Libguestfs - Aug 2020 - [PATCH v2] appliance: extract UUID from QCOW2 disk image

For the appliance of the QCOW2 format, get the UUID of the disk by
reading the first 256k bytes with 'qemu-img dd' command. Then pass the
read block to the 'file' command. In case of failure, run the
'file'
command again directly.

Suggested-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
---
v2:
  01: The order of the function calls to direct <file> command on the
appliance
      and the indirect one (through the temporary file) has been swapped.

 lib/appliance-kcmdline.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

diff --git a/lib/appliance-kcmdline.c b/lib/appliance-kcmdline.c
index fbeb4f4..c78524b 100644
--- a/lib/appliance-kcmdline.c
+++ b/lib/appliance-kcmdline.c
@@ -71,7 +71,7 @@ read_uuid (guestfs_h *g, void *retv, const char *line, size_t
len)
  * The L<file(1)> command does the hard work.
  */
 static char *
-get_root_uuid (guestfs_h *g, const char *appliance)
+do_get_root_uuid (guestfs_h *g, const char *appliance)
 {
   CLEANUP_CMD_CLOSE struct command *cmd = guestfs_int_new_command (g);
   char *ret = NULL;
@@ -96,6 +96,74 @@ get_root_uuid (guestfs_h *g, const char *appliance)
 }
 
 /**
+ * Read the first 256k bytes of the in_file with L<qemu-img(1)> command
+ * and write them into the out_file. That may be useful to get UUID of
+ * the QCOW2 disk image with further L<file(1)> command.
+ * The function returns zero if successful, otherwise -1.
+ */
+static int
+run_qemu_img_dd (guestfs_h *g, const char *in_file, char *out_file)
+{
+  CLEANUP_CMD_CLOSE struct command *cmd = guestfs_int_new_command (g);
+  int r;
+
+  guestfs_int_cmd_add_arg (cmd, "qemu-img");
+  guestfs_int_cmd_add_arg (cmd, "dd");
+  guestfs_int_cmd_add_arg_format (cmd, "if=%s", in_file);
+  guestfs_int_cmd_add_arg_format (cmd, "of=%s", out_file);
+  guestfs_int_cmd_add_arg (cmd, "bs=256k");
+  guestfs_int_cmd_add_arg (cmd, "count=1");
+
+  r = guestfs_int_cmd_run (cmd);
+  if (r == -1) {
+    error (g, "Failed to run qemu-img");
+    return -1;
+  }
+  if (!WIFEXITED (r) || WEXITSTATUS (r) != 0) {
+    guestfs_int_external_command_failed (g, r, "qemu-img dd", NULL);
+    return -1;
+  }
+
+  return 0;
+}
+
+/**
+ * Get the UUID from the appliance disk image.
+ */
+static char *
+get_root_uuid (guestfs_h *g, const char *appliance)
+{
+  char *uuid = NULL;
+  int ret;
+  char tmp_file[] = "/tmp/libguestfsXXXXXX";
+
+  uuid = do_get_root_uuid (g, appliance);
+  if (uuid) {
+      return uuid;
+  }
+
+  if (!mktemp (tmp_file)) {
+    error (g, "get_root_uuid: mktemp failed");
+    return NULL;
+  }
+
+  ret = run_qemu_img_dd (g, appliance, tmp_file);
+  if (ret == 0) {
+    uuid = do_get_root_uuid (g, tmp_file);
+    if (uuid) {
+      goto out;
+    }
+  }
+
+  /* We get here in case of failure to extract UUID from the temporary file. */
+  error (g, "Failed to get the appliance UUID");
+
+out:
+  unlink (tmp_file);
+  return uuid;
+}
+
+/**
  * Construct the Linux command line passed to the appliance.  This is
  * used by the C<direct> and C<libvirt> backends, and is simply
  * located in this file because it's a convenient place for this
-- 
1.8.3.1

