[Qemu-devel] [PATCH for-2.10? v3 0/2] block: Do OFD lock check at runtime

[Qemu-devel] [PATCH for-2.10? v3 0/2] block: Do OFD lock check at runtime

[Fam Zheng]

v3: Fix mingw build. [patchew]

v2: Probe /dev/null to save LOC. [Eric]
    Mention "new glibc + old kernel" in commit message. [Kevin, Daniel, Eric,
    Christian]

This fixes the image opening failure reported by Andrew Baumann:

> I'm running a recent Linux build of qemu on Windows Subsystem for Linux (WSL)
> which doesn't appear to implement file locking:
>
> $ qemu-system-aarch64 ... -drive file=test.vhdx,if=none,id=hd0 -device virtio-blk-pci,drive=hd0
> qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
> qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
> qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to lock byte 100

It appears to be that the binary is built for Linux targets, but the WSL
runtime doesn't recognize the ops (-EINVAL).

This is also a practical problem for Linux considering it's possible to run
QEMU which is built against a new glibc on an old kernel that doesn't have OFD
API.

Convert to runtime check to cope with that.

Fam Zheng (2):
  osdep: Add runtime OFD lock detection
  file-posix: Do runtime check for ofd lock API

 block/file-posix.c   | 19 +++++++--------
 include/qemu/osdep.h |  1 +
 util/osdep.c         | 66 ++++++++++++++++++++++++++++++++++++++++++++--------
 3 files changed, 65 insertions(+), 21 deletions(-)

-- 
2.13.4

[Qemu-devel] [PATCH for-2.10? v3 1/2] osdep: Add runtime OFD lock detection

[Fam Zheng]

Build time check of OFD lock is not sufficient and can cause image open
errors when the runtime environment doesn't support it.

Add a helper function to probe it at runtime, additionally. Also provide
a qemu_has_ofd_lock() for callers to check the status.

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 include/qemu/osdep.h |  1 +
 util/osdep.c         | 66 ++++++++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 57 insertions(+), 10 deletions(-)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 3b74f6fcb2..6855b94bbf 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -357,6 +357,7 @@ 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);
 
 #if defined(__HAIKU__) && defined(__i386__)
 #define FMT_pid "%ld"
diff --git a/util/osdep.c b/util/osdep.c
index a2863c8e53..a479fedc4a 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -38,14 +38,6 @@ extern int madvise(caddr_t, size_t, int);
 #include "qemu/error-report.h"
 #include "monitor/monitor.h"
 
-#ifdef F_OFD_SETLK
-#define QEMU_SETLK F_OFD_SETLK
-#define QEMU_GETLK F_OFD_GETLK
-#else
-#define QEMU_SETLK F_SETLK
-#define QEMU_GETLK F_GETLK
-#endif
-
 static bool fips_enabled = false;
 
 static const char *hw_version = QEMU_HW_VERSION;
@@ -82,6 +74,10 @@ int qemu_madvise(void *addr, size_t len, int advice)
 }
 
 #ifndef _WIN32
+
+static int fcntl_op_setlk = -1;
+static int fcntl_op_getlk = -1;
+
 /*
  * Dups an fd and sets the flags
  */
@@ -149,6 +145,54 @@ static int qemu_parse_fdset(const char *param)
     return qemu_parse_fd(param);
 }
 
+static void qemu_probe_lock_ops(void)
+{
+    if (fcntl_op_setlk == -1) {
+#ifdef F_OFD_SETLK
+        int fd;
+        int ret;
+        struct flock fl = {
+            .l_whence = SEEK_SET,
+            .l_start  = 0,
+            .l_len    = 0,
+            .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;
+        } else {
+            fcntl_op_setlk = F_SETLK;
+            fcntl_op_getlk = F_GETLK;
+        }
+#else
+        fcntl_op_setlk = F_SETLK;
+        fcntl_op_getlk = F_GETLK;
+#endif
+    }
+}
+
+bool qemu_has_ofd_lock(void)
+{
+    qemu_probe_lock_ops();
+#ifdef F_OFD_SETLK
+    return fcntl_op_setlk == F_OFD_SETLK;
+#else
+    return false;
+#endif
+}
+
 static int qemu_lock_fcntl(int fd, int64_t start, int64_t len, int fl_type)
 {
     int ret;
@@ -158,7 +202,8 @@ static int qemu_lock_fcntl(int fd, int64_t start, int64_t len, int fl_type)
         .l_len    = len,
         .l_type   = fl_type,
     };
-    ret = fcntl(fd, QEMU_SETLK, &fl);
+    qemu_probe_lock_ops();
+    ret = fcntl(fd, fcntl_op_setlk, &fl);
     return ret == -1 ? -errno : 0;
 }
 
