[PATCH v4 05/21] afs: Remove setting of AS_RELEASE_ALWAYS for symlinks and mountpoints

[David Howells <dhowells@redhat.com>]

Regular AFS files correctly use afs_file_aops which have release_folio
set as netfs_release_folio, so AS_RELEASE_ALWAYS is valid for them
when fscache is enabled (set via afs_vnode_set_cache()).
Symlinks and mountpoints in AFS use afs_dir_aops, which does not provide
a release_folio callback. However, afs_apply_status() unconditionally
calls mapping_set_release_always() for these.

In such case when memory management code attempts to release folios,
filemap_release_folio() checks folio_needs_release() which
returns true due to AS_RELEASE_ALWAYS being set. Since there is no
release_folio callback, it falls through to try_to_free_buffers(),
which at present expects buffer_heads to be not null. For symlinks
and mountpoints without buffer_heads, this causes pointer dereference.

[dh: Added more bits that were missed]

Fixes: eae9e78951bb ("afs: Use netfslib for symlinks, allowing them to be cached")
Signed-off-by: Deepakkumar Karn <dkarn@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
---
 fs/afs/inode.c    | 7 +++----
 fs/afs/internal.h | 2 --
 2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index a88995629d72..54ac6ec21daf 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -52,9 +52,9 @@ static noinline void dump_vnode(struct afs_vnode *vnode, struct afs_vnode *paren
 /*
  * Set parameters for the netfs library
  */
-static void afs_set_netfs_context(struct afs_vnode *vnode)
+static void afs_set_netfs_context(struct afs_vnode *vnode, bool is_file)
 {
-	netfs_inode_init(&vnode->netfs, &afs_req_ops, true);
+	netfs_inode_init(&vnode->netfs, &afs_req_ops, is_file);
 }
 
 /*
@@ -126,7 +126,6 @@ static int afs_inode_init_from_status(struct afs_operation *op,
 		}
 		inode->i_mapping->a_ops	= &afs_symlink_aops;
 		inode_nohighmem(inode);
-		mapping_set_release_always(inode->i_mapping);
 		break;
 	default:
 		dump_vnode(vnode, op->file[0].vnode != vnode ? op->file[0].vnode : NULL);
@@ -136,7 +135,7 @@ static int afs_inode_init_from_status(struct afs_operation *op,
 
 	i_size_write(inode, status->size);
 	inode_set_bytes(inode, status->size);
-	afs_set_netfs_context(vnode);
+	afs_set_netfs_context(vnode, status->type == AFS_FTYPE_FILE);
 
 	vnode->invalid_before	= status->data_version;
 	trace_afs_set_dv(vnode, status->data_version);
diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index 0b72a8566299..785c646856d7 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -750,8 +750,6 @@ static inline void afs_vnode_set_cache(struct afs_vnode *vnode,
 {
 #ifdef CONFIG_AFS_FSCACHE
 	vnode->netfs.cache = cookie;
-	if (cookie)
-		mapping_set_release_always(vnode->netfs.inode.i_mapping);
 #endif
 }