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

[Marco Elver]

On some architectures, like Arm, the alignment of a structure is that of
its largest member.

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.

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 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.

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