RE: [PATCH tip 1/2] signal, perf: Fix siginfo_t by avoiding u64 on 32-bit architectures
From: David Laight <hidden>
Date: 2021-04-22 09:48:51
Also in:
linux-arch, lkml
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
quoted hunk ↗ jump to hunk
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@samsung.com> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com> Reported-by: Jon Hunter <jonathanh@nvidia.com> Signed-off-by: Marco Elver <elver@google.com> --- 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.cb/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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