Messages in this thread | | | Date | Tue, 9 Apr 2024 09:15:17 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH] [RFC] xfrm: work around a clang-19 fortifiy-string false-positive |
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On Mon, Apr 08, 2024 at 09:06:21AM +0200, Arnd Bergmann wrote: > On Fri, Feb 16, 2024, at 22:19, Kees Cook wrote: > > On Fri, Feb 16, 2024 at 09:26:40PM +0100, Arnd Bergmann wrote: > >> From: Arnd Bergmann <arnd@arndb.de> > >> > >> clang-19 recently got branched from clang-18 and is not yet released. > >> The current version introduces exactly one new warning that I came > >> across in randconfig testing, in the copy_to_user_tmpl() function: > >> > >> include/linux/fortify-string.h:420:4: error: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror,-Wattribute-warning] > >> 420 | __write_overflow_field(p_size_field, size); > >> > >> I have not yet produced a minimized test case for it, but I have a > >> local workaround, which avoids the memset() here by replacing it with > >> an initializer. > >> > >> The memset is required to avoid leaking stack data to user space > >> and was added in commit 1f86840f8977 ("xfrm_user: fix info leak in > >> copy_to_user_tmpl()"). Simply changing the initializer to set all fields > >> still risks leaking data in the padding between them, which the compiler > >> is free to do here. To work around that problem, explicit padding fields > >> have to get added as well. > > > > Per C11, padding bits are zero initialized if there is an initializer, > > so "= { }" here should be sufficient -- no need to add the struct > > members. > > > >> Since this is a false positive, a better fix would likely be to > >> fix the compiler. > > > > As Nathan has found, this appears to be a loop unrolling bug in Clang. > > https://github.com/ClangBuiltLinux/linux/issues/1985 > > > > The shorter fix (in the issue) is to explicitly range-check before > > the loop: > > > > if (xp->xfrm_nr > XFRM_MAX_DEPTH) > > return -ENOBUFS; > > I ran into this issue again and I see that Nathan's fix has > made it into mainline and backports, but it's apparently > not sufficient. > > I don't see the warning with my patch from this thread, but > there may still be a better fix.
Is it the exact same warning? clang-19 or older? What architecture/configuration? If my change is not sufficient then maybe there are two separate issues here? I have not seen this warning appear in our CI since my change was applied.
Cheers, Nathan
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