RE: [PATCH tip 1/2] signal, perf: Fix siginfo_t by avoiding u64 on 32-bit architectures

[David Laight]

From: Marco Elver
> Sent: 22 April 2021 07:45
> 
> On some architectures, like Arm, the alignment of a structure is that of
> its largest member.

That is true everywhere.
(Apart from obscure ABI where structure have at least 4 byte alignment!)

> This means that there is no portable way to add 64-bit integers to
> siginfo_t on 32-bit architectures, because siginfo_t does not contain
> any 64-bit integers on 32-bit architectures.

Uh?

The actual problem is that adding a 64-bit aligned item to the union
forces the union to be 8 byte aligned and adds a 4 byte pad before it
(and possibly another one at the end of the structure).

> In the case of the si_perf field, word size is sufficient since there is
> no exact requirement on size, given the data it contains is user-defined
> via perf_event_attr::sig_data. On 32-bit architectures, any excess bits
> of perf_event_attr::sig_data will therefore be truncated when copying
> into si_perf.

Is that right on BE architectures?

> Since this field is intended to disambiguate events (e.g. encoding
> relevant information if there are more events of the same type), 32 bits
> should provide enough entropy to do so on 32-bit architectures.

What is the size of the field used to supply the data?
The size of the returned item really ought to match.

Much as I hate __packed, you could add __packed to the
definition of the structure member _perf.
The compiler will remove the padding before it and will
assume it has the alignment of the previous item.

So it will never use byte accesses.

	David

> 
> For 64-bit architectures, no change is intended.
> 
> Fixes: fb6cc127e0b6 ("signal: Introduce TRAP_PERF si_code and si_perf to siginfo")
> Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Reported-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> 
> Note: I added static_assert()s to verify the siginfo_t layout to
> arch/arm and arch/arm64, which caught the problem. I'll send them
> separately to arm&arm64 maintainers respectively.
> ---
>  include/linux/compat.h                                | 2 +-
>  include/uapi/asm-generic/siginfo.h                    | 2 +-
>  tools/testing/selftests/perf_events/sigtrap_threads.c | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/compat.h b/include/linux/compat.h
> index c8821d966812..f0d2dd35d408 100644
> --- a/include/linux/compat.h
> +++ b/include/linux/compat.h
> @@ -237,7 +237,7 @@ typedef struct compat_siginfo {
>  					u32 _pkey;
>  				} _addr_pkey;
>  				/* used when si_code=TRAP_PERF */
> -				compat_u64 _perf;
> +				compat_ulong_t _perf;
>  			};
>  		} _sigfault;
> 
> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> index d0bb9125c853..03d6f6d2c1fe 100644
> --- a/include/uapi/asm-generic/siginfo.h
> +++ b/include/uapi/asm-generic/siginfo.h
> @@ -92,7 +92,7 @@ union __sifields {
>  				__u32 _pkey;
>  			} _addr_pkey;
>  			/* used when si_code=TRAP_PERF */
> -			__u64 _perf;
> +			unsigned long _perf;
>  		};
>  	} _sigfault;
> 
> diff --git a/tools/testing/selftests/perf_events/sigtrap_threads.c
> b/tools/testing/selftests/perf_events/sigtrap_threads.c
> index 9c0fd442da60..78ddf5e11625 100644
> --- a/tools/testing/selftests/perf_events/sigtrap_threads.c
> +++ b/tools/testing/selftests/perf_events/sigtrap_threads.c
> @@ -44,7 +44,7 @@ static struct {
>  } ctx;
> 
>  /* Unique value to check si_perf is correctly set from perf_event_attr::sig_data. */
> -#define TEST_SIG_DATA(addr) (~(uint64_t)(addr))
> +#define TEST_SIG_DATA(addr) (~(unsigned long)(addr))
> 
>  static struct perf_event_attr make_event_attr(bool enabled, volatile void *addr)
>  {
> --
> 2.31.1.498.g6c1eba8ee3d-goog

-
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