fuse: handle extent map files in fuse_passthrough_fadvise()

This addresses a collision between the pending backing_file fadvise
patch and the new extent-mapping code for famfs (which falls into the
passthrough bucket.

A file opened with FOPEN_PASSTHROUGH on an extent map backing has no
backing file: fuse_passthrough_open() returns before setting
ff->passthrough and ff->cred.  fuse_file_fadvise() routes every
FOPEN_PASSTHROUGH file to fuse_passthrough_fadvise(), which installed
the NULL ff->cred on the task with override_creds() and then passed the
NULL backing file to vfs_fadvise().  Any posix_fadvise(), readahead()
or madvise(MADV_WILLNEED) on such a file oopsed.  fio triggers it at
file open with its default invalidate and fadvise_hint settings.

Fall back to generic_fadvise() when there is no backing file, as the
other passthrough helpers fall back to their extent map or generic
implementations.  generic_fadvise() already ignores advice on DAX
inodes while keeping the EINVAL and ESPIPE checks.

Reported-by: Aravind Ramesh <arramesh@micron.com>
Fixes: 69f77a06ffad ("fuse: Forward vfs_fadvise to backing_file in passthrough mode")
Tested-by: John Groves <john@groves.net>
Signed-off-by: John Groves <john@groves.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

3 files changed, 7 insertions, 3 deletions

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index da73d1e183c4..eb5ae11b6010 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -3129,7 +3129,7 @@ static int fuse_file_fadvise(struct file *file, loff_t offset, loff_t len, int a
 	struct fuse_file *ff = file->private_data;
 
 	if (fuse_is_passthrough(ff))
-		return fuse_passthrough_fadvise(ff, offset, len, advice);
+		return fuse_passthrough_fadvise(file, offset, len, advice);
 
 	return generic_fadvise(file, offset, len, advice);
 }
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 311252eef2e9..c91af283aef2 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1368,7 +1368,7 @@ ssize_t fuse_passthrough_splice_write(struct pipe_inode_info *pipe,
 				      struct file *out, loff_t *ppos,
 				      size_t len, unsigned int flags);
 ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma);
-int fuse_passthrough_fadvise(struct fuse_file *ff, loff_t offset, loff_t len, int advice);
+int fuse_passthrough_fadvise(struct file *file, loff_t offset, loff_t len, int advice);
 
 #ifdef CONFIG_SYSCTL
 extern int fuse_sysctl_register(void);
diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index 1c1cf1085c9c..0ed964c479d9 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -159,12 +159,16 @@ ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma)
 	return backing_file_mmap(backing_file, vma, &ctx);
 }
 
-int fuse_passthrough_fadvise(struct fuse_file *ff, loff_t offset, loff_t len, int advice)
+int fuse_passthrough_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
 {
+	struct fuse_file *ff = file->private_data;
 	struct file *backing_file = fuse_file_passthrough(ff);
 	const struct cred *old_cred;
 	int ret;
 
+	if (!backing_file)
+		return generic_fadvise(file, offset, len, advice);
+
 	old_cred = override_creds(ff->cred);
 	ret = vfs_fadvise(backing_file, offset, len, advice);
 	revert_creds(old_cred);