[PATCH 2/2] exfat: keep trailing dots in paths if keeptail is set

[David Disseldorp <ddiss@suse.de>]

exfat currently unconditionally strips trailing periods '.' when
performing path lookup. This is done intentionally, loosely following
Windows behaviour and specifications which state:

  #exFAT
  The concatenated file name has the same set of illegal characters as
  other FAT-based file systems (see Table 31).

  #FAT
  ...
  Leading and trailing spaces in a long name are ignored.
  Leading and embedded periods are allowed in a name and are stored in
  the long name. Trailing periods are ignored.

Note: Leading and trailing space ' ' characters are currently retained
by Linux kernel exfat, in conflict with the above specification.

Some implementations, such as fuse-exfat, don't perform path trailer
removal. When mounting images which contain trailing-dot paths, these
paths are unreachable, e.g.:

  + mount.exfat-fuse /dev/zram0 /mnt/test/
  FUSE exfat 1.3.0
  + cd /mnt/test/
  + touch fuse_created_dots... '  fuse_created_spaces  '
  + ls -l
  total 0
  -rwxrwxrwx 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
  -rwxrwxrwx 1 root 0 0 Aug 18 09:45  fuse_created_dots...
  + cd /
  + umount /mnt/test/
  + mount -t exfat /dev/zram0 /mnt/test
  + cd /mnt/test
  + ls -l
  ls: cannot access 'fuse_created_dots...': No such file or directory
  total 0
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
  -????????? ? ?    ? ?            ?  fuse_created_dots...
  + touch kexfat_created_dots... '  kexfat_created_spaces  '
  + ls -l
  ls: cannot access 'fuse_created_dots...': No such file or directory
  total 0
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  kexfat_created_spaces  '
  -????????? ? ?    ? ?            ?  fuse_created_dots...
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45  kexfat_created_dots
  + cd /
  + umount /mnt/test/

With this change, the "keeptail" mount option can be used to access
paths with trailing periods. E.g. continuing from the previous example:

  + mount -t exfat -o keeptail /dev/zram0 /mnt/test
  + cd /mnt/test
  + ls -l
  total 0
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32 '  fuse_created_spaces  '
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32 '  kexfat_created_spaces  '
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32  fuse_created_dots...
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32  kexfat_created_dots

Link: https://bugzilla.suse.com/show_bug.cgi?id=1188964
Link: https://lore.kernel.org/linux-fsdevel/003b01d755e4$31fb0d80$95f12880$@samsung.com/
Link: https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification#773-filename-field
Suggested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 fs/exfat/namei.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/fs/exfat/namei.c b/fs/exfat/namei.c
index 24b41103d1cc..e49455ce6da2 100644
--- a/fs/exfat/namei.c
+++ b/fs/exfat/namei.c
@@ -65,11 +65,14 @@ static int exfat_d_revalidate(struct dentry *dentry, unsigned int flags)
 	return ret;
 }
 
-/* returns the length of a struct qstr, ignoring trailing dots */
-static unsigned int exfat_striptail_len(unsigned int len, const char *name)
+/* returns the length of a struct qstr, possibly ignoring trailing dots */
+static unsigned int exfat_striptail_len(struct super_block *sb,
+					unsigned int len, const char *name)
 {
-	while (len && name[len - 1] == '.')
-		len--;
+	if (!EXFAT_SB(sb)->options.keeptail) {
+		while (len && name[len - 1] == '.')
+			len--;
+	}
 	return len;
 }
 
@@ -83,7 +86,7 @@ static int exfat_d_hash(const struct dentry *dentry, struct qstr *qstr)
 	struct super_block *sb = dentry->d_sb;
 	struct nls_table *t = EXFAT_SB(sb)->nls_io;
 	const unsigned char *name = qstr->name;
-	unsigned int len = exfat_striptail_len(qstr->len, qstr->name);
+	unsigned int len = exfat_striptail_len(sb, qstr->len, qstr->name);
 	unsigned long hash = init_name_hash(dentry);
 	int i, charlen;
 	wchar_t c;
@@ -104,8 +107,8 @@ static int exfat_d_cmp(const struct dentry *dentry, unsigned int len,
 {
 	struct super_block *sb = dentry->d_sb;
 	struct nls_table *t = EXFAT_SB(sb)->nls_io;
-	unsigned int alen = exfat_striptail_len(name->len, name->name);
-	unsigned int blen = exfat_striptail_len(len, str);
+	unsigned int alen = exfat_striptail_len(sb, name->len, name->name);
+	unsigned int blen = exfat_striptail_len(sb, len, str);
 	wchar_t c1, c2;
 	int charlen, i;
 
@@ -136,7 +139,7 @@ static int exfat_utf8_d_hash(const struct dentry *dentry, struct qstr *qstr)
 {
 	struct super_block *sb = dentry->d_sb;
 	const unsigned char *name = qstr->name;
-	unsigned int len = exfat_striptail_len(qstr->len, qstr->name);
+	unsigned int len = exfat_striptail_len(sb, qstr->len, qstr->name);
 	unsigned long hash = init_name_hash(dentry);
 	int i, charlen;
 	unicode_t u;
@@ -161,8 +164,8 @@ static int exfat_utf8_d_cmp(const struct dentry *dentry, unsigned int len,
 		const char *str, const struct qstr *name)
 {
 	struct super_block *sb = dentry->d_sb;
-	unsigned int alen = exfat_striptail_len(name->len, name->name);
-	unsigned int blen = exfat_striptail_len(len, str);
+	unsigned int alen = exfat_striptail_len(sb, name->len, name->name);
+	unsigned int blen = exfat_striptail_len(sb, len, str);
 	unicode_t u_a, u_b;
 	int charlen, i;
 
@@ -419,7 +422,7 @@ static int __exfat_resolve_path(struct inode *inode, const unsigned char *path,
 	struct exfat_inode_info *ei = EXFAT_I(inode);
 
 	/* strip all trailing periods */
-	namelen = exfat_striptail_len(strlen(path), path);
+	namelen = exfat_striptail_len(sb, strlen(path), path);
 	if (!namelen)
 		return -ENOENT;
 
-- 
2.31.1