Index: lib/libc/sys/procctl.2
===================================================================
--- lib/libc/sys/procctl.2
+++ lib/libc/sys/procctl.2
@@ -413,6 +413,16 @@
 .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/kern/subr_syscall.c
===================================================================
--- sys/kern/subr_syscall.c
+++ sys/kern/subr_syscall.c
@@ -199,6 +199,7 @@
                        ksi.ksi_signo = SIGTRAP;
                        ksi.ksi_errno = td->td_errno;
                        ksi.ksi_code = TRAP_CAP;
+                       ksi.ksi_info.si_syscall = sa->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__))