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vfs_donmount: in certain cases try r/o mount if r/w mount fails

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If the operation is not an update, if neither r/w nor r/o mode is
explicitly requested, if the error code hints at the possibility of the
media being read-only, and if the fallback is allowed, then we can try
to automatically downgrade to the readonly mode.

This is especially useful for auto-mounting of removable media that
sometimes can happen to be write-protected.

The fallback to r/o is not enabled by default.  It can be requested on a
per-mount basis with a new mount option, 'autoro'.  Or it can be
globally allowed by setting vfs.default_autoro.

Reviewed by:	cem, kib
MFC after:	3 weeks
Relnotes:	yes
Differential Revision: https://reviews.freebsd.org/D13361
This commit is contained in:
Andriy Gapon 2018-03-27 14:31:42 +00:00
parent 5a2fc46447
commit 31260bf042
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=331616
2 changed files with 62 additions and 3 deletions
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4
sbin/mount/mount.8
View File

@ -155,6 +155,10 @@ This flag indicates that the file system was mounted by
.Xr automountd 8 .
Automounted file systems are automatically unmounted by
.Xr autounmountd 8 .
.It Cm autoro
Mount the file system read-write.
If that fails with an error that suggests that the media could be read-only,
then automatically try to mount the file system read-only.
.It Cm current
When used with the
.Fl u

61
sys/kern/vfs_mount.c
View File

@ -81,6 +81,10 @@ static int usermount = 0;
SYSCTL_INT(_vfs, OID_AUTO, usermount, CTLFLAG_RW, &usermount, 0,
"Unprivileged users may mount and unmount file systems");
static bool default_autoro = false;
SYSCTL_BOOL(_vfs, OID_AUTO, default_autoro, CTLFLAG_RW, &default_autoro, 0,
"Retry failed r/w mount as r/o if no explicit ro/rw option is specified");
MALLOC_DEFINE(M_MOUNT, "mount", "vfs mount structure");
MALLOC_DEFINE(M_STATFS, "statfs", "statfs structure");
static uma_zone_t mount_zone;
@ -546,6 +550,31 @@ vfs_mount_destroy(struct mount *mp)
uma_zfree(mount_zone, mp);
}
static bool
vfs_should_downgrade_to_ro_mount(uint64_t fsflags, int error)
{
/* This is an upgrade of an exisiting mount. */
if ((fsflags & MNT_UPDATE) != 0)
return (false);
/* This is already an R/O mount. */
if ((fsflags & MNT_RDONLY) != 0)
return (false);
switch (error) {
case ENODEV: /* generic, geom, ... */
case EACCES: /* cam/scsi, ... */
case EROFS: /* md, mmcsd, ... */
/*
* These errors can be returned by the storage layer to signal
* that the media is read-only. No harm in the R/O mount
* attempt if the error was returned for some other reason.
*/
return (true);
default:
return (false);
}
}
int
vfs_donmount(struct thread *td, uint64_t fsflags, struct uio *fsoptions)
{
@ -553,10 +582,12 @@ vfs_donmount(struct thread *td, uint64_t fsflags, struct uio *fsoptions)
struct vfsopt *opt, *tmp_opt;
char *fstype, *fspath, *errmsg;
int error, fstypelen, fspathlen, errmsg_len, errmsg_pos;
bool autoro;
errmsg = fspath = NULL;
errmsg_len = fspathlen = 0;
errmsg_pos = -1;
autoro = default_autoro;
error = vfs_buildopts(fsoptions, &optlist);
if (error)
@ -648,16 +679,27 @@ vfs_donmount(struct thread *td, uint64_t fsflags, struct uio *fsoptions)
free(opt->name, M_MOUNT);
opt->name = strdup("nonosymfollow", M_MOUNT);
}
else if (strcmp(opt->name, "noro") == 0)
else if (strcmp(opt->name, "noro") == 0) {
fsflags &= ~MNT_RDONLY;
else if (strcmp(opt->name, "rw") == 0)
autoro = false;
}
else if (strcmp(opt->name, "rw") == 0) {
fsflags &= ~MNT_RDONLY;
else if (strcmp(opt->name, "ro") == 0)
autoro = false;
}
else if (strcmp(opt->name, "ro") == 0) {
fsflags |= MNT_RDONLY;
autoro = false;
}
else if (strcmp(opt->name, "rdonly") == 0) {
free(opt->name, M_MOUNT);
opt->name = strdup("ro", M_MOUNT);
fsflags |= MNT_RDONLY;
autoro = false;
}
else if (strcmp(opt->name, "autoro") == 0) {
vfs_freeopt(optlist, opt);
autoro = true;
}
else if (strcmp(opt->name, "suiddir") == 0)
fsflags |= MNT_SUIDDIR;
@ -682,6 +724,19 @@ vfs_donmount(struct thread *td, uint64_t fsflags, struct uio *fsoptions)
}
error = vfs_domount(td, fstype, fspath, fsflags, &optlist);
/*
* See if we can mount in the read-only mode if the error code suggests
* that it could be possible and the mount options allow for that.
* Never try it if "[no]{ro|rw}" has been explicitly requested and not
* overridden by "autoro".
*/
if (autoro && vfs_should_downgrade_to_ro_mount(fsflags, error)) {
printf("%s: R/W mount failed, possibly R/O media,"
" trying R/O mount\n", __func__);
fsflags |= MNT_RDONLY;
error = vfs_domount(td, fstype, fspath, fsflags, &optlist);
}
bail:
/* copyout the errmsg */
if (errmsg_pos != -1 && ((2 * errmsg_pos + 1) < fsoptions->uio_iovcnt)