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Date: Wed, 6 Apr 2022 01:09:13 +0800
From: Keyu Tao <>
Subject: getmntent() fails to parse when source is empty string


The first argument (const char *source) of mount(2) can be an empty 
string like this:

mount("", "/tmp/test1", "tmpfs", MS_NODEV | MS_NOSUID, NULL);

After mounting in Linux, the /etc/mtab shows this mountpoint like:

  /tmp/test1 tmpfs rw,nosuid,nodev,relatime,inode64 0 0

However, getmntent_r() in musl (in L42 of root/src/misc/mntent.c) uses 
sscanf() to parse mounted filesystem description:

cnt = sscanf(linebuf, " %n%*s%n %n%*s%n %n%*s%n %n%*s%n %d %d", ...)

When parsing the weird mountpoint, the value of cnt is 1: the mountpoint 
gets ignored and the do-while loop continues. I'm wondering whether it 
would be considered as a bug of musl.

Thank you.

(P.S: I'm not subscribed to the mail list and I would like to be Cc'd on 


Reproduction (in an Alpine 3.15 container on Debian 11, kernel version = 

$ sudo docker run -it --rm --privileged -v /tmp:/test alpine:3.15
/ # apk add build-base
/ # cat /test/mount.c
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <stdio.h>

int main(void) {
     int ret = mkdir("/tmp/test1", 0777);
     if (ret && errno != EEXIST) {
         perror("mkdir (1)");
     ret = mount("", "/tmp/test1", "tmpfs", MS_NODEV | MS_NOSUID, NULL);
     if (ret) {
         perror("mount (1)");
     ret = mkdir("/tmp/test2", 0777);
     if (ret && errno != EEXIST) {
         perror("mkdir (2)");
     ret = mount(NULL, "/tmp/test2", "tmpfs", MS_NODEV | MS_NOSUID, NULL);
     if (ret) {
         perror("mount (2)");
     // umount("/tmp/test1");
     // umount("/tmp/test2");
     return 0;
/ # gcc /test/mount.c
/ # ./a.out
/ # mount
(other mountpoints omitted)
devpts on /dev/console type devpts 
none on /tmp/test2 type tmpfs (rw,nosuid,nodev,relatime,inode64)
/ # cat /etc/mtab
(other mountpoints omitted)
devpts /dev/console devpts 
rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666 0 0
  /tmp/test1 tmpfs rw,nosuid,nodev,relatime,inode64 0 0
none /tmp/test2 tmpfs rw,nosuid,nodev,relatime,inode64 0 0

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