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

Nilay Shroff
2024-12-05T12:35:14Z

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.

Link: https://lore.kernel.org/all/7cbbd751-8332-4ab2-afa7-8c353834772a@linux.ibm.com/
Cc: briannorris@chromium.org
Cc: kees@kernel.org
Cc: nathan@kernel.org
Cc: yury.norov@gmail.com
Cc: steffen.klassert@secunet.com
Cc: daniel.m.jordan@oracle.com
Cc: linux-crypto@vger.kernel.org
Cc: linux@weissschuh.net
Cc: gjoyce@ibm.com
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
Changes from v1:
- Fix spell error in the commit message (Brian Norris)
- Add commentary around change to note that changes are needed to
avoid false positive on gcc 13+ (Brian Norris)
- Add the kerenl/padata.c file maintainers (Brian Norris)
---

kernel/Makefile | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/kernel/Makefile b/kernel/Makefile
index 87866b037fbe..03242d8870c7 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -120,6 +120,10 @@ obj-$(CONFIG_CFI_CLANG) += cfi.o
obj-$(CONFIG_PERF_EVENTS) += events/

obj-$(CONFIG_USER_RETURN_NOTIFIER) += user-return-notifier.o
+
+# Silence the false positive stringop-overread errors on GCC 13+
+CFLAGS_padata.o += $(call cc-disable-warning, stringop-overread)
+
obj-$(CONFIG_PADATA) += padata.o
obj-$(CONFIG_JUMP_LABEL) += jump_label.o
obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
--
2.45.2