commit 08b7e9988272113ca5640cf5e115ea51449fb392

Author: Ian Kent <ikent@redhat.com>

Date:   Mon Sep 2 13:26:14 2019 +0200

    Use autofs "ignore" mount hint in getmntent_r/getmntent

    Historically autofs mounts were not included in mount table

    listings. This is the case in other SysV autofs implementations

    and was also the case with Linux autofs.

    But now that /etc/mtab is a symlink to the proc filesystem

    mount table the autofs mount entries appear in the mount table

    on Linux.

    Prior to the symlinking of /etc/mtab mount table it was

    sufficient to call mount(2) and simply not update /etc/mtab

    to exclude autofs mounts from mount listings.

    Also, with the symlinking of /etc/mtab we have seen a shift in

    usage toward using the proc mount tables directly.

    But the autofs mount entries need to be retained when coming

    from the proc file system for applications that need them

    (largely autofs file system users themselves) so filtering out

    these entries within the kernel itself can't be done. So it

    needs be done in user space.

    There are three reasons to omit the autofs mount entries.

    One is that certain types of auto-mounts have an autofs mount

    for every entry in their autofs mount map and these maps can

    be quite large. This leads to mount table listings containing

    a lot of unnecessary entries.

    Also, this change in behaviour between autofs implementations

    can cause problems for applications that use getmntent(3) in

    other OS implementations as well as Linux.

    Lastly, there's very little that user space can do with autofs

    mount entries since this must be left to the autofs mount owner,

    typically the automount daemon. But it can also lead to attempts

    to access automount managed paths resulting mounts being triggered

    when they aren't needed or mounts staying mounted for much longer

    thay they need be. While the point of this change ins't to help

    with these problems (and it can be quite a problem) it may be

    a welcome side effect.

    So the Linux autofs file system has been modified to accept a

    pseudo mount option of "ignore" (as is used in other OS

    implementations) so that user space can use this as a hint to

    skip autofs entries on reading the mount table.

    The Linux autofs automount daemon used getmntent(3) itself and

    has been modified to use the proc file system directly so that

    it can "ignore" mount option.

    The use of this mount option is opt-in and a configuration

    option has been added which defaults to not use this option

    so if there are applications that need these entries, other

    than autofs itself, they can be retained. Also, since this

    filtering is based on an added mount option earlier versions

    of Linux autofs iand other autofs file system users will not

    use the option and so won't be affected by the change.

diff -rup a/misc/mntent_r.c b/misc/mntent_r.c

--- a/misc/mntent_r.c	2012-12-24 22:02:13.000000000 -0500

+++ b/misc/mntent_r.c	2020-01-20 15:55:23.417838854 -0500

@@ -18,6 +18,7 @@

 #include <alloca.h>

 #include <mntent.h>

+#include <stdbool.h>

 #include <stdio.h>

 #include <stdio_ext.h>

 #include <string.h>

@@ -112,26 +113,18 @@ decode_name (char *buf)

   return buf;

 }

-

-/* Read one mount table entry from STREAM.  Returns a pointer to storage

-   reused on the next call, or null for EOF or error (use feof/ferror to

-   check).  */

-struct mntent *

-__getmntent_r (FILE *stream, struct mntent *mp, char *buffer, int bufsiz)

+static bool

+get_mnt_entry (FILE *stream, struct mntent *mp, char *buffer, int bufsiz)

 {

   char *cp;

   char *head;

-  flockfile (stream);

   do

     {

       char *end_ptr;

       if (fgets_unlocked (buffer, bufsiz, stream) == NULL)

-	{

-	  funlockfile (stream);

-	  return NULL;

-	}

+	return false;

       end_ptr = strchr (buffer, '\n');

       if (end_ptr != NULL)	/* chop newline */

@@ -173,9 +166,40 @@ __getmntent_r (FILE *stream, struct mnte

     case 2:

       break;

     }

+

+  return true;

+}

+

+/* Read one mount table entry from STREAM.  Returns a pointer to storage

+   reused on the next call, or null for EOF or error (use feof/ferror to

+   check).  */

+struct mntent *

+__getmntent_r (FILE *stream, struct mntent *mp, char *buffer, int bufsiz)

+{

+  struct mntent *result;

+

+  flockfile (stream);

+  while (true)

+    if (get_mnt_entry (stream, mp, buffer, bufsiz))

+      {

+	/* If the file system is autofs look for a mount option hint

+	   ("ignore") to skip the entry.  */

+	if (strcmp (mp->mnt_type, "autofs") == 0 && __hasmntopt (mp, "ignore"))

+	  memset (mp, 0, sizeof (*mp));

+	else

+	  {

+	    result = mp;

+	    break;

+	  }

+      }

+    else

+      {

+	result = NULL;

+	break;

+      }

   funlockfile (stream);

-  return mp;

+  return result;

 }

 libc_hidden_def (__getmntent_r)

 weak_alias (__getmntent_r, getmntent_r)