[PATCH v2] fs/9p: Don't open remote file with APPEND mode when writeback cache is used

[Tingmao Wang <m@maowtm.org>]

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>
---

I haven't done a bisect to figure out if this regression was introduced by
a change or was this behaviour always present - 6.14 has the same problem
and 6.13 did not compile for me due to some problem with bool / true /
false being a keyword in c23, and it could not compile with c17 either.

 fs/9p/vfs_file.c       | 11 ++++++++---
 fs/9p/vfs_inode.c      |  3 +--
 fs/9p/vfs_inode_dotl.c |  2 +-
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/fs/9p/vfs_file.c b/fs/9p/vfs_file.c
index 612a230bc012..6f3880208587 100644
--- a/fs/9p/vfs_file.c
+++ b/fs/9p/vfs_file.c
@@ -43,14 +43,18 @@ 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));
@@ -58,9 +62,10 @@ int v9fs_file_open(struct inode *inode, struct file *file)
 			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");
diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index 8666c9c62258..97abe65bf7c1 100644
--- a/fs/9p/vfs_inode.c
+++ b/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,
 };
-
diff --git a/fs/9p/vfs_inode_dotl.c b/fs/9p/vfs_inode_dotl.c
index 1661a25f2772..643e759eacb2 100644
--- a/fs/9p/vfs_inode_dotl.c
+++ b/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");
 	}
-- 
2.51.2