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Message-ID: <87k08bdqm1.fsf@mpe.ellerman.id.au> Date: Mon, 18 Jul 2022 13:52:38 +1000 From: Michael Ellerman <mpe@...erman.id.au> To: Segher Boessenkool <segher@...nel.crashing.org>, Linus Torvalds <torvalds@...ux-foundation.org> Cc: Sudip Mukherjee <sudipm.mukherjee@...il.com>, Kees Cook <keescook@...omium.org>, linux-kernel <linux-kernel@...r.kernel.org>, Paul Mackerras <paulus@...ba.org>, linux-hardening@...r.kernel.org, linuxppc-dev <linuxppc-dev@...ts.ozlabs.org> Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check Segher Boessenkool <segher@...nel.crashing.org> writes: > On Sun, Jul 17, 2022 at 07:44:22AM -0700, Linus Torvalds wrote: >> On Sun, Jul 17, 2022 at 2:13 AM Sudip Mukherjee >> <sudipm.mukherjee@...il.com> wrote: >> > I was trying to check it. With gcc-11 the assembly code generated is >> > not using memset, but using __memset. >> > But with gcc-12, I can see the assembly code is using memset. One >> > example from the assembly: >> >> You could try making the 'args' array in 'struct prom_args' be marked >> 'volatile'. >> >> Ie something like this: >> >> --- a/arch/powerpc/kernel/prom_init.c >> +++ b/arch/powerpc/kernel/prom_init.c >> @@ -115,6 +115,6 @@ struct prom_args { >> __be32 service; >> __be32 nargs; >> __be32 nret; >> - __be32 args[10]; >> + volatile __be32 args[10]; >> }; >> >> because I think it's just the compilers turning the small loop over >> those fields into a "memset()". > > Yes. See <https://gcc.gnu.org/onlinedocs/gcc/Standards.html#C-Language> > near the end: > Most of the compiler support routines used by GCC are present in > libgcc, but there are a few exceptions. GCC requires the freestanding > environment provide memcpy, memmove, memset and memcmp. Finally, if > __builtin_trap is used, and the target does not implement the trap > pattern, then GCC emits a call to abort. > > Can't we simply have a small simple implementation of these functions in > arch/powerpc/boot/? This stuff is not performance-critical, and this is > not the first time we hit these problems. prom_init.c isn't in arch/powerpc/boot :) It's linked into the kernel proper, but we want it to behave like a pre-boot environment (because not all boot paths run it) which is why we restrict what symbols it can call. We could have a prom_memset() etc. but we'd need to do some tricks to rewrite references to memset() to prom_memset() before linking. cheers
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