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

Nilay Shroff
2024-12-08T14:10:11Z

On 12/8/24 19:27, Greg KH wrote:
> On Sun, Dec 08, 2024 at 07:21:48PM +0530, Nilay Shroff wrote:
>>
>>
>> On 12/8/24 18:58, Greg KH wrote:
>>> On Sun, Dec 08, 2024 at 03:51:10PM +0530, Nilay Shroff wrote:
>>>>
>>>>
>>>> On 12/7/24 17:14, Greg KH wrote:
>>>>> 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.
>>>> Thanks Greg for confirming that this is not isolated to PowerPC!!
>>>>>>
>>>>>> 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.
>>>> Okay so now we've an evidence confirming that this is no longer PowerPC specific
>>>> issue. Hence as Yury suggested, in another mail[1], fixing this error by disabling
>>>> stringop-overread globally for GCC13+ and CONFIG_FORTIFY_SOURCE=n, I will spin a
>>>> new patch and submit it.
>>>>
>>>> [1] https://lore.kernel.org/all/Z1HTdtvNjm-nZSvJ@yury-ThinkPad/
>>>
>>> That feels wrong, unless this is a compiler bug. And if it's a compiler
>>> bug, can we fix the compiler please or at least submit a bug to the gcc
>>> developers?
>>>
>> Yes this seems to be a compiler bug. Please see here :
>> [1] https://lore.kernel.org/all/202411021337.85E9BB06@keescook/
>> [2] https://gcc.gnu.org/pipermail/gcc-patches/2024-October/666872.html
>>
>>> I'm slowly moving all my boxes/builds over to using clang to build the
>>> kernel due to rust kernel work, so I guess I can do that here as well as
>>> I don't think this issue shows up for that compiler, right?
>>>
>> Yes this error doesn't show up for LLVM/clang. We've two options here:
>> 1) To disable this error globally for GCC-13+ until we find the root cause. Maybe when
>> GCC folks add more diagnostics and contexts around -Wstringop-* compiler warning as
>> discussed in [2] above.
>> or
>> 2) Silence this error only for file kernel/padata.c compiling which this error manifests
>> as of today.
>>
>> Yury suggested option #1 above so that we may avoid random victims hitting this error.
>> What do you suggest?
>
> I suggest the hardening maintainers should decide, as this is their area
> and feature they are supporting, not me :)
>
Alright, then I would go ahead with option #1, as Yury suggested, for now while I spin next patch
and keep in Cc all hardening maintainers to decide the disposition.

Thanks,
--Nilay