@@ -181,7 +226,8 @@ 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,
     };
-    ret = fcntl(fd, QEMU_GETLK, &fl);
+    qemu_probe_lock_ops();
+    ret = fcntl(fd, fcntl_op_getlk, &fl);
     if (ret == -1) {
         return -errno;
     } else {
-- 
2.13.4

[Qemu-devel] [PATCH for-2.10? v3 2/2] file-posix: Do runtime check for ofd lock API

[Fam Zheng]

It is reported that on Windows Subsystem for Linux, ofd operations fail
with -EINVAL. In other words, QEMU binary built with system headers that
exports F_OFD_SETLK doesn't necessarily run in an environment that
actually supports it:

$ qemu-system-aarch64 ... -drive file=test.vhdx,if=none,id=hd0 \
    -device virtio-blk-pci,drive=hd0
qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to lock byte 100

As a matter of fact this is not WSL specific. It can happen when running
a QEMU compiled against a newer glibc on an older kernel, such as in
a containerized environment.

Let's do a runtime check to cope with that.

Reported-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/file-posix.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index f4de022ae0..cb3bfce147 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -457,22 +457,19 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
     switch (locking) {
     case ON_OFF_AUTO_ON:
         s->use_lock = true;
-#ifndef F_OFD_SETLK
-        fprintf(stderr,
-                "File lock requested but OFD locking syscall is unavailable, "
-                "falling back to POSIX file locks.\n"
-                "Due to the implementation, locks can be lost unexpectedly.\n");
-#endif
+        if (!qemu_has_ofd_lock()) {
+            fprintf(stderr,
+                    "File lock requested but OFD locking syscall is "
+                    "unavailable, falling back to POSIX file locks.\n"
+                    "Due to the implementation, locks can be lost "
+                    "unexpectedly.\n");
+        }
         break;
     case ON_OFF_AUTO_OFF:
         s->use_lock = false;
         break;
     case ON_OFF_AUTO_AUTO:
-#ifdef F_OFD_SETLK
-        s->use_lock = true;
-#else
-        s->use_lock = false;
-#endif
+        s->use_lock = qemu_has_ofd_lock();
         break;
     default:
         abort();
-- 
2.13.4

Re: [Qemu-devel] [PATCH for-2.10? v3 0/2] block: Do OFD lock check at runtime

[Kevin Wolf]

Am 11.08.2017 um 13:44 hat Fam Zheng geschrieben:
> v3: Fix mingw build. [patchew]
> 
> v2: Probe /dev/null to save LOC. [Eric]
>     Mention "new glibc + old kernel" in commit message. [Kevin, Daniel, Eric,
>     Christian]
> 
> This fixes the image opening failure reported by Andrew Baumann:
> 
> > I'm running a recent Linux build of qemu on Windows Subsystem for Linux (WSL)
> > which doesn't appear to implement file locking:
> >
> > $ qemu-system-aarch64 ... -drive file=test.vhdx,if=none,id=hd0 -device virtio-blk-pci,drive=hd0
> > qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
> > qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
> > qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to lock byte 100
> 
> It appears to be that the binary is built for Linux targets, but the WSL
> runtime doesn't recognize the ops (-EINVAL).
> 
> This is also a practical problem for Linux considering it's possible to run
> QEMU which is built against a new glibc on an old kernel that doesn't have OFD
> API.
> 
> Convert to runtime check to cope with that.

Thanks, applied to the block branch.

Kevin