proc: simpler iterations for /proc/*/cmdline

"rv" variable is used both as a counter of bytes transferred and an
error value holder but it can be reduced solely to error values if
original start of userspace buffer is stashed and used at the very end.

[akpm@linux-foundation.org: simplify cleanup code]
Link: http://lkml.kernel.org/r/20180221193009.GA28678@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -215,8 +215,9 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 	unsigned long arg_start, arg_end, env_start, env_end;
 	unsigned long len1, len2, len;
 	unsigned long p;
+	char __user *buf0 = buf;
 	char c;
-	ssize_t rv;
+	int rv;
 
 	BUG_ON(*pos < 0);
 
@@ -253,25 +254,20 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 	len2 = env_end - env_start;
 
 	/* Empty ARGV. */
-	if (len1 == 0) {
-		rv = 0;
-		goto out_free_page;
-	}
+	if (len1 == 0)
+		goto end;
+
 	/*
 	 * Inherently racy -- command line shares address space
 	 * with code and data.
 	 */
-	if (access_remote_vm(mm, arg_end - 1, &c, 1, FOLL_ANON) != 1) {
-		rv = 0;
-		goto out_free_page;
-	}
-
-	rv = 0;
+	if (access_remote_vm(mm, arg_end - 1, &c, 1, FOLL_ANON) != 1)
+		goto end;
 
 	if (c == '\0') {
 		/* Command line (set of strings) occupies whole ARGV. */
 		if (len1 <= *pos)
-			goto out_free_page;
+			goto end;
 
 		p = arg_start + *pos;
 		len = len1 - *pos;
@@ -281,7 +277,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			nr_read = min3(count, len, PAGE_SIZE);
 			nr_read = access_remote_vm(mm, p, page, nr_read, FOLL_ANON);
 			if (nr_read == 0)
-				goto out_free_page;
+				goto end;
 
 			if (copy_to_user(buf, page, nr_read)) {
 				rv = -EFAULT;
@@ -292,7 +288,6 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			len	-= nr_read;
 			buf	+= nr_read;
 			count	-= nr_read;
-			rv	+= nr_read;
 		}
 	} else {
 		/*
@@ -323,7 +318,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 				nr_read = min3(count, len, PAGE_SIZE);
 				nr_read = access_remote_vm(mm, p, page, nr_read, FOLL_ANON);
 				if (nr_read == 0)
-					goto out_free_page;
+					goto end;
 
 				/*
 				 * Command line can be shorter than whole ARGV
@@ -340,10 +335,9 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 				len	-= nr_write;
 				buf	+= nr_write;
 				count	-= nr_write;
-				rv	+= nr_write;
 
 				if (nr_write < nr_read)
-					goto out_free_page;
+					goto end;
 			}
 
 			/* Only first chunk can be read partially. */
@@ -352,12 +346,13 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 		}
 	}
 
+end:
+	*pos += buf - buf0;
+	rv = buf - buf0;
 out_free_page:
 	free_page((unsigned long)page);
 out_mmput:
 	mmput(mm);
-	if (rv > 0)
-		*pos += rv;
 	return rv;
 }