[PATCH v3 10/33] exfat: keep the current mount options on remount

[Christian Brauner <brauner@kernel.org>]

exfat_init_fs_context() only inherits uid, gid, fmask and dmask for a
reconfigure. But exfat_reconfigure() refuses to change iocharset,
keep_last_dots, sys_tz, time_offset and allow_utime. So every remount
that doesn't repeat a non-default value for one of them fails with
EINVAL, including emergency remount (sysrq u) which passes no options at
all. And an option-less remount silently resets errors=, zero_size_dir
and discard.

So start a reconfigure from the current options like gfs2 does. The
iocharset string stays NULL in the new context because
->init_fs_context() runs without s_umount for mount(2) and a concurrent
remount could free it. It's only compared if the remount names one. A
shutdown turns discard off for good so exfat_reconfigure() keeps it off.

The next patch stops exfat from setting SB_RDONLY after errors. An
errored exfat then stays read-write and emergency remount would run into
this as well.

Fixes: e6fd5d3a4317 ("exfat: support modifying mount options via remount")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/exfat/super.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/fs/exfat/super.c b/fs/exfat/super.c
index 4943cef977418..44d2000007dfb 100644
--- a/fs/exfat/super.c
+++ b/fs/exfat/super.c
@@ -797,14 +797,12 @@ static int exfat_reconfigure(struct fs_context *fc)
 		mutex_unlock(&sbi->s_lock);
 	}
 
-	if (new_opts->allow_utime == (unsigned short)-1)
-		new_opts->allow_utime = ~new_opts->fs_dmask & 0022;
-
 	/*
 	 * Since the old settings of these mount options are cached in
 	 * inodes or dentries, they cannot be modified dynamically.
 	 */
-	if (strcmp(new_opts->iocharset, cur_opts->iocharset) ||
+	if ((new_opts->iocharset &&
+	     strcmp(new_opts->iocharset, cur_opts->iocharset)) ||
 	    new_opts->keep_last_dots != cur_opts->keep_last_dots ||
 	    new_opts->sys_tz != cur_opts->sys_tz ||
 	    new_opts->time_offset != cur_opts->time_offset ||
@@ -815,6 +813,10 @@ static int exfat_reconfigure(struct fs_context *fc)
 	    new_opts->allow_utime != cur_opts->allow_utime)
 		return -EINVAL;
 
+	/* A shutdown turned discard off for good. */
+	if (exfat_forced_shutdown(sb))
+		new_opts->discard = 0;
+
 	if (new_opts->discard != cur_opts->discard &&
 	    new_opts->discard &&
 	    !bdev_max_discard_sectors(sb->s_bdev)) {
@@ -823,6 +825,8 @@ static int exfat_reconfigure(struct fs_context *fc)
 	}
 
 	swap(*cur_opts, *new_opts);
+	/* The iocharset can't change, keep the current string. */
+	swap(cur_opts->iocharset, new_opts->iocharset);
 
 	return 0;
 }
@@ -849,23 +853,24 @@ static int exfat_init_fs_context(struct fs_context *fc)
 
 	if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE && fc->root) {
 		struct super_block *sb = fc->root->d_sb;
-		struct exfat_mount_options *cur_opts = &EXFAT_SB(sb)->options;
 
-		sbi->options.fs_uid = cur_opts->fs_uid;
-		sbi->options.fs_gid = cur_opts->fs_gid;
-		sbi->options.fs_fmask = cur_opts->fs_fmask;
-		sbi->options.fs_dmask = cur_opts->fs_dmask;
+		/*
+		 * Start from the current options so a remount only changes
+		 * what it specifies. The iocharset string stays with the
+		 * superblock, see exfat_reconfigure().
+		 */
+		sbi->options = EXFAT_SB(sb)->options;
+		sbi->options.iocharset = NULL;
 	} else {
 		sbi->options.fs_uid = current_uid();
 		sbi->options.fs_gid = current_gid();
 		sbi->options.fs_fmask = current->fs->umask;
 		sbi->options.fs_dmask = current->fs->umask;
+		sbi->options.allow_utime = -1;
+		sbi->options.errors = EXFAT_ERRORS_RO;
+		exfat_set_iocharset(&sbi->options, exfat_default_iocharset);
 	}
 
-	sbi->options.allow_utime = -1;
-	sbi->options.errors = EXFAT_ERRORS_RO;
-	exfat_set_iocharset(&sbi->options, exfat_default_iocharset);
-
 	fc->s_fs_info = sbi;
 	fc->ops = &exfat_context_ops;
 	return 0;

-- 
2.53.0