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Date: Fri, 27 Aug 2021 10:26:35 -0300
From: Érico Nogueira <ericonr@...root.org>
To: <musl@...ts.openwall.com>, <2010267516@...com>
Subject: Re: What if the line in /proc/mounts is too long when
calling getmntent_r?
Apparently I failed to CC you in my original reply, sorry.
Forwarded message from Érico Nogueira on Fri Aug 27, 2021 at 10:05 AM:
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!
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