[PATCH] virtiofsd: Fix data corruption with O_APPEND wirte in writeback mode

[Misono Tomohiro]

When writeback mode is enabled (-o writeback), O_APPEND handling is
done in kernel. Therefore virtiofsd clears O_APPEND flag when open.
Otherwise O_APPEND flag takes precedence over pwrite() and write
data may corrupt.

Currently clearing O_APPEND flag is done in lo_open(), but we also
need the same operation in lo_create(). So, factor out the flag
update operation in lo_open() to update_open_flags() and call it
in both lo_open() and lo_create().

This fixes the failure of xfstest generic/069 in writeback mode
(which tests O_APPEND write data integrity).

Signed-off-by: Misono Tomohiro <address@hidden>
---
 contrib/virtiofsd/passthrough_ll.c | 56 +++++++++++++++---------------
 1 file changed, 28 insertions(+), 28 deletions(-)

diff --git a/contrib/virtiofsd/passthrough_ll.c 
b/contrib/virtiofsd/passthrough_ll.c
index e8892c3c32..79fb78ecce 100644
--- a/contrib/virtiofsd/passthrough_ll.c
+++ b/contrib/virtiofsd/passthrough_ll.c
@@ -1733,6 +1733,32 @@ static void lo_releasedir(fuse_req_t req, fuse_ino_t 
ino, struct fuse_file_info
        fuse_reply_err(req, 0);
 }
 
+static void update_open_flags(int writeback, struct fuse_file_info *fi)
+{
+       /* With writeback cache, kernel may send read requests even
+          when userspace opened write-only */
+       if (writeback && (fi->flags & O_ACCMODE) == O_WRONLY) {
+               fi->flags &= ~O_ACCMODE;
+               fi->flags |= O_RDWR;
+       }
+
+       /* With writeback cache, O_APPEND is handled by the kernel.
+          This breaks atomicity (since the file may change in the
+          underlying filesystem, so that the kernel's idea of the
+          end of the file isn't accurate anymore). In this example,
+          we just accept that. A more rigorous filesystem may want
+          to return an error here */
+       if (writeback && (fi->flags & O_APPEND))
+               fi->flags &= ~O_APPEND;
+
+       /*
+        * O_DIRECT in guest should not necessarily mean bypassing page
+        * cache on host as well. If somebody needs that behavior, it
+        * probably should be a configuration knob in daemon.
+        */
+       fi->flags &= ~O_DIRECT;
+}
+
 static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
                      mode_t mode, struct fuse_file_info *fi)
 {
@@ -1760,12 +1786,7 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, 
const char *name,
        if (err)
                goto out;
 
-       /*
-        * O_DIRECT in guest should not necessarily mean bypassing page
-        * cache on host as well. If somebody needs that behavior, it
-        * probably should be a configuration knob in daemon.
-        */
-       fi->flags &= ~O_DIRECT;
+       update_open_flags(lo->writeback, fi);
 
        fd = openat(parent_inode->fd, name,
                    (fi->flags | O_CREAT) & ~O_NOFOLLOW, mode);
@@ -1966,28 +1987,7 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, 
struct fuse_file_info *fi)
 
        fuse_log(FUSE_LOG_DEBUG, "lo_open(ino=%" PRIu64 ", flags=%d)\n", ino, 
fi->flags);
 
-       /* With writeback cache, kernel may send read requests even
-          when userspace opened write-only */
-       if (lo->writeback && (fi->flags & O_ACCMODE) == O_WRONLY) {
-               fi->flags &= ~O_ACCMODE;
-               fi->flags |= O_RDWR;
-       }
-
-       /* With writeback cache, O_APPEND is handled by the kernel.
-          This breaks atomicity (since the file may change in the
-          underlying filesystem, so that the kernel's idea of the
-          end of the file isn't accurate anymore). In this example,
-          we just accept that. A more rigorous filesystem may want
-          to return an error here */
-       if (lo->writeback && (fi->flags & O_APPEND))
-               fi->flags &= ~O_APPEND;
-
-       /*
-        * O_DIRECT in guest should not necessarily mean bypassing page
-        * cache on host as well. If somebody needs that behavior, it
-        * probably should be a configuration knob in daemon.
-        */
-       fi->flags &= ~O_DIRECT;
+       update_open_flags(lo->writeback, fi);
 
        sprintf(buf, "%i", lo_fd(req, ino));
        fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW);
-- 
2.21.0