A new on by default warning in clang [1] aims to flags instances where const variables without static or thread local storage or const members in aggregate types are not initialized because it can lead to an indeterminate value. This is quite noisy for the kernel due to instances originating from header files such as:
drivers/gpu/drm/i915/gt/intel_ring.h:62:2: error: default initialization of an object of type 'typeof (ring->size)' (aka 'const unsigned int') leaves the object uninitialized [-Werror,-Wdefault-const-init-var-unsafe] 62 | typecheck(typeof(ring->size), next); | ^ include/linux/typecheck.h:10:9: note: expanded from macro 'typecheck' 10 | ({ type __dummy; \ | ^
include/net/ip.h:478:14: error: default initialization of an object of type 'typeof (rt->dst.expires)' (aka 'const unsigned long') leaves the object uninitialized [-Werror,-Wdefault-const-init-var-unsafe] 478 | if (mtu && time_before(jiffies, rt->dst.expires)) | ^ include/linux/jiffies.h:138:26: note: expanded from macro 'time_before' 138 | #define time_before(a,b) time_after(b,a) | ^ include/linux/jiffies.h:128:3: note: expanded from macro 'time_after' 128 | (typecheck(unsigned long, a) && \ | ^ include/linux/typecheck.h:11:12: note: expanded from macro 'typecheck' 11 | typeof(x) __dummy2; \ | ^
include/linux/list.h:409:27: warning: default initialization of an object of type 'union (unnamed union at include/linux/list.h:409:27)' with const member leaves the object uninitialized [-Wdefault-const-init-field-unsafe] 409 | struct list_head *next = smp_load_acquire(&head->next); | ^ include/asm-generic/barrier.h:176:29: note: expanded from macro 'smp_load_acquire' 176 | #define smp_load_acquire(p) __smp_load_acquire(p) | ^ arch/arm64/include/asm/barrier.h:164:59: note: expanded from macro '__smp_load_acquire' 164 | union { __unqual_scalar_typeof(*p) __val; char __c[1]; } __u; \ | ^ include/linux/list.h:409:27: note: member '__val' declared 'const' here
crypto/scatterwalk.c:66:22: error: default initialization of an object of type 'struct scatter_walk' with const member leaves the object uninitialized [-Werror,-Wdefault-const-init-field-unsafe] 66 | struct scatter_walk walk; | ^ include/crypto/algapi.h:112:15: note: member 'addr' declared 'const' here 112 | void *const addr; | ^
fs/hugetlbfs/inode.c:733:24: error: default initialization of an object of type 'struct vm_area_struct' with const member leaves the object uninitialized [-Werror,-Wdefault-const-init-field-unsafe] 733 | struct vm_area_struct pseudo_vma; | ^ include/linux/mm_types.h:803:20: note: member 'vm_flags' declared 'const' here 803 | const vm_flags_t vm_flags; | ^
Silencing the instances from typecheck.h is difficult because '= {}' is not available in older but supported compilers and '= {0}' would cause warnings about a literal 0 being treated as NULL. While it might be possible to come up with a local hack to silence the warning for clang-21+, it may not be worth it since -Wuninitialized will still trigger if an uninitialized const variable is actually used.
In all audited cases of the "field" variant of the warning, the members are either not used in the particular call path, modified through other means such as memset() / memcpy() because the containing object is not const, or are within a union with other non-const members.
Since this warning does not appear to have a high signal to noise ratio, just disable it.
Cc: stable@vger.kernel.org Link: https://github.com/llvm/llvm-project/commit/576161cb6069e2c7656a8ef530727a0f... [1] Reported-by: Linux Kernel Functional Testing lkft@linaro.org Closes: https://lore.kernel.org/CA+G9fYuNjKcxFKS_MKPRuga32XbndkLGcY-PVuoSwzv6VWbY=w@... Reported-by: Marcus Seyfarth m.seyfarth@gmail.com Closes: https://github.com/ClangBuiltLinux/linux/issues/2088 Signed-off-by: Nathan Chancellor nathan@kernel.org --- Changes in v2: - Disable -Wdefault-const-init-var-unsafe as well, as '= {}' does not work in typecheck() for all supported compilers and it may not be worth a local hack. - Link to v1: https://lore.kernel.org/r/20250501-default-const-init-clang-v1-0-3d2c6c185db... --- scripts/Makefile.extrawarn | 12 ++++++++++++ 1 file changed, 12 insertions(+)
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn index d88acdf40855..fd649c68e198 100644 --- a/scripts/Makefile.extrawarn +++ b/scripts/Makefile.extrawarn @@ -37,6 +37,18 @@ KBUILD_CFLAGS += -Wno-gnu # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219 KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf) KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf) + +# Clang may emit a warning when a const variable, such as the dummy variables +# in typecheck(), or const member of an aggregate type are not initialized, +# which can result in unexpected behavior. However, in many audited cases of +# the "field" variant of the warning, this is intentional because the field is +# never used within a particular call path, the field is within a union with +# other non-const members, or the containing object is not const so the field +# can be modified via memcpy() / memset(). While the variable warning also gets +# disabled with this same switch, there should not be too much coverage lost +# because -Wuninitialized will still flag when an uninitialized const variable +# is used. +KBUILD_CFLAGS += $(call cc-disable-warning, default-const-init-unsafe) else
# gcc inanely warns about local variables called 'main'
--- base-commit: 92a09c47464d040866cf2b4cd052bc60555185fb change-id: 20250430-default-const-init-clang-b6e21b8d03b6
Best regards,