fault-inject: parse as natural 1-based value for fail-nth write interface

The value written to fail-nth file is parsed as 0-based.  Parsing as
one-based is more natural to understand and it enables to cancel the
previous setup by simply writing '0'.

This change also converts task->fail_nth from signed to unsigned int.

Link: http://lkml.kernel.org/r/1491490561-10485-3-git-send-email-akinobu.mita@gmail.com
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/fault-injection/fault-injection.txt

@@ -138,8 +138,8 @@ o proc entries
 
 - /proc/self/task/<current-tid>/fail-nth:
 
-	Write to this file of integer N makes N-th call in the current task fail
-	(N is 0-based). Read from this file returns a single char 'Y' or 'N'
+	Write to this file of integer N makes N-th call in the task fail.
+	Read from this file returns a single char 'Y' or 'N'
 	that says if the fault setup with a previous write to this file was
 	injected or not, and disables the fault if it wasn't yet injected.
 	Note that this file enables all types of faults (slab, futex, etc).
@@ -320,7 +320,7 @@ int main()
 	system("echo N > /sys/kernel/debug/failslab/ignore-gfp-wait");
 	sprintf(buf, "/proc/self/task/%ld/fail-nth", syscall(SYS_gettid));
 	fail_nth = open(buf, O_RDWR);
-	for (i = 0;; i++) {
+	for (i = 1;; i++) {
 		sprintf(buf, "%d", i);
 		write(fail_nth, buf, strlen(buf));
 		res = socketpair(AF_LOCAL, SOCK_STREAM, 0, fds);
@@ -339,7 +339,6 @@ int main()
 
 An example output:
 
-0-th fault Y: res=-1/23
 1-th fault Y: res=-1/23
 2-th fault Y: res=-1/23
 3-th fault Y: res=-1/12

fs/proc/base.c

@@ -1360,7 +1360,8 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
 				   size_t count, loff_t *ppos)
 {
 	struct task_struct *task;
-	int err, n;
+	int err;
+	unsigned int n;
 
 	task = get_proc_task(file_inode(file));
 	if (!task)
@@ -1368,12 +1369,10 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
 	put_task_struct(task);
 	if (task != current)
 		return -EPERM;
-	err = kstrtoint_from_user(buf, count, 0, &n);
+	err = kstrtouint_from_user(buf, count, 0, &n);
 	if (err)
 		return err;
-	if (n < 0 || n == INT_MAX)
-		return -EINVAL;
-	current->fail_nth = n + 1;
+	current->fail_nth = n;
 	return count;
 }
 

include/linux/sched.h

@@ -974,7 +974,7 @@ struct task_struct {
 
 #ifdef CONFIG_FAULT_INJECTION
 	int				make_it_fail;
-	int fail_nth;
+	unsigned int			fail_nth;
 #endif
 	/*
 	 * When (nr_dirtied >= nr_dirtied_pause), it's time to call