| Message ID | 20210422191823.79012-1-elver@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [tip,v2,1/2] signal, perf: Fix siginfo_t by avoiding u64 on 32-bit architectures | expand |
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) {
The alignment of a structure is that of its largest member. On architectures like 32-bit Arm (but not e.g. 32-bit x86) 64-bit integers will require 64-bit alignment and not its natural word size. This means that there is no portable way to add 64-bit integers to siginfo_t on 32-bit architectures without breaking the ABI, because siginfo_t does not yet (and therefore likely never will) contain 64-bit fields on 32-bit architectures. Adding a 64-bit integer could change the alignment of the union after the 3 initial int si_signo, si_errno, si_code, thus introducing 4 bytes of padding shifting the entire union, which would break the ABI. One alternative would be to use the __packed attribute, however, it is non-standard C. Given siginfo_t has definitions outside the Linux kernel in various standard libraries that can be compiled with any number of different compilers (not just those we rely on), using non-standard attributes on siginfo_t should be avoided to ensure portability. 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. Since si_perf 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. 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> Tested-by: Jon Hunter <jonathanh@nvidia.com> Signed-off-by: Marco Elver <elver@google.com> --- v2: * Update commit message wording to be clearer and mentioned __packed, as pointed out by David Laight. I'm sure some time in the future somebody will wonder and perhaps run into the same issue, so let's try to give as much background as we can... v1: https://lkml.kernel.org/r/20210422064437.3577327-1-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(-)