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(RHEL-6223) fstab-generator: Chase symlinks where possible (#6293) #146

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Sep 25, 2023
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(RHEL-6223) fstab-generator: Chase symlinks where possible (#6293) #146

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8 changes: 8 additions & 0 deletions man/systemd-fstab-generator.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -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>
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5 changes: 3 additions & 2 deletions man/systemd.mount.xml
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Expand Up @@ -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>
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42 changes: 34 additions & 8 deletions src/fstab-generator/fstab-generator.c
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Expand Up @@ -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,
Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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;

Expand All @@ -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");
Expand All @@ -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,
Expand Down Expand Up @@ -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,
Expand Down Expand Up @@ -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,
Expand Down