[PATCH 097/104] virtiofsd: Fix data corruption with O_APPEND wirte in writeback mode

[Dr. David Alan Gilbert (git)]

From: Misono Tomohiro <address@hidden>

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>
---
 tools/virtiofsd/passthrough_ll.c | 66 ++++++++++++++++----------------
 1 file changed, 33 insertions(+), 33 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 6b3d396b6f..1bf251a91d 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -1676,6 +1676,37 @@ static void lo_releasedir(fuse_req_t req, fuse_ino_t ino,
     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)
 {
@@ -1705,12 +1736,7 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, 
const char *name,
         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);
@@ -1920,33 +1946,7 @@ 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);
-- 
2.23.0