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Message-ID: <20250509132805.GA4132662@ax162>
Date: Fri, 9 May 2025 14:28:05 +0100
From: Nathan Chancellor <nathan@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nicolas Schier <nicolas.schier@...ux.dev>,
Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
Bill Wendling <morbo@...gle.com>,
Justin Stitt <justinstitt@...gle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
llvm@...ts.linux.dev, patches@...ts.linux.dev,
stable@...r.kernel.org,
Linux Kernel Functional Testing <lkft@...aro.org>,
Marcus Seyfarth <m.seyfarth@...il.com>
Subject: Re: [PATCH v2] kbuild: Disable -Wdefault-const-init-unsafe
On Fri, May 09, 2025 at 10:04:23PM +0900, Masahiro Yamada wrote:
> On Wed, May 7, 2025 at 6:06 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >
> > 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@...r.kernel.org
> > Link: https://github.com/llvm/llvm-project/commit/576161cb6069e2c7656a8ef530727a0f4aefff30 [1]
> > Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>
> > Closes: https://lore.kernel.org/CA+G9fYuNjKcxFKS_MKPRuga32XbndkLGcY-PVuoSwzv6VWbY=w@mail.gmail.com/
> > Reported-by: Marcus Seyfarth <m.seyfarth@...il.com>
> > Closes: https://github.com/ClangBuiltLinux/linux/issues/2088
> > Signed-off-by: Nathan Chancellor <nathan@...nel.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-v1-0-3d2c6c185dbb@kernel.org
>
>
>
> Applied to linux-kbuild.
> Thanks.
>
> I fixed up the conflict with the -Wdefault-const-init-field-unsafe patch.
>
> Please check if it is correct.
This patch should replace the -Wdefault-const-init-field-unsafe patch,
not be applied on top. -Wdefault-const-init-unsafe contains both
-Wdefault-const-init-field-unsafe and -Wdefault-const-init-var-unsafe.
Cheers,
Nathan
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