Re: [musl] What if the line in /proc/mounts is too long when calling getmntent_r?

["Érico Nogueira" <ericonr@disroot.org>]

Unfortunately your message was sent all garbled (please try to stick to
plain text email ;), so I'm reproducing it cleanly underneath with my
answer:

>Hi!
>  I want to get cgroups mount information from /proc/mounts, but when i calling struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int buflen), i got nothing...
>  I run the program in a container.
>
>  alpine docker image:  amd64/alpine:3.14
>  musl: 1.2.2
>  program:  
>
>    #include <stdio.h>
>
>    #include <stdlib.h>
>
>    #include <mntent.h>
>
>
>    #define CGROUP_MAX_VAL 512
>
>
>    int main(void)
>
>    {
>
>      struct mntent ent;
>
>      FILE *f;
>
>      char buf[CGROUP_MAX_VAL];
>
>
>      f = setmntent("/proc/mounts", "r");
>
>      if (f == NULL) {
>
>        perror("setmntent");
>
>        exit(1);
>
>      }
>
>
>      while (getmntent_r(f, &ent, buf, sizeof(buf)) != NULL) {
>
>        printf("%s %s\n", ent.mnt_type, ent.mnt_opts);
>
>      }

The man page specifies that getmntent_r can return NULL on error, you
should check errno to see if anything happened. In this case, it would
be ERANGE, which tells you your buffer was too small.

>
>
>      endmntent(f);
>
>    }
>
>  contents of file "/proc/mounts"
>
>    overlay / overlay rw,relatime,lowerdir=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/955/fs:/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/954/fs:/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/953/fs:/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/952/fs:/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/941/fs:/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/940/fs:/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/879/fs:/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/325/fs,upperdir=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/956/fs,workdir=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/956/work 0 0
>
>    proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
>
>    tmpfs /dev tmpfs rw,nosuid,size=65536k,mode=755 0 0
>
>    devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666 0 0
>
>    mqueue /dev/mqueue mqueue rw,nosuid,nodev,noexec,relatime 0 0
>
>    sysfs /sys sysfs ro,nosuid,nodev,noexec,relatime 0 0
>
>    tmpfs /sys/fs/cgroup tmpfs rw,nosuid,nodev,noexec,relatime,mode=755 0 0
>
>    cgroup /sys/fs/cgroup/systemd cgroup rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/usr/lib/systemd/systemd-cgroups-agent,name=systemd 0 0
>
>    cgroup /sys/fs/cgroup/net_cls,net_prio cgroup rw,nosuid,nodev,noexec,relatime,net_prio,net_cls 0 0
>
>    cgroup /sys/fs/cgroup/pids cgroup rw,nosuid,nodev,noexec,relatime,pids 0 0
>
>    cgroup /sys/fs/cgroup/hugetlb cgroup rw,nosuid,nodev,noexec,relatime,hugetlb 0 0
>
>    cgroup /sys/fs/cgroup/cpuset cgroup rw,nosuid,nodev,noexec,relatime,cpuset 0 0
>
>    cgroup /sys/fs/cgroup/blkio cgroup rw,nosuid,nodev,noexec,relatime,blkio 0 0
>
>    cgroup /sys/fs/cgroup/devices cgroup rw,nosuid,nodev,noexec,relatime,devices 0 0
>
>    cgroup /sys/fs/cgroup/freezer cgroup rw,nosuid,nodev,noexec,relatime,freezer 0 0
>
>    cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,relatime,cpuacct,cpu 0 0
>
>    cgroup /sys/fs/cgroup/memory cgroup rw,nosuid,nodev,noexec,relatime,memory 0 0
>
>    cgroup /sys/fs/cgroup/perf_event cgroup rw,nosuid,nodev,noexec,relatime,perf_event 0 0
>
>    ...
>
>
>  I find the first line of the file /proc/mounts has 822 characters(In theory the 'overlay' could be even longer), more than CGROUP_MAX_VAL(512) defined in the proagram. Function fget in getmntent_r cann't get the whole line into linebuf, neither the character '\n'. And the function strchr(linebuf, '\n') returns false, causing program returnd. 
>  The function struct mntent *getmntent(FILE *f) is a good chioce to deal this. But it can not be used in multiple threads, right?

Correct, getmntent isn't thread safe.

>  Maybe the implementation of GNU libc struct mntent *__getmntent_r (FILE *stream, struct mntent *mp, char *buffer, int bufsiz) can be referenced.

From what I can see, glibc silently throws away any and all chars that
don't fit in the provided buffer until it finds a newline. getmntent_r
isn't actually specified, so I guess its behavior is a bit up to the
implementation. Anyhow, musl's reports ERANGE properly (maybe the man
page can be fixed to mention it?) and you should use a dynamic buffer in
your program if you expect to deal with huge entries, and resize it if
getmntent_r fails with ERANGE.

>
>  thanks!