pmc: Initialise and check the pm_flags field for CONFIGURELOG

Whilst the former is not breaking, the latter is, and so this will be
followed by a bump to the pmc major version. This will allow the flags
to actually be usable in future, as otherwise we cannot distinguish
uninitialised stack junk from a deliberately-initialised value.

Reviewed by:	jhb, mhorne
Differential Revision:	https://reviews.freebsd.org/D40049

lib/libpmc/libpmc.c

@@ -1195,6 +1195,7 @@ pmc_configure_logfile(int fd)
 {
 	struct pmc_op_configurelog cla;
 
+	cla.pm_flags = 0;
 	cla.pm_logfd = fd;
 	if (PMC_CALL(CONFIGURELOG, &cla) < 0)
 		return (-1);

share/man/man4/hwpmc.4

@@ -598,6 +598,12 @@ An invalid CPU number was passed in for a
 .Dv PMC_OP_GETPMCINFO
 operation.
 .It Bq Er EINVAL
+The
+.Ar pm_flags
+argument to a
+.Dv PMC_OP_CONFIGURELOG
+request contained unknown flags.
+.It Bq Er EINVAL
 A
 .Dv PMC_OP_CONFIGURELOG
 request to de-configure a log file was issued without a log file

sys/dev/hwpmc/hwpmc_mod.c

@@ -3438,6 +3438,12 @@ pmc_syscall_handler(struct thread *td, void *syscall_args)
 			break;
 		}
 
+		/* No flags currently implemented */
+		if (cl.pm_flags != 0) {
+			error = EINVAL;
+			break;
+		}
+
 		/* mark this process as owning a log file */
 		p = td->td_proc;
 		if ((po = pmc_find_owner_descriptor(p)) == NULL)