lkml.org 
[lkml]   [2022]   [Oct]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH v3] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN
    Date
    WG14 N2350 made very clear that it is an UB having type definitions with
    in "offsetof". This patch change the implementation of 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.

    I've made a clang patch that rejects any definitions within
    __builtin_offsetof (usually #defined with "offsetof"), and tested
    compiling with this patch, there are no error if this patch applied.

    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 we're replacing has behavior that matches
    _Alignof rather than __alignof__.

    Signed-off-by: YingChi Long <me@inclyc.cn>
    Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
    Link: https://godbolt.org/z/sPs1GEhbT
    Link: https://gcc.gnu.org/onlinedocs/gcc/Alignment.html
    Link: https://reviews.llvm.org/D133574
    ---
    v3:
    - commit message changes suggested by Nick and David

    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 621f4b6cac4a..de96c11e1fe9 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:
    --
    2.35.1
    \
     
     \ /
      Last update: 2022-10-06 16:19    [W:4.028 / U:0.368 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site