[PATCH] fat: ignore .. subdir and always add a link to dirs

[Thadeu Lima de Souza Cascardo <cascardo@igalia.com>]

The tools used for creating images for the Lego Mindstrom EV3 are not
adding '.' and '..' entry in the 'Projects' directory.

Without this fix, the kernel can not fill the inode structure for
'Projects' directory.

See https://github.com/microsoft/pxt-ev3/issues/980
And https://github.com/microsoft/uf2-linux/issues/6

When counting the number of subdirs, ignore .. subdir and add one when
setting the initial link count for directories. This way, when .. is
present, it is still accounted for, and when neither . or .. are present, a
single link is still done, as it should, since this link would be the one
from the parent directory.

With this fix applied, we can mount an image with such empty directories,
access them, create subdirectories and remove them.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Cc: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/all/20220204062232.3410036-1-gwendal@chromium.org/
Cc: dlunev@chromium.org
---
 fs/fat/dir.c   |  3 ++-
 fs/fat/inode.c | 12 +++++++++---
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index 00235b8a1823..fcdb652efc53 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -937,7 +937,8 @@ int fat_subdirs(struct inode *dir)
 	bh = NULL;
 	cpos = 0;
 	while (fat_get_short_entry(dir, &cpos, &bh, &de) >= 0) {
-		if (de->attr & ATTR_DIR)
+		if (de->attr & ATTR_DIR &&
+		    strncmp(de->name, MSDOS_DOTDOT, MSDOS_NAME))
 			count++;
 	}
 	brelse(bh);
diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 1fac3dabf130..9a3bd38a4494 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -494,8 +494,14 @@ static int fat_validate_dir(struct inode *dir)
 {
 	struct super_block *sb = dir->i_sb;
 
-	if (dir->i_nlink < 2) {
-		/* Directory should have "."/".." entries at least. */
+	if (dir->i_nlink < 1) {
+		/*
+		 * Though it is expected that directories have at least
+		 * "."/".." entries, there are filesystems in the field that
+		 * don't have either. Even in those cases, at least one link
+		 * is necessary, as otherwise, when trying to increment it,
+		 * VFS would BUG.
+		 */
 		fat_fs_error(sb, "corrupted directory (invalid entries)");
 		return -EIO;
 	}
@@ -534,7 +540,7 @@ int fat_fill_inode(struct inode *inode, struct msdos_dir_entry *de)
 			return error;
 		MSDOS_I(inode)->mmu_private = inode->i_size;
 
-		set_nlink(inode, fat_subdirs(inode));
+		set_nlink(inode, fat_subdirs(inode) + 1);
 
 		error = fat_validate_dir(inode);
 		if (error < 0)
-- 
2.34.1