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Date: Mon, 18 Jul 2022 14:41:23 +1000 From: Michael Ellerman <mpe@...erman.id.au> To: Sudip Mukherjee <sudipm.mukherjee@...il.com>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Kees Cook <keescook@...omium.org> Cc: linuxppc-dev@...ts.ozlabs.org, linux-kernel <linux-kernel@...r.kernel.org>, linux-hardening@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check Sudip Mukherjee <sudipm.mukherjee@...il.com> writes: > On Thu, Jul 14, 2022 at 9:55 AM Sudip Mukherjee (Codethink) > <sudipm.mukherjee@...il.com> wrote: >> >> Hi All, >> >> Not sure if it has been reported before but the latest mainline kernel >> branch fails to build for powerpc allmodconfig with gcc-12 and the error is: >> >> Error: External symbol 'memset' referenced from prom_init.c >> make[2]: *** [arch/powerpc/kernel/Makefile:204: arch/powerpc/kernel/prom_init_check] Error 1 > > 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: > > call_prom: > .quad .call_prom,.TOC.@...base,0 > .previous > .size call_prom,24 > .type .call_prom,@function > .call_prom: > mflr 0 #, > std 29,-24(1) #, > std 30,-16(1) #, > std 31,-8(1) #, > mr 29,3 # tmp166, service > mr 31,4 # nargs, tmp167 > mr 30,5 # tmp168, nret > # arch/powerpc/kernel/prom_init.c:396: struct prom_args args; > li 4,254 #, Here we load 254 into r4, which is the 2nd parameter to memset (c). > li 5,52 #, This is r5, the 3rd parameter (n), ie. the size of the structure. That tells us we're memsetting the entire structure, ie. the 10 x 4 bytes of args.args plus 3 x 4 bytes for the other members. > # arch/powerpc/kernel/prom_init.c:394: { > std 0,16(1) #, > stdu 1,-208(1) #,, > # arch/powerpc/kernel/prom_init.c:396: struct prom_args args; > addi 3,1,112 # tmp174,, Here we load (calculate) the address of "args" into r3, the first parameter to memset. > # arch/powerpc/kernel/prom_init.c:394: { > std 9,304(1) #, > std 10,312(1) #, > std 6,280(1) #, > std 7,288(1) #, > std 8,296(1) #, > # arch/powerpc/kernel/prom_init.c:396: struct prom_args args; > bl .memset # So we're memsetting all of args to 254, not zero. That's happening because allmodconfig with gcc 12 enables CONFIG_INIT_STACK_ALL_PATTERN, whereas gcc 11 doesn't. I think the simplest fix in the short term is to just disable stack initialisation for prom_init.c. It only runs at boot so there's no real security impact to disabling it. cheers
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