virtiofsd: Add basic support for FUSE_SYNCFS request

Honor the expected behavior of syncfs() to synchronously flush all data
and metadata to disk on linux systems.

If virtiofsd is started with '-o announce_submounts', the client is
expected to send a FUSE_SYNCFS request for each individual submount.
In this case, we just create a new file descriptor on the submount
inode with lo_inode_open(), call syncfs() on it and close it. The
intermediary file is needed because O_PATH descriptors aren't
backed by an actual file and syncfs() would fail with EBADF.

If virtiofsd is started without '-o announce_submounts' or if the
client doesn't have the FUSE_CAP_SUBMOUNTS capability, the client
only sends a single FUSE_SYNCFS request for the root inode. The
server would thus need to track submounts internally and call
syncfs() on each of them. This will be implemented later.

Note that syncfs() might suffer from a time penalty if the submounts
are being hammered by some unrelated workload on the host. The only
solution to prevent that is to avoid shared mounts.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20220215181529.164070-2-groug@kaod.org>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/fuse_lowlevel.c

@@ -1967,6 +1967,16 @@ static void do_lseek(fuse_req_t req, fuse_ino_t nodeid,
     }
 }
 
+static void do_syncfs(fuse_req_t req, fuse_ino_t nodeid,
+                      struct fuse_mbuf_iter *iter)
+{
+    if (req->se->op.syncfs) {
+        req->se->op.syncfs(req, nodeid);
+    } else {
+        fuse_reply_err(req, ENOSYS);
+    }
+}
+
 static void do_init(fuse_req_t req, fuse_ino_t nodeid,
                     struct fuse_mbuf_iter *iter)
 {
@@ -2399,6 +2409,7 @@ static struct {
     [FUSE_RENAME2] = { do_rename2, "RENAME2" },
     [FUSE_COPY_FILE_RANGE] = { do_copy_file_range, "COPY_FILE_RANGE" },
     [FUSE_LSEEK] = { do_lseek, "LSEEK" },
+    [FUSE_SYNCFS] = { do_syncfs, "SYNCFS" },
 };
 
 #define FUSE_MAXOP (sizeof(fuse_ll_ops) / sizeof(fuse_ll_ops[0]))

tools/virtiofsd/fuse_lowlevel.h

@@ -1226,6 +1226,19 @@ struct fuse_lowlevel_ops {
      */
     void (*lseek)(fuse_req_t req, fuse_ino_t ino, off_t off, int whence,
                   struct fuse_file_info *fi);
+
+    /**
+     * Synchronize file system content
+     *
+     * If this request is answered with an error code of ENOSYS,
+     * this is treated as success and future calls to syncfs() will
+     * succeed automatically without being sent to the filesystem
+     * process.
+     *
+     * @param req request handle
+     * @param ino the inode number
+     */
+    void (*syncfs)(fuse_req_t req, fuse_ino_t ino);
 };
 
 /**

tools/virtiofsd/passthrough_ll.c

@@ -3699,6 +3699,49 @@ static void lo_lseek(fuse_req_t req, fuse_ino_t ino, off_t off, int whence,
     }
 }
 
+static int lo_do_syncfs(struct lo_data *lo, struct lo_inode *inode)
+{
+    int fd, ret = 0;
+
+    fuse_log(FUSE_LOG_DEBUG, "lo_do_syncfs(ino=%" PRIu64 ")\n",
+             inode->fuse_ino);
+
+    fd = lo_inode_open(lo, inode, O_RDONLY);
+    if (fd < 0) {
+        return -fd;
+    }
+
+    if (syncfs(fd) < 0) {
+        ret = errno;
+    }
+
+    close(fd);
+    return ret;
+}
+
+static void lo_syncfs(fuse_req_t req, fuse_ino_t ino)
+{
+    struct lo_data *lo = lo_data(req);
+    struct lo_inode *inode = lo_inode(req, ino);
+    int err;
+
+    if (!inode) {
+        fuse_reply_err(req, EBADF);
+        return;
+    }
+
+    err = lo_do_syncfs(lo, inode);
+    lo_inode_put(lo, &inode);
+
+    /*
+     * If submounts aren't announced, the client only sends a request to
+     * sync the root inode. TODO: Track submounts internally and iterate
+     * over them as well.
+     */
+
+    fuse_reply_err(req, err);
+}
+
 static void lo_destroy(void *userdata)
 {
     struct lo_data *lo = (struct lo_data *)userdata;
@@ -3759,6 +3802,7 @@ static struct fuse_lowlevel_ops lo_oper = {
     .copy_file_range = lo_copy_file_range,
 #endif
     .lseek = lo_lseek,
+    .syncfs = lo_syncfs,
     .destroy = lo_destroy,
 };
 

tools/virtiofsd/passthrough_seccomp.c

@@ -111,6 +111,7 @@ static const int syscall_allowlist[] = {
     SCMP_SYS(set_robust_list),
     SCMP_SYS(setxattr),
     SCMP_SYS(symlinkat),
+    SCMP_SYS(syncfs),
     SCMP_SYS(time), /* Rarely needed, except on static builds */
     SCMP_SYS(tgkill),
     SCMP_SYS(unlinkat),