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Re: [libvirt] [PATCH] Add support for probing filesystem with libblkid



On 11/01/2011 09:02 AM, Daniel P. Berrange wrote:
From: "Daniel P. Berrange"<berrange redhat com>

The LXC code for mounting container filesystems from block devices
tries all filesystems in /etc/filesystems and possibly those in
/proc/filesystems. The regular mount binary, however, first tries
using libblkid to detect the format. Add support for doing the same
in libvirt, since Fedora's /etc/filesystems is missing many formats,
most notably ext4 which is the default filesystem Fedora uses!

* src/Makefile.am: Link libvirt_lxc to libblkid
* src/lxc/lxc_container.c: Probe filesystem format with liblkid

s/liblkid/libblkid/

---
  src/Makefile.am         |    4 ++
  src/lxc/lxc_container.c |  102 ++++++++++++++++++++++++++++++++++++++++++++++-
  2 files changed, 105 insertions(+), 1 deletions(-)

configure.ac already unconditionally checked for libblkid, so this is simple enough to start using.

+#ifdef HAVE_LIBBLKID
+static int
+lxcContainerMountDetectFilesystem(const char *src, char **type)
+{
+    int fd;
+    int ret = -1;
+    int rc;
+    const char *data = NULL;
+    blkid_probe blkid = NULL;
+
+    *type = NULL;
+
+    if ((fd = open(src, O_RDONLY))<  0) {
+        virReportSystemError(errno,
+                             _("Unable to open filesystem %s"), src);
+        return -1;

If we fail to open() the file, can't we at least fall back to the mount probing? That is, what do we gain by returning failure here, instead of returning 0 and letting the fallback code be attempted?

+    }
+
+    if (!(blkid = blkid_new_probe())) {
+        virReportSystemError(errno, "%s",
+                             _("Unable to create blkid library handle"));
+        goto cleanup;
+    }
+    if (blkid_probe_set_device(blkid, fd, 0, 0)<  0) {
+        virReportSystemError(EINVAL,
+                             _("Unable to associated device %s with blkid library"),
+                             src);

s/associated/associate/

+        goto cleanup;
+    }
+
+    blkid_probe_enable_superblocks(blkid, 1);
+
+    blkid_probe_set_superblocks_flags(blkid, BLKID_SUBLKS_TYPE);
+
+    rc = blkid_do_safeprobe(blkid);
+    if (rc != 0) {
+        if (rc == 1) /* Nothing found, return success with *type == NULL */
+            goto done;
+
+        if (rc == -2) {
+            virReportSystemError(EINVAL,
+                                 _("Too many filesystems detected for %s"),
+                                 src);
+        } else {
+            virReportSystemError(errno,
+                                 _("Unable to detect filesystem for %s"),
+                                 src);
+        }
+        goto cleanup;
+    }
+
+    if (blkid_probe_lookup_value(blkid, "TYPE",&data, NULL)<  0) {
+        virReportSystemError(ENOENT,
+                             _("Unable to find filesystem type for %s"),
+                             src);
+        goto cleanup;
+    }
+
+    if (!(*type = strdup(data))) {
+        virReportOOMError();
+        goto cleanup;
+    }
+
+done:
+    ret = 0;
+cleanup:
+    VIR_FORCE_CLOSE(fd);
+    blkid_free_probe(blkid);

Is this safe? In storage_backed_fs.c, you check that probe != NULL before calling blkid_free_probe. If this function is free-like, we should add it to cfg.mk and update storage_backend_fs.c to get rid of the useless conditional; if not, then this needs fixing to avoid crashing libblkid on a NULL deref.

+    return ret;
+}
+#else /* ! HAVE_LIBBLKID */
+static int
+lxcContainerMountDetectFilesystem(const char *src ATTRIBUTE_UNUSED,
+                                  char **type)
+{
+    /* No libblkid, so just return success with no detected type */
+    *type = NULL;
+    return 0;
+}
+#endif /* ! HAVE_LIBBLKID */

  /*
   * This functions attempts to do automatic detection of filesystem
@@ -771,6 +855,8 @@ static int lxcContainerMountFSBlockHelper(virDomainFSDefPtr fs,
  {
      int fsflags = 0;
      int ret = -1;
+    char *format = NULL;
+
      if (fs->readonly)
          fsflags |= MS_RDONLY;

@@ -781,7 +867,21 @@ static int lxcContainerMountFSBlockHelper(virDomainFSDefPtr fs,
          goto cleanup;
      }

-    ret = lxcContainerMountFSBlockAuto(fs, fsflags, src, srcprefix);
+    if (lxcContainerMountDetectFilesystem(src,&format)<  0)
+        goto cleanup;

Given my earlier question, about the only failures that should avoid the fallback would be OOM failure. But since OOM is a real possibility, I agree with the signature (return 0 on success, whether or not format == NULL).

+
+    if (format) {
+        VIR_DEBUG("Mount %s with detected format %s", src, format);
+        if (mount(src, fs->dst, format, fsflags, NULL)<  0) {
+            virReportSystemError(errno,
+                                 _("Failed to mount device %s to %s as %s"),
+                                 src, fs->dst, format);
+            goto cleanup;
+        }
+        ret = 0;
+    } else {
+        ret = lxcContainerMountFSBlockAuto(fs, fsflags, src, srcprefix);
+    }

This part makes sense. Overall, I like the idea, but it may be worth a v2 depending on the answers to my questions above.

--
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


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