[PATCH v2] readdir: require opt-in for d_type flags

[Amir Goldstein <amir73il@gmail.com>]

Commit c31f91c6af96 ("fuse: don't allow signals to interrupt getdents
copying") introduced the use of high bits in d_type as flags. However,
overlayfs was not adapted to handle this change.

In ovl_cache_entry_new(), the code checks if d_type == DT_CHR to
determine if an entry might be a whiteout. When fuse is used as the
lower layer and sets high bits in d_type, this comparison fails,
causing whiteout files to not be recognized properly and resulting in
incorrect overlayfs behavior.

Fix this by requiring callers of iterate_dir() to opt-in for getting
flag bits in d_type outside of S_DT_MASK.

Fixes: c31f91c6af96 ("fuse: don't allow signals to interrupt getdents copying")
Link: https://lore.kernel.org/all/20260107034551.439-1-luochunsheng@ustc.edu/
Link: https://github.com/containerd/stargz-snapshotter/issues/2214
Reported-by: Chunsheng Luo <luochunsheng@ustc.edu>
Reviewed-by: Chunsheng Luo <luochunsheng@ustc.edu>
Tested-by: Chunsheng Luo <luochunsheng@ustc.edu>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Christian,

"Not pretty, but fine." [0]
This is what you had to say on the Fixes commit ;)
Maybe this will be finer...

I was considering whether or not a mention in porting.rst is due.

My conclusion was that the regressing commit might have needed to
mention a change of vfs API, but this fix brings the vfs API back to
conform to pre v6.16 semantics, so no porting instructions apply.

Thanks,
Amir.

Chages sinse v1:
- Rename s/dt_flag_mask/dt_flags_mask/
- Add Test/Reviewd-by

[0] https://lore.kernel.org/linux-fsdevel/20250515-antlitz-aufzwingen-cdba155ce864@brauner/

 fs/readdir.c       | 3 +++
 include/linux/fs.h | 6 +++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/readdir.c b/fs/readdir.c
index 7764b86389788..73707b6816e9a 100644
--- a/fs/readdir.c
+++ b/fs/readdir.c
@@ -316,6 +316,7 @@ SYSCALL_DEFINE3(getdents, unsigned int, fd,
 	struct getdents_callback buf = {
 		.ctx.actor = filldir,
 		.ctx.count = count,
+		.ctx.dt_flags_mask = FILLDIR_FLAG_NOINTR,
 		.current_dir = dirent
 	};
 	int error;
@@ -400,6 +401,7 @@ SYSCALL_DEFINE3(getdents64, unsigned int, fd,
 	struct getdents_callback64 buf = {
 		.ctx.actor = filldir64,
 		.ctx.count = count,
+		.ctx.dt_flags_mask = FILLDIR_FLAG_NOINTR,
 		.current_dir = dirent
 	};
 	int error;
@@ -569,6 +571,7 @@ COMPAT_SYSCALL_DEFINE3(getdents, unsigned int, fd,
 	struct compat_getdents_callback buf = {
 		.ctx.actor = compat_filldir,
 		.ctx.count = count,
+		.ctx.dt_flags_mask = FILLDIR_FLAG_NOINTR,
 		.current_dir = dirent,
 	};
 	int error;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f5c9cf28c4dcf..a01621fa636a6 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1855,6 +1855,8 @@ struct dir_context {
 	 * INT_MAX  unlimited
 	 */
 	int count;
+	/* @actor supports these flags in d_type high bits */
+	unsigned int dt_flags_mask;
 };
 
 /* If OR-ed with d_type, pending signals are not checked */
@@ -3524,7 +3526,9 @@ static inline bool dir_emit(struct dir_context *ctx,
 			    const char *name, int namelen,
 			    u64 ino, unsigned type)
 {
-	return ctx->actor(ctx, name, namelen, ctx->pos, ino, type);
+	unsigned int dt_mask = S_DT_MASK | ctx->dt_flags_mask;
+
+	return ctx->actor(ctx, name, namelen, ctx->pos, ino, type & dt_mask);
 }
 static inline bool dir_emit_dot(struct file *file, struct dir_context *ctx)
 {
-- 
2.52.0