proc: proc_skip_spaces() shouldn't think it is working on C strings

proc_skip_spaces() seems to think it is working on C strings, and ends
up being just a wrapper around skip_spaces() with a really odd calling
convention.

Instead of basing it on skip_spaces(), it should have looked more like
proc_skip_char(), which really is the exact same function (except it
skips a particular character, rather than whitespace).  So use that as
inspiration, odd coding and all.

Now the calling convention actually makes sense and works for the
intended purpose.
Reported-and-tested-by: Kyle Zeng <zengyhkyle@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sysctl.c

@@ -267,13 +267,14 @@ int proc_dostring(struct ctl_table *table, int write,
 			ppos);
 }
 
-static size_t proc_skip_spaces(char **buf)
+static void proc_skip_spaces(char **buf, size_t *size)
 {
-	size_t ret;
-	char *tmp = skip_spaces(*buf);
-	ret = tmp - *buf;
-	*buf = tmp;
-	return ret;
+	while (*size) {
+		if (!isspace(**buf))
+			break;
+		(*size)--;
+		(*buf)++;
+	}
 }
 
 static void proc_skip_char(char **buf, size_t *size, const char v)
@@ -520,7 +521,7 @@ static int __do_proc_dointvec(void *tbl_data, struct ctl_table *table,
 		bool neg;
 
 		if (write) {
-			left -= proc_skip_spaces(&p);
+			proc_skip_spaces(&p, &left);
 
 			if (!left)
 				break;
@@ -547,7 +548,7 @@ static int __do_proc_dointvec(void *tbl_data, struct ctl_table *table,
 	if (!write && !first && left && !err)
 		proc_put_char(&buffer, &left, '\n');
 	if (write && !err && left)
-		left -= proc_skip_spaces(&p);
+		proc_skip_spaces(&p, &left);
 	if (write && first)
 		return err ? : -EINVAL;
 	*lenp -= left;
@@ -589,7 +590,7 @@ static int do_proc_douintvec_w(unsigned int *tbl_data,
 	if (left > PAGE_SIZE - 1)
 		left = PAGE_SIZE - 1;
 
-	left -= proc_skip_spaces(&p);
+	proc_skip_spaces(&p, &left);
 	if (!left) {
 		err = -EINVAL;
 		goto out_free;
@@ -609,7 +610,7 @@ static int do_proc_douintvec_w(unsigned int *tbl_data,
 	}
 
 	if (!err && left)
-		left -= proc_skip_spaces(&p);
+		proc_skip_spaces(&p, &left);
 
 out_free:
 	if (err)
@@ -1074,7 +1075,7 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table,
 		if (write) {
 			bool neg;
 
-			left -= proc_skip_spaces(&p);
+			proc_skip_spaces(&p, &left);
 			if (!left)
 				break;
 
@@ -1103,7 +1104,7 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table,
 	if (!write && !first && left && !err)
 		proc_put_char(&buffer, &left, '\n');
 	if (write && !err)
-		left -= proc_skip_spaces(&p);
+		proc_skip_spaces(&p, &left);
 	if (write && first)
 		return err ? : -EINVAL;
 	*lenp -= left;