Index: lib/libc/sys/procctl.2
===================================================================
--- lib/libc/sys/procctl.2
+++ lib/libc/sys/procctl.2
@@ -454,6 +454,16 @@
 .Va si_code
 member is set to
 .Dv TRAP_CAP .
+The system call number is stored in the
+.Va si_syscall
+field of the
+.Fa siginfo
+signal handler parameter.
+The other system call parameters can be read from the
+.Fa ucontext_t
+but the system call number is typically stored in the register
+that also contains the return value and so is unavailable in the
+signal handler.
 .Pp
 See
 .Xr capsicum 4
Index: sys/amd64/amd64/trap.c
===================================================================
--- sys/amd64/amd64/trap.c
+++ sys/amd64/amd64/trap.c
@@ -1030,6 +1030,7 @@
 	sa = &td->td_sa;
 
 	sa->code = frame->tf_rax;
+	sa->original_code = sa->code;
 
 	if (__predict_false(sa->code == SYS_syscall ||
 	    sa->code == SYS___syscall ||
Index: sys/amd64/cloudabi32/cloudabi32_sysvec.c
===================================================================
--- sys/amd64/cloudabi32/cloudabi32_sysvec.c
+++ sys/amd64/cloudabi32/cloudabi32_sysvec.c
@@ -101,6 +101,7 @@
 
 	/* Obtain system call number. */
 	sa->code = frame->tf_rax;
+	sa->original_code = sa->code;
 	if (sa->code >= CLOUDABI32_SYS_MAXSYSCALL)
 		return (ENOSYS);
 	sa->callp = &cloudabi32_sysent[sa->code];
Index: sys/amd64/cloudabi64/cloudabi64_sysvec.c
===================================================================
--- sys/amd64/cloudabi64/cloudabi64_sysvec.c
+++ sys/amd64/cloudabi64/cloudabi64_sysvec.c
@@ -98,6 +98,7 @@
 
 	/* Obtain system call number. */
 	sa->code = frame->tf_rax;
+	sa->original_code = sa->code;
 	if (sa->code >= CLOUDABI64_SYS_MAXSYSCALL)
 		return (ENOSYS);
 	sa->callp = &cloudabi64_sysent[sa->code];
Index: sys/amd64/ia32/ia32_syscall.c
===================================================================
--- sys/amd64/ia32/ia32_syscall.c
+++ sys/amd64/ia32/ia32_syscall.c
@@ -150,6 +150,7 @@
 
 	params = (caddr_t)frame->tf_rsp + sizeof(u_int32_t);
 	sa->code = frame->tf_rax;
+	sa->original_code = sa->code;
 
 	/*
 	 * Need to check if this is a 32 bit or 64 bit syscall.
Index: sys/amd64/include/proc.h
===================================================================
--- sys/amd64/include/proc.h
+++ sys/amd64/include/proc.h
@@ -91,6 +91,7 @@
 #define	KINFO_PROC32_SIZE 768
 
 struct syscall_args {
+	u_int original_code;
 	u_int code;
 	struct sysent *callp;
 	register_t args[8];
Index: sys/amd64/linux/linux_sysvec.c
===================================================================
--- sys/amd64/linux/linux_sysvec.c
+++ sys/amd64/linux/linux_sysvec.c
@@ -192,6 +192,7 @@
 	sa->args[4] = frame->tf_r8;
 	sa->args[5] = frame->tf_r9;
 	sa->code = frame->tf_rax;
+	sa->original_code = sa->code;
 
 	if (sa->code >= p->p_sysent->sv_size)
 		/* nosys */
Index: sys/amd64/linux32/linux32_sysvec.c
===================================================================
--- sys/amd64/linux32/linux32_sysvec.c
+++ sys/amd64/linux32/linux32_sysvec.c
@@ -656,6 +656,7 @@
 	sa->args[4] = frame->tf_rdi;
 	sa->args[5] = frame->tf_rbp;	/* Unconfirmed */
 	sa->code = frame->tf_rax;
+	sa->original_code = sa->code;
 
 	if (sa->code >= p->p_sysent->sv_size)
 		/* nosys */
Index: sys/kern/subr_syscall.c
===================================================================
--- sys/kern/subr_syscall.c
+++ sys/kern/subr_syscall.c
@@ -230,6 +230,7 @@
 			ksi.ksi_signo = SIGTRAP;
 			ksi.ksi_errno = td->td_errno;
 			ksi.ksi_code = TRAP_CAP;
+			ksi.ksi_info.si_syscall = sa->original_code;
 			trapsignal(td, &ksi);
 		}
 	}
Index: sys/sys/signal.h
===================================================================
--- sys/sys/signal.h
+++ sys/sys/signal.h
@@ -255,6 +255,9 @@
 		struct {
 			long	_band;		/* band event for SIGPOLL */
 		} _poll;			/* was this ever used ? */
+		struct {
+			int	_syscall;/* Syscall number for cap faults */
+		} _capsicum;
 		struct {
 			long	__spare1__;
 			int	__spare2__[7];
@@ -267,6 +270,7 @@
 #define si_overrun	_reason._timer._overrun
 #define si_mqd		_reason._mesgq._mqd
 #define si_band		_reason._poll._band
+#define si_syscall	_reason._capsicum._syscall
 
 #if defined(_WANT_LWPINFO32) || (defined(_KERNEL) && defined(__LP64__))
 struct siginfo32 {