proc: return on proc_readdir error

Commit f0c3b509 ("[readdir] convert procfs") introduced a bug on the
listing of the proc file-system.  The return value of proc_readdir()
isn't tested anymore in the proc_root_readdir function.

This lead to an "interesting" behaviour when we are using the getdents()
system call with a buffer too small: instead of failing, it returns the
first entries of /proc (enough to fill the given buffer), plus the PID
directories.

This is not triggered on glibc (as getdents is called with a 32KB
buffer), but on uclibc, the buffer size is only 1KB, thus some proc
entries are missing.

See https://lkml.org/lkml/2013/8/12/288 for more background.
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/root.c

@@ -205,7 +205,9 @@ static struct dentry *proc_root_lookup(struct inode * dir, struct dentry * dentr
 static int proc_root_readdir(struct file *file, struct dir_context *ctx)
 {
 	if (ctx->pos < FIRST_PROCESS_ENTRY) {
-		proc_readdir(file, ctx);
+		int error = proc_readdir(file, ctx);
+		if (unlikely(error <= 0))
+			return error;
 		ctx->pos = FIRST_PROCESS_ENTRY;
 	}