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Re: [Qemu-devel] [PATCH v2] linux-user: Suppress address-of-packed-membe
From: |
Laurent Vivier |
Subject: |
Re: [Qemu-devel] [PATCH v2] linux-user: Suppress address-of-packed-member warnings in __get/put_user_e |
Date: |
Tue, 9 Oct 2018 19:01:27 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 |
Le 09/10/2018 à 18:18, Peter Maydell a écrit :
> Our __get_user_e() and __put_user_e() macros cause newer versions
> of clang to generate false-positive -Waddress-of-packed-member
> warnings if they are passed the address of a member of a packed
> struct (see https://bugs.llvm.org/show_bug.cgi?id=39113).
> Suppress these using the _Pragma() operator. Unfortunately
> _Pragma() support in gcc is broken in some gcc versions and
> in some usage contexts, so we limit the pragma usage here to clang.
>
> To put in the pragmas we need to convert the macros from
> expressions to statements, but all the callsites effectively
> treat them as statements already so this is OK.
>
> Signed-off-by: Peter Maydell <address@hidden>
> ---
> Changes v1->v2: _Pragma() in gcc appears to be a disaster area;
> limit the use of it to clang only, since it's just clang that
> emits the bogus warning in this case. Tested on clang-3.8.0,
> clang-7, gcc 5.4.0 and gcc 8.0.1.
>
> linux-user/qemu.h | 70 ++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 51 insertions(+), 19 deletions(-)
>
> diff --git a/linux-user/qemu.h b/linux-user/qemu.h
> index b4959e41c6e..1beb6a2cfc4 100644
> --- a/linux-user/qemu.h
> +++ b/linux-user/qemu.h
> @@ -461,27 +461,59 @@ static inline int access_ok(int type, abi_ulong addr,
> abi_ulong size)
> These are usually used to access struct data members once the struct has
> been locked - usually with lock_user_struct. */
>
> -/* Tricky points:
> - - Use __builtin_choose_expr to avoid type promotion from ?:,
> - - Invalid sizes result in a compile time error stemming from
> - the fact that abort has no parameters.
> - - It's easier to use the endian-specific unaligned load/store
> - functions than host-endian unaligned load/store plus tswapN. */
> +/*
> + * Tricky points:
> + * - Use __builtin_choose_expr to avoid type promotion from ?:,
> + * - Invalid sizes result in a compile time error stemming from
> + * the fact that abort has no parameters.
> + * - It's easier to use the endian-specific unaligned load/store
> + * functions than host-endian unaligned load/store plus tswapN.
> + * - The pragmas are necessary only to silence a clang false-positive
> + * warning: see https://bugs.llvm.org/show_bug.cgi?id=39113 .
> + * - We have to disable -Wpragmas warnings to avoid a complaint about
> + * an unknown warning type from older compilers that don't know about
> + * -Waddress-of-packed-member.
> + * - gcc has bugs in its _Pragma() support in some versions, eg
> + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83256 -- so we only
> + * include the warning-suppression pragmas for clang
> + */
> +#ifdef __clang__
> +#define PRAGMA_DISABLE_PACKED_WARNING \
> + _Pragma("GCC diagnostic push"); \
> + _Pragma("GCC diagnostic ignored \"-Wpragmas\""); \
> + _Pragma("GCC diagnostic ignored \"-Waddress-of-packed-member\"")
>
> -#define __put_user_e(x, hptr, e) \
> - (__builtin_choose_expr(sizeof(*(hptr)) == 1, stb_p, \
> - __builtin_choose_expr(sizeof(*(hptr)) == 2, stw_##e##_p, \
> - __builtin_choose_expr(sizeof(*(hptr)) == 4, stl_##e##_p, \
> - __builtin_choose_expr(sizeof(*(hptr)) == 8, stq_##e##_p, abort)))) \
> - ((hptr), (x)), (void)0)
> +#define PRAGMA_REENABLE_PACKED_WARNING \
> + _Pragma("GCC diagnostic pop")
> +
> +#else
> +#define PRAGMA_DISABLE_PACKED_WARNING
> +#define PRAGMA_REENABLE_PACKED_WARNING
> +#endif
> +
> +#define __put_user_e(x, hptr, e) \
> + do { \
> + PRAGMA_DISABLE_PACKED_WARNING; \
> + (__builtin_choose_expr(sizeof(*(hptr)) == 1, stb_p, \
> + __builtin_choose_expr(sizeof(*(hptr)) == 2, stw_##e##_p, \
> + __builtin_choose_expr(sizeof(*(hptr)) == 4, stl_##e##_p, \
> + __builtin_choose_expr(sizeof(*(hptr)) == 8, stq_##e##_p, abort)))) \
> + ((hptr), (x)), (void)0); \
> + PRAGMA_REENABLE_PACKED_WARNING; \
> + } while (0)
> +
> +#define __get_user_e(x, hptr, e) \
> + do { \
> + PRAGMA_DISABLE_PACKED_WARNING; \
> + ((x) = (typeof(*hptr))( \
> + __builtin_choose_expr(sizeof(*(hptr)) == 1, ldub_p, \
> + __builtin_choose_expr(sizeof(*(hptr)) == 2, lduw_##e##_p, \
> + __builtin_choose_expr(sizeof(*(hptr)) == 4, ldl_##e##_p, \
> + __builtin_choose_expr(sizeof(*(hptr)) == 8, ldq_##e##_p, abort)))) \
> + (hptr)), (void)0); \
> + PRAGMA_REENABLE_PACKED_WARNING; \
> + } while (0)
>
> -#define __get_user_e(x, hptr, e) \
> - ((x) = (typeof(*hptr))( \
> - __builtin_choose_expr(sizeof(*(hptr)) == 1, ldub_p, \
> - __builtin_choose_expr(sizeof(*(hptr)) == 2, lduw_##e##_p, \
> - __builtin_choose_expr(sizeof(*(hptr)) == 4, ldl_##e##_p, \
> - __builtin_choose_expr(sizeof(*(hptr)) == 8, ldq_##e##_p, abort)))) \
> - (hptr)), (void)0)
>
> #ifdef TARGET_WORDS_BIGENDIAN
> # define __put_user(x, hptr) __put_user_e(x, hptr, be)
>
Reviewed-by: Laurent Vivier <address@hidden>