kees Cook <keescook@...omium.org> To: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com> Cc: Christine Caulfield <ccaulfie@...hat.com>, David Teigland <teigland@...hat.com>, cluster-devel@...hat.com, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH v2][next] dlm: Replace one-element array with flexible-array member On Sun, Oct 09, 2022 at 03:05:17PM +1300, Paulo Miguel Almeida wrote: > On Sat, Oct 08, 2022 at 05:18:35PM -0700, Kees Cook wrote: > > This is allocating 1 more byte than before, since the struct size didn't change. But this has always allocated too much space, due to the struct padding. For a "no binary changes" patch, the above "+ 1" needs to be left off. > > That's true. I agree that leaving "+ 1" would work and produce a > no-binary-changes patch due to the existing padding that the structure > has. OTOH, I thought that relying on that space could bite us in the > future if anyone tweaks the struct again...so my reaction was to ensure > that the NUL-terminator space was always guaranteed to be there. > Hence, the change on c693 (objdump above). > > What do you think? Should we keep or leave the above > "+ 1" after the rationale above? I think it depends on what's expected from this allocation. Christine or David, can you speak to this? > > I would expect the correct allocation size to be: > > offsetof(typeof(*ls), ls_name) + namelen > > Fair point, I will make this change. Well, only do that if we don't depend on the padding nor a trailing %NUL. :) > > Question, though: is ls_name _expected_ to be %NUL terminated > > Yes, it is. I tracked down ls_name's utilisations and it is passed down to > a bunch of routines that expects it to be NUL-terminated such as > snprintf and vsnprintf. Agreed: I see the string functions it gets passed to. So, then the next question I have is does "namelen" take into account the %NUL, and is "name" %NUL terminated? Those answers appear to be "no" and "yes", respectively: static int new_lockspace(const char *name, ...) { ... int namelen = strlen(name); The comparisons for ls->ls_namelen are all done without the %NUL count: if (ls->ls_namelen != namelen) continue; if (memcmp(ls->ls_name, name, namelen)) continue; > >, and was the prior 3 bytes of extra allocation accidentally required? > > > > I am assuming that you are refering to ls_namelen in the struct dlm_ls > (please correct me if this isn't what you meant). No, I meant ls_name (the pahole output shows the trailing 3 bytes of padding before. And with your patch it becomes 4 bytes of trailing padding. So I think this is "accidentally correct", since it's so carefully using memcmp() and not strcmp(). Given the existing padding on the structure, through, it likely needs to keep a certain amount of minimum padding. original size was actually this, so you could use this for the new calculation to get the same values as before: offsetof(typeof(*ls), ls_name) + 4 + namelen; In reality, it may be possible to do this to get exactly what is needed, but no less than the struct itself: max(offsetof(typeof(*ls), ls_name) + 1 + namelen, sizeof(*ls)); -Kees -- Kees Cook
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