siginfo: Move si_trapno inside the union inside _si_fault

It turns out that linux uses si_trapno very sparingly, and as such it
can be considered extra information for a very narrow selection of
signals, rather than information that is present with every fault
reported in siginfo.

As such move si_trapno inside the union inside of _si_fault.  This
results in no change in placement, and makes it eaiser
to extend _si_fault in the future as this reduces the number of
special cases.  In particular with si_trapno included in the union it
is no longer a concern that the union must be pointer aligned on most
architectures because the union follows immediately after si_addr
which is a pointer.

This change results in a difference in siginfo field placement on
sparc and alpha for the fields si_addr_lsb, si_lower, si_upper,
si_pkey, and si_perf.  These architectures do not implement the
signals that would use si_addr_lsb, si_lower, si_upper, si_pkey, and
si_perf.  Further these architecture have not yet implemented the
userspace that would use si_perf.

The point of this change is in fact to correct these placement issues
before sparc or alpha grow userspace that cares.  This change was
discussed[1] and the agreement is that this change is currently safe.

[1]: https://lkml.kernel.org/r/CAK8P3a0+uKYwL1NhY6Hvtieghba2hKYGD6hcKx5n8=4Gtt+pHA@mail.gmail.comAcked-by: Marco Elver <elver@google.com>
v1: https://lkml.kernel.org/r/m1tunns7yf.fsf_-_@fess.ebiederm.org
v2: https://lkml.kernel.org/r/20210505141101.11519-5-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20210517195748.8880-1-ebiederm@xmission.comSigned-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

arch/x86/kernel/signal_compat.c

@@ -127,6 +127,9 @@ static inline void signal_compat_build_tests(void)
 	BUILD_BUG_ON(offsetof(siginfo_t, si_addr) != 0x10);
 	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_addr) != 0x0C);
 
+	BUILD_BUG_ON(offsetof(siginfo_t, si_trapno) != 0x18);
+	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_trapno) != 0x10);
+
 	BUILD_BUG_ON(offsetof(siginfo_t, si_addr_lsb) != 0x18);
 	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_addr_lsb) != 0x10);
 

include/linux/compat.h

@@ -214,12 +214,11 @@ typedef struct compat_siginfo {
 		/* SIGILL, SIGFPE, SIGSEGV, SIGBUS, SIGTRAP, SIGEMT */
 		struct {
 			compat_uptr_t _addr;	/* faulting insn/memory ref. */
-#ifdef __ARCH_SI_TRAPNO
-			int _trapno;	/* TRAP # which caused the signal */
-#endif
 #define __COMPAT_ADDR_BND_PKEY_PAD  (__alignof__(compat_uptr_t) < sizeof(short) ? \
 				     sizeof(short) : __alignof__(compat_uptr_t))
 			union {
+				/* used on alpha and sparc */
+				int _trapno;	/* TRAP # which caused the signal */
 				/*
 				 * used when si_code=BUS_MCEERR_AR or
 				 * used when si_code=BUS_MCEERR_AO

include/uapi/asm-generic/siginfo.h

@@ -63,9 +63,6 @@ union __sifields {
 	/* SIGILL, SIGFPE, SIGSEGV, SIGBUS, SIGTRAP, SIGEMT */
 	struct {
 		void __user *_addr; /* faulting insn/memory ref. */
-#ifdef __ARCH_SI_TRAPNO
-		int _trapno;	/* TRAP # which caused the signal */
-#endif
 #ifdef __ia64__
 		int _imm;		/* immediate value for "break" */
 		unsigned int _flags;	/* see ia64 si_flags */
@@ -75,6 +72,8 @@ union __sifields {
 #define __ADDR_BND_PKEY_PAD  (__alignof__(void *) < sizeof(short) ? \
 			      sizeof(short) : __alignof__(void *))
 		union {
+			/* used on alpha and sparc */
+			int _trapno;	/* TRAP # which caused the signal */
 			/*
 			 * used when si_code=BUS_MCEERR_AR or
 			 * used when si_code=BUS_MCEERR_AO
@@ -150,9 +149,7 @@ typedef struct siginfo {
 #define si_int		_sifields._rt._sigval.sival_int
 #define si_ptr		_sifields._rt._sigval.sival_ptr
 #define si_addr		_sifields._sigfault._addr
-#ifdef __ARCH_SI_TRAPNO
 #define si_trapno	_sifields._sigfault._trapno
-#endif
 #define si_addr_lsb	_sifields._sigfault._addr_lsb
 #define si_lower	_sifields._sigfault._addr_bnd._lower
 #define si_upper	_sifields._sigfault._addr_bnd._upper

kernel/signal.c

@@ -4607,6 +4607,7 @@ static inline void siginfo_buildtime_checks(void)
 
 	/* sigfault */
 	CHECK_OFFSET(si_addr);
+	CHECK_OFFSET(si_trapno);
 	CHECK_OFFSET(si_addr_lsb);
 	CHECK_OFFSET(si_lower);
 	CHECK_OFFSET(si_upper);