arm64: Expose ESR in mcontext

At present, the exception syndrome register (ESR) is not accessible via
the user context on FreeBSD AArch64, which makes it difficult to
determine the cause of an exception. For example, a signal handler might
get a `SIGSEGV` with `SEGV_ACCERR`, but it can't know whether that
occurred due to a bad read or write. The change implemented here
includes ESR in `struct __mcontext`, thereby allowing access via
`ucontext_t` for use by signal handlers, sanitizers, debuggers, etc.

Note that this addition consumes one of the seven spare 64-bit slots in
`struct __mcontext`.

Signed-off-by: Alex Arslan <ararslan@comcast.net>
Reviewed by:	andrew
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2053

sys/arm64/arm64/exec_machdep.c

@@ -484,6 +484,7 @@ int
 get_mcontext(struct thread *td, mcontext_t *mcp, int clear_ret)
 {
 	struct trapframe *tf = td->td_frame;
+	ksiginfo_t *ksi = td->td_proc->p_ksi;
 
 	if (clear_ret & GET_MC_CLEAR_RET) {
 		mcp->mc_gpregs.gp_x[0] = 0;
@@ -499,6 +500,10 @@ get_mcontext(struct thread *td, mcontext_t *mcp, int clear_ret)
 	mcp->mc_gpregs.gp_sp = tf->tf_sp;
 	mcp->mc_gpregs.gp_lr = tf->tf_lr;
 	mcp->mc_gpregs.gp_elr = tf->tf_elr;
+	if (ksi != NULL && (ksi->ksi_flags & KSI_EXCEPT) != 0) {
+		mcp->mc_esr = tf->tf_esr;
+		mcp->mc_flags |= _MC_ESR_VALID;
+	}
 	get_fpcontext(td, mcp);
 
 	return (0);

sys/arm64/include/ucontext.h

@@ -76,9 +76,11 @@ struct __mcontext {
 	struct fpregs	mc_fpregs;
 	int		mc_flags;
 #define	_MC_FP_VALID	0x1		/* Set when mc_fpregs has valid data */
+#define	_MC_ESR_VALID	0x2		/* Set when mc_esr has valid data */
 	int		mc_pad;		/* Padding */
 	__uint64_t	mc_ptr;		/* Address of extra_regs struct */
-	__uint64_t	mc_spare[7];	/* Space for expansion, set to zero */
+	__register_t	mc_esr;		/* Exception syndrome register */
+	__uint64_t	mc_spare[6];	/* Space for expansion, set to zero */
 };