Messages in this thread Patch in this message | | | From | YingChi Long <> | Subject | [PATCH v4] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN | Date | Thu, 3 Nov 2022 02:14:25 +0800 |
| |
Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
WG14 N2350 made very clear that it is an undefined behavior having type definitions with in "offsetof". Replace the macro "TYPE_ALIGN" to builtin "_Alignof" to avoid undefined behavior.
I've grepped all source files to find any type definitions within "offsetof".
offsetof\(struct .*\{ .*,
This implementation of macro "TYPE_ALIGN" seemes to be the only case of type definitions within offsetof in the kernel codebase.
Link: https://reviews.llvm.org/D133574
A clang patch has been made that rejects any definitions within __builtin_offsetof (usually #defined with "offsetof"), and tested compiling the kernel using clang, there are no error if this patch applied.
Link: https://gcc.gnu.org/onlinedocs/gcc/Alignment.html Link: https://godbolt.org/z/sPs1GEhbT
ISO C11 _Alignof is subtly different from the GNU C extension __alignof__. __alignof__ is the preferred alignment and _Alignof the minimal alignment. For 'long long' on x86 these are 8 and 4 respectively.
The macro TYPE_ALIGN has behavior that matches _Alignof rather than __alignof__.
Signed-off-by: YingChi Long <me@inclyc.cn> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> --- v4: - commit message changes suggested by Borislav Petkov
v3: - commit message changes suggested by Nick and David
v3: https://lore.kernel.org/all/20221006141442.2475978-1-me@inclyc.cn/
v2: https://lore.kernel.org/all/20220927153338.4177854-1-me@inclyc.cn/ --- arch/x86/kernel/fpu/init.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c index 8946f89761cc..851eb13edc01 100644 --- a/arch/x86/kernel/fpu/init.c +++ b/arch/x86/kernel/fpu/init.c @@ -133,9 +133,6 @@ static void __init fpu__init_system_generic(void) fpu__init_system_mxcsr(); }
-/* Get alignment of the TYPE. */ -#define TYPE_ALIGN(TYPE) offsetof(struct { char x; TYPE test; }, test) - /* * Enforce that 'MEMBER' is the last field of 'TYPE'. * @@ -143,8 +140,8 @@ static void __init fpu__init_system_generic(void) * because that's how C aligns structs. */ #define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER) \ - BUILD_BUG_ON(sizeof(TYPE) != ALIGN(offsetofend(TYPE, MEMBER), \ - TYPE_ALIGN(TYPE))) + BUILD_BUG_ON(sizeof(TYPE) != \ + ALIGN(offsetofend(TYPE, MEMBER), _Alignof(TYPE)))
/* * We append the 'struct fpu' to the task_struct: -- 2.37.4
| |