Re: [PATCH v3 04/12] signal: Verify the alignment and size of siginfo_t

[Marco Elver]

On Wed, 5 May 2021 at 16:11, Eric W. Beiderman <ebiederm@xxxxxxxxxxxx> wrote:
> From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>
> Update the static assertions about siginfo_t to also describe
> it's alignment and size.
>
> While investigating if it was possible to add a 64bit field into
> siginfo_t[1] it became apparent that the alignment of siginfo_t
> is as much a part of the ABI as the size of the structure.
>
> If the alignment changes siginfo_t when embedded in another structure
> can move to a different offset.  Which is not acceptable from an ABI
> structure.
>
> So document that fact and add static assertions to notify developers
> if they change change the alignment by accident.
>
> [1] https://lkml.kernel.org/r/YJEZdhe6JGFNYlum@xxxxxxxxxxxxxxxx
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

Acked-by: Marco Elver <elver@xxxxxxxxxx>

> ---
>  arch/arm/kernel/signal.c           | 2 ++
>  arch/arm64/kernel/signal.c         | 2 ++
>  arch/arm64/kernel/signal32.c       | 2 ++
>  arch/sparc/kernel/signal32.c       | 2 ++
>  arch/sparc/kernel/signal_64.c      | 2 ++
>  arch/x86/kernel/signal_compat.c    | 6 ++++++
>  include/uapi/asm-generic/siginfo.h | 5 +++++
>  7 files changed, 21 insertions(+)
>
> diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
> index 2dac5d2c5cf6..643bcb0f091b 100644
> --- a/arch/arm/kernel/signal.c
> +++ b/arch/arm/kernel/signal.c
> @@ -737,6 +737,8 @@ static_assert(NSIGBUS       == 5);
>  static_assert(NSIGTRAP == 6);
>  static_assert(NSIGCHLD == 6);
>  static_assert(NSIGSYS  == 2);
> +static_assert(sizeof(siginfo_t) == 128);
> +static_assert(__alignof__(siginfo_t) == 4);
>  static_assert(offsetof(siginfo_t, si_signo)    == 0x00);
>  static_assert(offsetof(siginfo_t, si_errno)    == 0x04);
>  static_assert(offsetof(siginfo_t, si_code)     == 0x08);
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index af8bd2af1298..ad4bd27fc044 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -985,6 +985,8 @@ static_assert(NSIGBUS       == 5);
>  static_assert(NSIGTRAP == 6);
>  static_assert(NSIGCHLD == 6);
>  static_assert(NSIGSYS  == 2);
> +static_assert(sizeof(siginfo_t) == 128);

Would using SI_MAX_SIZE be appropriate? Perhaps not.. in case somebody
changes it, given these static asserts are meant to double-check.

I leave it to you to decide what makes more sense.

> +static_assert(__alignof__(siginfo_t) == 8);
>  static_assert(offsetof(siginfo_t, si_signo)    == 0x00);
>  static_assert(offsetof(siginfo_t, si_errno)    == 0x04);
>  static_assert(offsetof(siginfo_t, si_code)     == 0x08);
> diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
> index b6afb646515f..ee6c7484e130 100644
> --- a/arch/arm64/kernel/signal32.c
> +++ b/arch/arm64/kernel/signal32.c
> @@ -469,6 +469,8 @@ static_assert(NSIGBUS       == 5);
>  static_assert(NSIGTRAP == 6);
>  static_assert(NSIGCHLD == 6);
>  static_assert(NSIGSYS  == 2);
> +static_assert(sizeof(compat_siginfo_t) == 128);
> +static_assert(__alignof__(compat_siginfo_t) == 4);
>  static_assert(offsetof(compat_siginfo_t, si_signo)     == 0x00);
>  static_assert(offsetof(compat_siginfo_t, si_errno)     == 0x04);
>  static_assert(offsetof(compat_siginfo_t, si_code)      == 0x08);
> diff --git a/arch/sparc/kernel/signal32.c b/arch/sparc/kernel/signal32.c
> index 778ed5c26d4a..32b977f253e3 100644
> --- a/arch/sparc/kernel/signal32.c
> +++ b/arch/sparc/kernel/signal32.c
> @@ -757,6 +757,8 @@ static_assert(NSIGBUS       == 5);
>  static_assert(NSIGTRAP == 6);
>  static_assert(NSIGCHLD == 6);
>  static_assert(NSIGSYS  == 2);
> +static_assert(sizeof(compat_siginfo_t) == 128);
> +static_assert(__alignof__(compat_siginfo_t) == 4);
>  static_assert(offsetof(compat_siginfo_t, si_signo)     == 0x00);
>  static_assert(offsetof(compat_siginfo_t, si_errno)     == 0x04);
>  static_assert(offsetof(compat_siginfo_t, si_code)      == 0x08);
> diff --git a/arch/sparc/kernel/signal_64.c b/arch/sparc/kernel/signal_64.c
> index c9bbf5f29078..e9dda9db156c 100644
> --- a/arch/sparc/kernel/signal_64.c
> +++ b/arch/sparc/kernel/signal_64.c
> @@ -567,6 +567,8 @@ static_assert(NSIGBUS       == 5);
>  static_assert(NSIGTRAP == 6);
>  static_assert(NSIGCHLD == 6);
>  static_assert(NSIGSYS  == 2);
> +static_assert(sizeof(siginfo_t) == 128);
> +static_assert(__alignof__(siginfo_t) == 8);
>  static_assert(offsetof(siginfo_t, si_signo)    == 0x00);
>  static_assert(offsetof(siginfo_t, si_errno)    == 0x04);
>  static_assert(offsetof(siginfo_t, si_code)     == 0x08);
> diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
> index 0e5d0a7e203b..e735bc129331 100644
> --- a/arch/x86/kernel/signal_compat.c
> +++ b/arch/x86/kernel/signal_compat.c
> @@ -34,7 +34,13 @@ static inline void signal_compat_build_tests(void)
>         BUILD_BUG_ON(NSIGSYS  != 2);
>
>         /* This is part of the ABI and can never change in size: */
> +       BUILD_BUG_ON(sizeof(siginfo_t) != 128);
>         BUILD_BUG_ON(sizeof(compat_siginfo_t) != 128);
> +
> +       /* This is a part of the ABI and can never change in alignment */
> +       BUILD_BUG_ON(__alignof__(siginfo_t) != 8);
> +       BUILD_BUG_ON(__alignof__(compat_siginfo_t) != 4);
> +
>         /*
>          * The offsets of all the (unioned) si_fields are fixed
>          * in the ABI, of course.  Make sure none of them ever
> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> index 03d6f6d2c1fe..91c80d0c10c5 100644
> --- a/include/uapi/asm-generic/siginfo.h
> +++ b/include/uapi/asm-generic/siginfo.h
> @@ -29,6 +29,11 @@ typedef union sigval {
>  #define __ARCH_SI_ATTRIBUTES
>  #endif
>
> +/*
> + * Be careful when extending this union.  On 32bit siginfo_t is 32bit
> + * aligned.  Which means that a 64bit field or any other field that
> + * would increase the alignment of siginfo_t will break the ABI.
> + */
>  union __sifields {
>         /* kill() */
>         struct {
> --
> 2.30.1
>