From a4c51f1bb38f272fb9c3dbaa33f1f383639ce345 Mon Sep 17 00:00:00 2001

From: Colin Walters <walters@verbum.org>

Date: Tue, 11 Jul 2017 12:48:57 -0400

Subject: [PATCH] fstab-generator: Chase symlinks where possible (#6293)

This has a long history; see see 5261ba901845c084de5a8fd06500ed09bfb0bd80

which originally introduced the behavior.  Unfortunately that commit

doesn't include any rationale, but IIRC the basic issue is that

systemd wants to model the real mount state as units, and symlinks

make canonicalization much more difficult.

At the same time, on a RHEL6 system (upstart), one can make e.g. `/home` a

symlink, and things work as well as they always did; but one doesn't have

access to the sophistication of mount units (dependencies, introspection, etc.)

Supporting symlinks here will hence make it easier for people to do upgrades to

RHEL7 and beyond.

The `/home` as symlink case also appears prominently for OSTree; see

https://ostree.readthedocs.io/en/latest/manual/adapting-existing/

Further work has landed in the nspawn case for this; see e.g.

d944dc9553009822deaddec76814f5642a6a8176

A basic limitation with doing this in the fstab generator (and that I hit while

doing some testing) is that we obviously can't chase symlinks into mounts,

since the generator runs early before mounts. Or at least - doing so would

require multiple passes over the fstab data (as well as looking at existing

mount units), and potentially doing multi-phase generation. I'm not sure it's

worth doing that without a real world use case. For now, this will fix at least

the OSTree + `/home` <https://bugzilla.redhat.com/show_bug.cgi?id=1382873> case

mentioned above, and in general anyone who for whatever reason has symlinks in

their `/etc/fstab`.

(cherry picked from commit 634735b56b82bdde3c67193ba7b470bab80fdcbd)

Resolves: RHEL-6223

---

 man/systemd-fstab-generator.xml       |  8 +++++

 man/systemd.mount.xml                 |  5 ++--

 src/fstab-generator/fstab-generator.c | 42 ++++++++++++++++++++++-----

 3 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/man/systemd-fstab-generator.xml b/man/systemd-fstab-generator.xml

index bdc2dc1d0e..6f45c680cc 100644

--- a/man/systemd-fstab-generator.xml

+++ b/man/systemd-fstab-generator.xml

@@ -71,6 +71,14 @@

     for more information about special <filename>/etc/fstab</filename>

     mount options this generator understands.</para>

+    <para>One special topic is handling of symbolic links.  Historical init

+    implementations supported symlinks in <filename>/etc/fstab</filename>.

+    Because mount units will refuse mounts where the target is a symbolic link,

+    this generator will resolve any symlinks as far as possible when processing

+    <filename>/etc/fstab</filename> in order to enhance backwards compatibility.

+    If a symlink target does not exist at the time that this generator runs, it

+    is assumed that the symlink target is the final target of the mount.</para>

+

     <para><filename>systemd-fstab-generator</filename> implements

     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

   </refsect1>

diff --git a/man/systemd.mount.xml b/man/systemd.mount.xml

index 1590c44cec..5021db5584 100644

--- a/man/systemd.mount.xml

+++ b/man/systemd.mount.xml

@@ -295,8 +295,9 @@

       <varlistentry>

         <term><varname>Where=</varname></term>

-        <listitem><para>Takes an absolute path of a directory of the

-        mount point. If the mount point does not exist at the time of

+        <listitem><para>Takes an absolute path of a directory for the

+        mount point; in particular, the destination cannot be a symbolic

+        link.  If the mount point does not exist at the time of

         mounting, it is created. This string must be reflected in the

         unit filename. (See above.) This option is

         mandatory.</para></listitem>

diff --git a/src/fstab-generator/fstab-generator.c b/src/fstab-generator/fstab-generator.c

index 23b5457e43..91603b08f4 100644

--- a/src/fstab-generator/fstab-generator.c

+++ b/src/fstab-generator/fstab-generator.c

@@ -240,6 +240,7 @@ static int write_idle_timeout(FILE *f, const char *where, const char *opts) {

 static int add_mount(

                 const char *what,

                 const char *where,

+                const char *original_where,

                 const char *fstype,

                 const char *opts,

                 int passno,

@@ -327,12 +328,14 @@ static int add_mount(

         }

         fprintf(f,

-                "\n"

+                 "\n"

                 "[Mount]\n"

-                "What=%s\n"

-                "Where=%s\n",

-                what,

-                where);

+                "What=%s\n",

+                 what);

+

+        if (original_where)

+                fprintf(f, "# Canonicalized from %s\n", original_where);

+        fprintf(f, "Where=%s\n", where);

         if (!isempty(fstype) && !streq(fstype, "auto"))

                 fprintf(f, "Type=%s\n", fstype);

@@ -436,7 +439,7 @@ static int parse_fstab(bool initrd) {

         }

         while ((me = getmntent(f))) {

-                _cleanup_free_ char *where = NULL, *what = NULL;

+                _cleanup_free_ char *where = NULL, *what = NULL, *canonical_where = NULL;

                 bool noauto, nofail;

                 int k;

@@ -456,8 +459,28 @@ static int parse_fstab(bool initrd) {

                 if (!where)

                         return log_oom();

-                if (is_path(where))

+                if (is_path(where)) {

                         path_kill_slashes(where);

+                        /* Follow symlinks here; see 5261ba901845c084de5a8fd06500ed09bfb0bd80 which makes sense for

+                         * mount units, but causes problems since it historically worked to have symlinks in e.g.

+                         * /etc/fstab. So we canonicalize here. Note that we use CHASE_NONEXISTENT to handle the case

+                         * where a symlink refers to another mount target; this works assuming the sub-mountpoint

+                         * target is the final directory.

+                         */

+                        r = chase_symlinks(where, initrd ? "/sysroot" : NULL,

+                                           CHASE_PREFIX_ROOT | CHASE_NONEXISTENT,

+                                           &canonical_where);

+                        if (r < 0)

+                                /* In this case for now we continue on as if it wasn't a symlink */

+                                log_warning_errno(r, "Failed to read symlink target for %s: %m", where);

+                        else {

+                                if (streq(canonical_where, where))

+                                        canonical_where = mfree(canonical_where);

+                                else

+                                        log_debug("Canonicalized what=%s where=%s to %s",

+                                                  what, where, canonical_where);

+                        }

+                }

                 noauto = fstab_test_yes_no_option(me->mnt_opts, "noauto\0" "auto\0");

                 nofail = fstab_test_yes_no_option(me->mnt_opts, "nofail\0" "fail\0");

@@ -482,7 +505,8 @@ static int parse_fstab(bool initrd) {

                                 post = SPECIAL_LOCAL_FS_TARGET;

                         k = add_mount(what,

-                                      where,

+                                      canonical_where ?: where,

+                                      canonical_where ? where: NULL,

                                       me->mnt_type,

                                       me->mnt_opts,

                                       me->mnt_passno,

@@ -526,6 +550,7 @@ static int add_sysroot_mount(void) {

         log_debug("Found entry what=%s where=/sysroot type=%s", what, strna(arg_root_fstype));

         return add_mount(what,

                          "/sysroot",

+                         NULL,

                          arg_root_fstype,

                          opts,

                          1,

@@ -583,6 +608,7 @@ static int add_sysroot_usr_mount(void) {

         log_debug("Found entry what=%s where=/sysroot/usr type=%s", what, strna(arg_usr_fstype));

         return add_mount(what,

                          "/sysroot/usr",

+                         NULL,

                          arg_usr_fstype,

                          opts,

                          1,