signal, perf: Fix siginfo_t by avoiding u64 on 32-bit architectures

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: fb6cc127 ("signal: Introduce TRAP_PERF si_code and si_perf to siginfo")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reported-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://lkml.kernel.org/r/20210422191823.79012-1-elver@google.com

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;
 

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;
 

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)
 {