[RFC PATCH] fuse: fix race in fuse_notify_store()

[Luis Henriques <luis@igalia.com>]

Userspace filesystems can push data for a specific inode without it being
explicitly requested.  This can be accomplished by using NOTIFY_STORE.  However,
this may race against another process performing different operations on the
same inode.

If, for example, there is a process reading from it, it may happen that it will
block waiting for data to be available (locking the folio), while the FUSE
server will also block trying to lock the same folio to update it with the inode
data.

The easiest solution, as suggested by Miklos, is to allow the userspace
filesystem to skip locked folios.

Link: https://lore.kernel.org/CH2PR14MB41040692ABC50334F500789ED6C89@CH2PR14MB4104.namprd14.prod.outlook.com
Reported-by: Teng Qin <tqin@jumptrading.com>
Originally-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Luis Henriques <luis@igalia.com>
---
Hi!

Instead of sending the usual 'ping' to the original thread, I've decided to
resend the patch as an RFC.

As I mentioned before, this is an attempt to forward port the original patch
from Miklos to the folios world.  Also, the same question:

if we fail to get a folio and need to skip it, 'this_num' needs to be
updated; but I'm not 100% sure if it's OK to use PAGE_SIZE in that case.

Obviously, libfuse will need to support this new NOWAIT flag (I can look at
that, of course).  But I was wondering if the NOTIFY_STORE behaviour
shouldn't *always* skip locked folios instead of doing it only when the flag
is set.

(By the way, I'm not sure if I'm using the 'Originally-by:' tag correctly;
I just want to make sure the authorship is preserved.  Please let me know if
that's not correct.)

Cheers,
-- 
Luis

 fs/fuse/dev.c             | 29 ++++++++++++++++++++++-------
 include/uapi/linux/fuse.h |  8 +++++++-
 2 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 27ccae63495d..9a0cd88a9bb9 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1630,6 +1630,7 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 	unsigned int num;
 	loff_t file_size;
 	loff_t end;
+	int fgp_flags = FGP_LOCK | FGP_ACCESSED | FGP_CREAT;
 
 	err = -EINVAL;
 	if (size < sizeof(outarg))
@@ -1645,6 +1646,9 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 
 	nodeid = outarg.nodeid;
 
+	if (outarg.flags & FUSE_NOTIFY_STORE_NOWAIT)
+		fgp_flags |= FGP_NOWAIT;
+
 	down_read(&fc->killsb);
 
 	err = -ENOENT;
@@ -1668,14 +1672,25 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 		struct page *page;
 		unsigned int this_num;
 
-		folio = filemap_grab_folio(mapping, index);
-		err = PTR_ERR(folio);
-		if (IS_ERR(folio))
-			goto out_iput;
+		folio = __filemap_get_folio(mapping, index, fgp_flags,
+					    mapping_gfp_mask(mapping));
+		err = PTR_ERR_OR_ZERO(folio);
+		if (err) {
+			if (!(outarg.flags & FUSE_NOTIFY_STORE_NOWAIT))
+				goto out_iput;
+			page = NULL;
+			/* XXX */
+			this_num = min_t(unsigned int, num, PAGE_SIZE - offset);
+		} else {
+			page = &folio->page;
+			this_num = min_t(unsigned int, num,
+					 folio_size(folio) - offset);
+		}
 
-		page = &folio->page;
-		this_num = min_t(unsigned, num, folio_size(folio) - offset);
 		err = fuse_copy_page(cs, &page, offset, this_num, 0);
+		if (!page)
+			goto skip;
+
 		if (!folio_test_uptodate(folio) && !err && offset == 0 &&
 		    (this_num == folio_size(folio) || file_size == end)) {
 			folio_zero_segment(folio, this_num, folio_size(folio));
@@ -1683,7 +1698,7 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 		}
 		folio_unlock(folio);
 		folio_put(folio);
-
+skip:
 		if (err)
 			goto out_iput;
 
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index e9e78292d107..59725f89340e 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -576,6 +576,12 @@ struct fuse_file_lock {
  */
 #define FUSE_EXPIRE_ONLY		(1 << 0)
 
+/**
+ * notify_store flags
+ * FUSE_NOTIFY_STORE_NOWAIT: skip locked pages
+ */
+#define FUSE_NOTIFY_STORE_NOWAIT	(1 << 0)
+
 /**
  * extension type
  * FUSE_MAX_NR_SECCTX: maximum value of &fuse_secctx_header.nr_secctx
@@ -1075,7 +1081,7 @@ struct fuse_notify_store_out {
 	uint64_t	nodeid;
 	uint64_t	offset;
 	uint32_t	size;
-	uint32_t	padding;
+	uint32_t	flags;
 };
 
 struct fuse_notify_retrieve_out {