Re: [PATCHv2] cpumask: work around false-postive stringop-overread errors

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Dec 07, 2024 at 12:43:19PM +0100, Greg KH wrote:
> On Thu, Dec 05, 2024 at 06:04:09PM +0530, Nilay Shroff wrote:
> > While building the powerpc code using gcc 13, I came across following
> > errors generated for kernel/padata.c file:
> > 
> >   CC      kernel/padata.o
> > In file included from ./include/linux/string.h:390,
> >                  from ./arch/powerpc/include/asm/paca.h:16,
> >                  from ./arch/powerpc/include/asm/current.h:13,
> >                  from ./include/linux/thread_info.h:23,
> >                  from ./include/asm-generic/preempt.h:5,
> >                  from ./arch/powerpc/include/generated/asm/preempt.h:1,
> >                  from ./include/linux/preempt.h:79,
> >                  from ./include/linux/spinlock.h:56,
> >                  from ./include/linux/swait.h:7,
> >                  from ./include/linux/completion.h:12,
> >                  from kernel/padata.c:14:
> > In function ‘bitmap_copy’,
> >     inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2,
> >     inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2:
> > ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread]
> >   114 | #define __underlying_memcpy     __builtin_memcpy
> >       |                                 ^
> > ./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’
> >   633 |         __underlying_##op(p, q, __fortify_size);                        \
> >       |         ^~~~~~~~~~~~~
> > ./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’
> >   678 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
> >       |                          ^~~~~~~~~~~~~~~~~~~~
> > ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’
> >   259 |                 memcpy(dst, src, len);
> >       |                 ^~~~~~
> > kernel/padata.c: In function ‘__padata_set_cpumasks’:
> > kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256]
> >   713 |                                  cpumask_var_t pcpumask,
> >       |                                  ~~~~~~~~~~~~~~^~~~~~~~
> > In function ‘bitmap_copy’,
> >     inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2,
> >     inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2:
> > ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread]
> >   114 | #define __underlying_memcpy     __builtin_memcpy
> >       |                                 ^
> > ./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’
> >   633 |         __underlying_##op(p, q, __fortify_size);                        \
> >       |         ^~~~~~~~~~~~~
> > ./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’
> >   678 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
> >       |                          ^~~~~~~~~~~~~~~~~~~~
> > ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’
> >   259 |                 memcpy(dst, src, len);
> >       |                 ^~~~~~
> > kernel/padata.c: In function ‘__padata_set_cpumasks’:
> > kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256]
> >   713 |                                  cpumask_var_t pcpumask,
> >       |                                  ~~~~~~~~~~~~~~^~~~~~~~
> > 
> > Apparently, above errors only manifests with GCC 13.x and config option
> > CONFIG_FORTIFY_SOURCE. Furthermore, if I use gcc 11.x or gcc 12.x then I
> > don't encounter above errors. Prima facie, these errors appear to be false-
> > positive. Brian informed me that currently some efforts are underway by
> > GCC developers to emit more verbose information when GCC detects string
> > overflow errors and that might help to further narrow down the root cause
> > of this error. So for now, silence these errors using -Wno-stringop-
> > overread gcc option while building kernel/padata.c file until we find the
> > root cause.
> 
> I'm hitting this now on Linus's tree using gcc14 on x86-64 so this isn't
> just a problem with your arch.
> 
> Let me try this patch locally and see if it helps...

Yes, fixes the build for me, so either this is a real fix, or something
else needs to be done for it, so I'll give a:

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

for now.

thanks,

greg k-h