Re: mainline build failure of powerpc allmodconfig for prom_init_check

Sudip Mukherjee [off-list ref] writes:

On Thu, Jul 14, 2022 at 9:55 AM Sudip Mukherjee (Codethink)
[off-list ref] wrote:

quoted

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.@tocbase,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