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

[Li Feng <fengli@smartx.com>]

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, make the 'qemu_has_ofd_lock' with a filename be more generic
and reasonable.

Signed-off-by: Li Feng <fengli@smartx.com>
---
 block/file-posix.c         | 32 +++++++++++++++-------------
 include/qemu/osdep.h       |  2 +-
 tests/test-image-locking.c |  2 +-
 util/osdep.c               | 43 ++++++++++++++++++++++++--------------
 4 files changed, 47 insertions(+), 32 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 806764f7e3..03be1b188c 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -595,7 +595,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
     switch (locking) {
     case ON_OFF_AUTO_ON:
         s->use_lock = true;
-        if (!qemu_has_ofd_lock()) {
+        if (!qemu_has_ofd_lock(filename)) {
             warn_report("File lock requested but OFD locking syscall is "
                         "unavailable, falling back to POSIX file locks");
             error_printf("Due to the implementation, locks can be lost "
@@ -606,7 +606,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
         s->use_lock = false;
         break;
     case ON_OFF_AUTO_AUTO:
-        s->use_lock = qemu_has_ofd_lock();
+        s->use_lock = qemu_has_ofd_lock(filename);
         break;
     default:
         abort();
@@ -2388,6 +2388,7 @@ 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 +2429,22 @@ 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_ofd_lock(file_opts->filename);
+    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..349adad465 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -512,7 +512,7 @@ int qemu_dup(int fd);
 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_ofd_lock(const char *filename);
 #endif
 
 #if defined(__HAIKU__) && defined(__i386__)
diff --git a/tests/test-image-locking.c b/tests/test-image-locking.c
index ba057bd66c..3e80246081 100644
--- a/tests/test-image-locking.c
+++ b/tests/test-image-locking.c
@@ -149,7 +149,7 @@ int main(int argc, char **argv)
 
     g_test_init(&argc, &argv, NULL);
 
-    if (qemu_has_ofd_lock()) {
+    if (qemu_has_ofd_lock(NULL)) {
         g_test_add_func("/image-locking/basic", test_image_locking_basic);
         g_test_add_func("/image-locking/set-perm-abort", test_set_perm_abort);
     }
diff --git a/util/osdep.c b/util/osdep.c
index 66d01b9160..e7e502edd1 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -42,6 +42,7 @@ extern int madvise(char *, size_t, int);
 static bool fips_enabled = false;
 
 static const char *hw_version = QEMU_HW_VERSION;
+static const char *null_device = "/dev/null";
 
 int socket_set_cork(int fd, int v)
 {
@@ -187,11 +188,10 @@ static int qemu_parse_fdset(const char *param)
     return qemu_parse_fd(param);
 }
 
-static void qemu_probe_lock_ops(void)
+static void qemu_probe_lock_ops_fd(int fd)
 {
     if (fcntl_op_setlk == -1) {
 #ifdef F_OFD_SETLK
-        int fd;
         int ret;
         struct flock fl = {
             .l_whence = SEEK_SET,
@@ -200,17 +200,7 @@ static void qemu_probe_lock_ops(void)
             .l_type   = F_WRLCK,
         };
 
-        fd = open("/dev/null", O_RDWR);
-        if (fd < 0) {
-            fprintf(stderr,
-                    "Failed to open /dev/null for OFD lock probing: %s\n",
-                    strerror(errno));
-            fcntl_op_setlk = F_SETLK;
-            fcntl_op_getlk = F_GETLK;
-            return;
-        }
         ret = fcntl(fd, F_OFD_GETLK, &fl);
-        close(fd);
         if (!ret) {
             fcntl_op_setlk = F_OFD_SETLK;
             fcntl_op_getlk = F_OFD_GETLK;
@@ -225,9 +215,30 @@ static void qemu_probe_lock_ops(void)
     }
 }
 
-bool qemu_has_ofd_lock(void)
+static void qemu_probe_lock_ops(const char *filename)
+{
+    int fd;
+    if (filename) {
+        fd = open(filename, O_RDWR);
+    } else {
+        fd = open(null_device, O_RDONLY);
+    }
+    if (fd < 0) {
+        fprintf(stderr,
+                "Failed to open %s for OFD lock probing: %s\n",
+                filename ? filename : null_device,
+                strerror(errno));
+        fcntl_op_setlk = F_SETLK;
+        fcntl_op_getlk = F_GETLK;
+        return;
+    }
+    qemu_probe_lock_ops_fd(fd);
+    close(fd);
+}
+
+bool qemu_has_ofd_lock(const char *filename)
 {
-    qemu_probe_lock_ops();
+    qemu_probe_lock_ops(filename);
 #ifdef F_OFD_SETLK
     return fcntl_op_setlk == F_OFD_SETLK;
 #else
@@ -244,7 +255,7 @@ static int qemu_lock_fcntl(int fd, int64_t start, int64_t len, int fl_type)
         .l_len    = len,
         .l_type   = fl_type,
     };
-    qemu_probe_lock_ops();
+    qemu_probe_lock_ops_fd(fd);
     do {
         ret = fcntl(fd, fcntl_op_setlk, &fl);
     } while (ret == -1 && errno == EINTR);
@@ -270,7 +281,7 @@ int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive)
         .l_len    = len,
         .l_type   = exclusive ? F_WRLCK : F_RDLCK,
     };
-    qemu_probe_lock_ops();
+    qemu_probe_lock_ops_fd(fd);
     ret = fcntl(fd, fcntl_op_getlk, &fl);
     if (ret == -1) {
         return -errno;
-- 
2.24.3