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

[Li Feng]

Daniel P. Berrangé <berrange@redhat.com> 于2020年12月8日周二 下午9:45写道：
>
> On Tue, Dec 08, 2020 at 08:59:37PM +0800, Li Feng wrote:
> > 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.
>
> IIUC, the problem you're describing is one of whether the filesystem
> supports fcntl locking at all, which is indeed a per-FS check.
>
> The QEMU code being changed though is just about detecting whether
> the host OS supports OFD to not, which is supposed to be a kernel
> level feature applied  universally to all FS types.
>
> >
> > 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);
> > +}
> > +
>
> This method now does a test whose results will vary based on the
> filename passed in, but it is updating a global variable to say
> whether to use OFD locks.  This is looks badly broken when using
> files across different filesystems.
Daniel, thanks for your reply.
I understand your concern, however, this global variable actually maybe
not suitable if files are across different filesystems.

>
> IMHO the raw_co_create method just needs to use a dedicated method
> to check whether fcntl locks are supposed, and all this broken
> refactoring of the OFD check should be removed.
>
OK, I will just keep the fixed part in v2.

> > +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
> >
> >
>
> Regards,
> Daniel
> --
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