[PATCH v4] file-posix: detect the lock using the real file

[Li Feng]

This patch addresses this issue:
When accessing a volume on an NFS filesystem without supporting the file lock,
tools, like qemu-img, will complain "Failed to lock byte 100".

In the original code, the qemu_has_ofd_lock will test the lock on the
"/dev/null" pseudo-file. Actually, the file.locking is per-drive property,
which depends on the underlay filesystem.

In this patch, add a new 'qemu_has_file_lock' to detect whether the
file supports the file lock. And disable the lock when the underlay file
system doesn't support locks.

Signed-off-by: Li Feng <fengli@smartx.com>
---
v4: use the fd as the qemu_has_file_lock argument.
v3: don't call the qemu_has_ofd_lock, use a new function instead.
v2: remove the refactoring.
---
 block/file-posix.c   | 66 +++++++++++++++++++++++++-------------------
 include/qemu/osdep.h |  1 +
 util/osdep.c         | 19 +++++++++++++
 3 files changed, 58 insertions(+), 28 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 806764f7e3..9708212f01 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -584,6 +584,21 @@ static int raw_open_common(BlockDriverState *bs, QDict 
*options,
     s->use_linux_io_uring = (aio == BLOCKDEV_AIO_OPTIONS_IO_URING);
 #endif
 
+    s->open_flags = open_flags;
+    raw_parse_flags(bdrv_flags, &s->open_flags, false);
+
+    s->fd = -1;
+    fd = qemu_open(filename, s->open_flags, errp);
+    ret = fd < 0 ? -errno : 0;
+
+    if (ret < 0) {
+        if (ret == -EROFS) {
+            ret = -EACCES;
+        }
+        goto fail;
+    }
+    s->fd = fd;
+
     locking = qapi_enum_parse(&OnOffAuto_lookup,
                               qemu_opt_get(opts, "locking"),
                               ON_OFF_AUTO_AUTO, &local_err);
@@ -607,6 +622,13 @@ static int raw_open_common(BlockDriverState *bs, QDict 
*options,
         break;
     case ON_OFF_AUTO_AUTO:
         s->use_lock = qemu_has_ofd_lock();
+        if (s->use_lock && !qemu_has_file_lock(s->fd)) {
+            /*
+             * When the os supports the OFD lock, but the filesystem doesn't
+             * support, just disable the file lock.
+             */
+            s->use_lock = false;
+        }
         break;
     default:
         abort();
@@ -625,22 +647,6 @@ static int raw_open_common(BlockDriverState *bs, QDict 
*options,
     s->drop_cache = qemu_opt_get_bool(opts, "drop-cache", true);
     s->check_cache_dropped = qemu_opt_get_bool(opts, "x-check-cache-dropped",
                                                false);
-
-    s->open_flags = open_flags;
-    raw_parse_flags(bdrv_flags, &s->open_flags, false);
-
-    s->fd = -1;
-    fd = qemu_open(filename, s->open_flags, errp);
-    ret = fd < 0 ? -errno : 0;
-
-    if (ret < 0) {
-        if (ret == -EROFS) {
-            ret = -EACCES;
-        }
-        goto fail;
-    }
-    s->fd = fd;
-
     /* Check s->open_flags rather than bdrv_flags due to auto-read-only */
     if (s->open_flags & O_RDWR) {
         ret = check_hdev_writable(s->fd);
@@ -2388,6 +2394,7 @@ raw_co_create(BlockdevCreateOptions *options, Error 
**errp)
     int fd;
     uint64_t perm, shared;
     int result = 0;
+    bool use_lock;
 
     /* Validate options and set default values */
     assert(options->driver == BLOCKDEV_DRIVER_FILE);
@@ -2428,19 +2435,22 @@ raw_co_create(BlockdevCreateOptions *options, Error 
**errp)
     perm = BLK_PERM_WRITE | BLK_PERM_RESIZE;
     shared = BLK_PERM_ALL & ~BLK_PERM_RESIZE;
 
-    /* Step one: Take locks */
-    result = raw_apply_lock_bytes(NULL, fd, perm, ~shared, false, errp);
-    if (result < 0) {
-        goto out_close;
-    }
+    use_lock = qemu_has_file_lock(fd);
+    if (use_lock) {
+        /* Step one: Take locks */
+        result = raw_apply_lock_bytes(NULL, fd, perm, ~shared, false, errp);
+        if (result < 0) {
+            goto out_close;
+        }
 
-    /* Step two: Check that nobody else has taken conflicting locks */
-    result = raw_check_lock_bytes(fd, perm, shared, errp);
-    if (result < 0) {
-        error_append_hint(errp,
-                          "Is another process using the image [%s]?\n",
-                          file_opts->filename);
-        goto out_unlock;
+        /* Step two: Check that nobody else has taken conflicting locks */
+        result = raw_check_lock_bytes(fd, perm, shared, errp);
+        if (result < 0) {
+            error_append_hint(errp,
+                              "Is another process using the image [%s]?\n",
+                              file_opts->filename);
+            goto out_unlock;
+        }
     }
 
     /* Clear the file by truncating it to 0 */
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index f9ec8c84e9..c7587be99d 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -513,6 +513,7 @@ int qemu_lock_fd(int fd, int64_t start, int64_t len, bool 
exclusive);
 int qemu_unlock_fd(int fd, int64_t start, int64_t len);
 int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive);
 bool qemu_has_ofd_lock(void);
+bool qemu_has_file_lock(int fd);
 #endif
 
 #if defined(__HAIKU__) && defined(__i386__)
diff --git a/util/osdep.c b/util/osdep.c
index 66d01b9160..07de97e2c5 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -225,6 +225,25 @@ static void qemu_probe_lock_ops(void)
     }
 }
 
+bool qemu_has_file_lock(int fd)
+{
+#ifdef F_OFD_SETLK
+    int cmd = F_OFD_GETLK;
+#else
+    int cmd = F_GETLK;
+#endif
+    int ret;
+    struct flock fl = {
+        .l_whence = SEEK_SET,
+        .l_start  = 0,
+        .l_len    = 0,
+        .l_type   = F_WRLCK,
+    };
+
+    ret = fcntl(fd, cmd, &fl);
+    return ret == 0;
+}
+
 bool qemu_has_ofd_lock(void)
 {
     qemu_probe_lock_ops();
-- 
2.24.3