/proc/PID/syscall

This adds /proc/PID/syscall and /proc/PID/task/TID/syscall magic files.
These use task_current_syscall() to show the task's current system call
number and argument registers, stack pointer and PC.  For a task blocked
but not in a syscall, the file shows "-1" in place of the syscall number,
followed by only the SP and PC.  For a task that's not blocked, it shows
"running".
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -509,6 +509,26 @@ static int proc_pid_limits(struct task_struct *task, char *buffer)
 	return count;
 }
 
+#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
+static int proc_pid_syscall(struct task_struct *task, char *buffer)
+{
+	long nr;
+	unsigned long args[6], sp, pc;
+
+	if (task_current_syscall(task, &nr, args, 6, &sp, &pc))
+		return sprintf(buffer, "running\n");
+
+	if (nr < 0)
+		return sprintf(buffer, "%ld 0x%lx 0x%lx\n", nr, sp, pc);
+
+	return sprintf(buffer,
+		       "%ld 0x%lx 0x%lx 0x%lx 0x%lx 0x%lx 0x%lx 0x%lx 0x%lx\n",
+		       nr,
+		       args[0], args[1], args[2], args[3], args[4], args[5],
+		       sp, pc);
+}
+#endif /* CONFIG_HAVE_ARCH_TRACEHOOK */
+
 /************************************************************************/
 /*                       Here the fs part begins                        */
 /************************************************************************/
@@ -2477,6 +2497,9 @@ static const struct pid_entry tgid_base_stuff[] = {
 	INF("limits",	  S_IRUSR, pid_limits),
 #ifdef CONFIG_SCHED_DEBUG
 	REG("sched",      S_IRUGO|S_IWUSR, pid_sched),
+#endif
+#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
+	INF("syscall",    S_IRUSR, pid_syscall),
 #endif
 	INF("cmdline",    S_IRUGO, pid_cmdline),
 	ONE("stat",       S_IRUGO, tgid_stat),
@@ -2809,6 +2832,9 @@ static const struct pid_entry tid_base_stuff[] = {
 	INF("limits",	 S_IRUSR, pid_limits),
 #ifdef CONFIG_SCHED_DEBUG
 	REG("sched",     S_IRUGO|S_IWUSR, pid_sched),
+#endif
+#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
+	INF("syscall",   S_IRUSR, pid_syscall),
 #endif
 	INF("cmdline",   S_IRUGO, pid_cmdline),
 	ONE("stat",      S_IRUGO, tid_stat),