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libmount: ignore "bind" from fstab on command line "remount" 

The current code always apply all flags from /etc/fstab on remount.
Unfortunately remount+bind has special semantic and it's impossible
from command line to avoid interaction with the "bind" from fstab.

Example, fstab:

	/dev/sda1 /bar ext4 defaults 0 1
	/bar /foo none bind 0 0

Command line:

    # mount /foo -o remount,rw

produces:

    mount(... MS_REMOUNT|MS_BIND ) syscall

This changes the per-mountpoint (VFS) ro flag to rw, but doesn't
change the filesystem itself.

This patch forces libmount to ignore "bind" from fstab when "-o
remount" specified on command line. If you need remount+bind semantic
you have to specify the "bind" flag on command line. This allow to
differentiate between

    # mount /foo -o remount,bind,rw  --> mount(MS_REMOUNT|MS_BIND)
and

    # mount /foo -o remount,rw       --> mount(MS_REMOUNT)

Suggested-by: NeilBrown <neilb@suse.com>
Signed-off-by: Karel Zak <kzak@redhat.com>
This commit is contained in:
Karel Zak 2017-06-20 12:46:02 +02:00
parent 22eb2f0190
commit 9b76b0e98b
2 changed files with 29 additions and 3 deletions
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14
libmount/src/context.c
View File

@ -2045,7 +2045,7 @@ static int apply_table(struct libmnt_context *cxt, struct libmnt_table *tb,
*/
int mnt_context_apply_fstab(struct libmnt_context *cxt)
{
int rc = -1, isremount = 0;
int rc = -1, isremount = 0, iscmdbind = 0;
struct libmnt_table *tab = NULL;
const char *src = NULL, *tgt = NULL;
unsigned long mflags = 0;
@ -2070,8 +2070,10 @@ int mnt_context_apply_fstab(struct libmnt_context *cxt)
cxt->optsmode &= ~MNT_OMODE_FORCE;
}
if (mnt_context_get_mflags(cxt, &mflags) == 0 && mflags & MS_REMOUNT)
isremount = 1;
if (mnt_context_get_mflags(cxt, &mflags) == 0) {
isremount = !!(mflags & MS_REMOUNT);
iscmdbind = !!(mflags & MS_BIND);
}
if (cxt->fs) {
src = mnt_fs_get_source(cxt->fs);
@ -2138,6 +2140,12 @@ int mnt_context_apply_fstab(struct libmnt_context *cxt)
* not found are not so important and may be misinterpreted by
* applications... */
rc = -MNT_ERR_NOFSTAB;
} else if (isremount && !iscmdbind) {
/* remove "bind" from fstab (or no-op if not present) */
mnt_optstr_remove_option(&cxt->fs->optstr, "bind");
}
return rc;
}

18
sys-utils/mount.8
View File

@ -385,6 +385,20 @@ will be read-only.
It's also possible to change nosuid, nodev, noexec, noatime, nodiratime and
relatime VFS entry flags by "remount,bind" operation. It's impossible to change
mount options recursively (for example with \fB-o rbind,ro\fR).
.BR mount (8)
since v2.31 ignores the \fBbind\fR flag from
.I /etc/fstab
on
.B remount operation
(if "-o remount" specified on command line). This is necessary to fully control
mount options on remount by command line. In the previous versions the bind
flag has been always applied and it was impossible to re-define mount options
without interaction with the bind semantic. This
.BR mount (8)
behavior does not affect situations when "remount,bind" is specified in the
.I /etc/fstab
file.
.RE
.SS The move operation
@ -1088,6 +1102,10 @@ Attempt to remount an already-mounted filesystem. This is commonly
used to change the mount flags for a filesystem, especially to make a
readonly filesystem writable. It does not change device or mount point.
The remount operation together with the
.B bind
flag has special semantic. See above, the subsection \fBBind mounts\fR.
The remount functionality follows the standard way the mount command works
with options from fstab. This means that \fBmount\fR does not
read fstab (or mtab) only when both