fs/9p: Don't open remote file with APPEND mode when writeback cache is used

When page cache is used, writebacks are done on a page granularity, and it
is expected that the underlying filesystem (such as v9fs) should respect
the write position.  However, currently v9fs will passthrough O_APPEND to
the server even on cached mode.  This causes data corruption if a sync or
fstat gets between two writes to the same file.

This patch removes the APPEND flag from the open request we send to the
server when writeback caching is involved.  I believe keeping server-side
APPEND is probably fine for uncached mode (even if two fds are opened, one
without O_APPEND and one with it, this should still be fine since they
would use separate fid for the writes).

Signed-off-by: Tingmao Wang <m@maowtm.org>
Fixes: 4eb3117 ("fs/9p: Rework cache modes and add new options to Documentation")
Message-ID: <20251102235631.8724-1-m@maowtm.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: micromaomao

fs/9p/vfs_file.c

@@ -43,24 +43,29 @@ int v9fs_file_open(struct inode *inode, struct file *file)
 	struct v9fs_session_info *v9ses;
 	struct p9_fid *fid;
 	int omode;
+	int o_append;
 
 	p9_debug(P9_DEBUG_VFS, "inode: %p file: %p\n", inode, file);
 	v9ses = v9fs_inode2v9ses(inode);
-	if (v9fs_proto_dotl(v9ses))
+	if (v9fs_proto_dotl(v9ses)) {
 		omode = v9fs_open_to_dotl_flags(file->f_flags);
-	else
+		o_append = P9_DOTL_APPEND;
+	} else {
 		omode = v9fs_uflags2omode(file->f_flags,
 					v9fs_proto_dotu(v9ses));
+		o_append = P9_OAPPEND;
+	}
 	fid = file->private_data;
 	if (!fid) {
 		fid = v9fs_fid_clone(file_dentry(file));
 		if (IS_ERR(fid))
 			return PTR_ERR(fid);
 
 		if ((v9ses->cache & CACHE_WRITEBACK) && (omode & P9_OWRITE)) {
-			int writeback_omode = (omode & ~P9_OWRITE) | P9_ORDWR;
+			int writeback_omode = (omode & ~(P9_OWRITE | o_append)) | P9_ORDWR;
 
 			p9_debug(P9_DEBUG_CACHE, "write-only file with writeback enabled, try opening O_RDWR\n");
+
 			err = p9_client_open(fid, writeback_omode);
 			if (err < 0) {
 				p9_debug(P9_DEBUG_CACHE, "could not open O_RDWR, disabling caches\n");

fs/9p/vfs_inode.c

@@ -786,7 +786,7 @@ v9fs_vfs_atomic_open(struct inode *dir, struct dentry *dentry,
 	p9_omode = v9fs_uflags2omode(flags, v9fs_proto_dotu(v9ses));
 
 	if ((v9ses->cache & CACHE_WRITEBACK) && (p9_omode & P9_OWRITE)) {
-		p9_omode = (p9_omode & ~P9_OWRITE) | P9_ORDWR;
+		p9_omode = (p9_omode & ~(P9_OWRITE | P9_OAPPEND)) | P9_ORDWR;
 		p9_debug(P9_DEBUG_CACHE,
 			"write-only file with writeback enabled, creating w/ O_RDWR\n");
 	}
@@ -1393,4 +1393,3 @@ static const struct inode_operations v9fs_symlink_inode_operations = {
 	.getattr = v9fs_vfs_getattr,
 	.setattr = v9fs_vfs_setattr,
 };
-

fs/9p/vfs_inode_dotl.c

@@ -282,7 +282,7 @@ v9fs_vfs_atomic_open_dotl(struct inode *dir, struct dentry *dentry,
 	}
 
 	if ((v9ses->cache & CACHE_WRITEBACK) && (p9_omode & P9_OWRITE)) {
-		p9_omode = (p9_omode & ~P9_OWRITE) | P9_ORDWR;
+		p9_omode = (p9_omode & ~(P9_OWRITE | P9_DOTL_APPEND)) | P9_ORDWR;
 		p9_debug(P9_DEBUG_CACHE,
 			"write-only file with writeback enabled, creating w/ O_RDWR\n");
 	}