On Wednesday, 12 August 2020 17:36:10 CEST Andrey Shinkevich
wrote:
> For the appliance of the QCOW2 format, get the UUID of the disk by
> reading the first 256k bytes with 'qemu-img dd' command. Then pass
the
> read block to the 'file' command. In case of failure, run the
'file'
> command again directly.
> 
> Suggested-by: Denis V. Lunev <den@openvz.org>
> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> ---
> v2:
>   01: The order of the function calls to direct <file> command on the
appliance
>       and the indirect one (through the temporary file) has been swapped.
> 
>  lib/appliance-kcmdline.c | 70
+++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/appliance-kcmdline.c b/lib/appliance-kcmdline.c
> index fbeb4f4..c78524b 100644
> --- a/lib/appliance-kcmdline.c
> +++ b/lib/appliance-kcmdline.c
> @@ -71,7 +71,7 @@ read_uuid (guestfs_h *g, void *retv, const char *line,
size_t len)
>   * The L<file(1)> command does the hard work.
>   */
>  static char *
> -get_root_uuid (guestfs_h *g, const char *appliance)
> +do_get_root_uuid (guestfs_h *g, const char *appliance)
get_root_uuid_with_file seems a better name, and gives less confusion
than get_root_uuid & do_get_root_uuid.
>  {
>    CLEANUP_CMD_CLOSE struct command *cmd = guestfs_int_new_command (g);
>    char *ret = NULL;
> @@ -96,6 +96,74 @@ get_root_uuid (guestfs_h *g, const char *appliance)
>  }
>  
>  /**
> + * Read the first 256k bytes of the in_file with L<qemu-img(1)>
command
> + * and write them into the out_file. That may be useful to get UUID of
> + * the QCOW2 disk image with further L<file(1)> command.
> + * The function returns zero if successful, otherwise -1.
> + */
> +static int
> +run_qemu_img_dd (guestfs_h *g, const char *in_file, char *out_file)
> +{
> +  CLEANUP_CMD_CLOSE struct command *cmd = guestfs_int_new_command (g);
> +  int r;
> +
> +  guestfs_int_cmd_add_arg (cmd, "qemu-img");
> +  guestfs_int_cmd_add_arg (cmd, "dd");
> +  guestfs_int_cmd_add_arg_format (cmd, "if=%s", in_file);
> +  guestfs_int_cmd_add_arg_format (cmd, "of=%s", out_file);
> +  guestfs_int_cmd_add_arg (cmd, "bs=256k");
> +  guestfs_int_cmd_add_arg (cmd, "count=1");
> +
> +  r = guestfs_int_cmd_run (cmd);
> +  if (r == -1) {
> +    error (g, "Failed to run qemu-img");
> +    return -1;
> +  }
> +  if (!WIFEXITED (r) || WEXITSTATUS (r) != 0) {
> +    guestfs_int_external_command_failed (g, r, "qemu-img dd",
NULL);
> +    return -1;
> +  }
> +
> +  return 0;
> +}
> +
> +/**
> + * Get the UUID from the appliance disk image.
> + */
> +static char *
> +get_root_uuid (guestfs_h *g, const char *appliance)
> +{
> +  char *uuid = NULL;
> +  int ret;
> +  char tmp_file[] = "/tmp/libguestfsXXXXXX";
Please do not hardcode a location, and use the internal temporary
directory instead. The internal guestfs_int_make_temp_path will do
the right job.

Thanks,
-- 
Pino Toscano

So the patch is generally fine now, but there's one thing
I didn't spot in the earlier version:
> +/**
> + * Get the UUID from the appliance disk image.
> + */
> +static char *
> +get_root_uuid (guestfs_h *g, const char *appliance)
> +{
> +  char *uuid = NULL;
> +  int ret;
> +  char tmp_file[] = "/tmp/libguestfsXXXXXX";
> +
> +  uuid = do_get_root_uuid (g, appliance);
> +  if (uuid) {
> +      return uuid;
> +  }
> +
> +  if (!mktemp (tmp_file)) {
> +    error (g, "get_root_uuid: mktemp failed");
> +    return NULL;
> +  }
mktemp isn't safe to use in modern code.  Libguestfs already makes a
temporary directory for handles to use so you can create a file in
that directory like this:

  CLEANUP_FREE char *file = NULL;

  file = guestfs_int_make_temp_path (g, "root", "raw");
  if (!file) return -1;

Note you don't have to delete the file explicitly.

Rich.

-- 
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