From 08b7e9988272113ca5640cf5e115ea51449fb392 Mon Sep 17 00:00:00 2001 From: Ian Kent Date: Mon, 2 Sep 2019 13:26:14 +0200 Subject: [PATCH] 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. --- ChangeLog | 8 ++++++++ misc/mntent_r.c | 48 ++++++++++++++++++++++++++++++++++++------------ 2 files changed, 44 insertions(+), 12 deletions(-) diff --git a/ChangeLog b/ChangeLog index 3d33931..de0d7ff 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,11 @@ +2019-09-02 Ian Kent + + Use autofs "ignore" mount hint in getmntent_r/getmntent. + * misc/mntent_r.c (get_mnt_entry): New function, extracted from + getmntent_r. + (__getmntent_r): Call it. Filter out autofs entries with an + "ignore" mount option. + 2019-08-30 Wilco Dijkstra * benchtests/bench-memcpy.c (simple_memcpy): Remove. diff --git a/misc/mntent_r.c b/misc/mntent_r.c index 5d88c45..d90e8d7 100644 --- a/misc/mntent_r.c +++ b/misc/mntent_r.c @@ -18,6 +18,7 @@ #include #include +#include #include #include #include @@ -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 */ @@ -181,9 +174,40 @@ __getmntent_r (FILE *stream, struct mntent *mp, char *buffer, int bufsiz) 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) -- 2.7.4