It was found that 'qemu-nbd' is not able to work with some disk images
exported from Azure. Looking at the 512b footer (which contains VPC
metadata):

00000000  63 6f 6e 65 63 74 69 78  00 00 00 02 00 01 00 00  |conectix........|
00000010  ff ff ff ff ff ff ff ff  2e c7 9b 96 77 61 00 00  |............wa..|
00000020  00 07 00 00 57 69 32 6b  00 00 00 01 40 00 00 00  |....Wi2k....@...|
00000030  00 00 00 01 40 00 00 00  28 a2 10 3f 00 00 00 02  |....@...(..?....|
00000040  ff ff e7 47 8c 54 df 94  bd 35 71 4c 94 5f e5 44  |...G.T...5qL._.D|
00000050  44 53 92 1a 00 00 00 00  00 00 00 00 00 00 00 00  |DS..............|
00000060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

we can see that Azure uses a different 'Creator application' --
'wa\0\0' (offset 0x1c, likely reads as 'Windows Azure') and QEMU uses this
field to determine how it can get image size. Apparently, Azure uses 'new'
method, just like Hyper-V.

Overall, it seems that only VPC and old QEMUs need to be ignored as all new
creator apps seem to have reliable current_size. Invert the logic and make
'current_size' method the default to avoid adding every new creator app to
the list.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
Changes since v1/v2: invert the logic and make 'vpc' and 'qemu' use CHS
while defaulting to current_size.
---
 block/vpc.c | 65 ++++++++++++++++++++++++++++-------------------------
 1 file changed, 35 insertions(+), 30 deletions(-)

diff --git a/block/vpc.c b/block/vpc.c
index d95a204612b7..e22d4bfe3fc1 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -216,6 +216,39 @@ static void vpc_parse_options(BlockDriverState *bs, QemuOpts *opts,
     }
 }
 
+/*
+ * Microsoft Virtual PC and Microsoft Hyper-V produce and read
+ * VHD image sizes differently.  VPC will rely on CHS geometry,
+ * while Hyper-V and disk2vhd use the size specified in the footer.
+ *
+ * We use a couple of approaches to try and determine the correct method:
+ * look at the Creator App field, and look for images that have CHS
+ * geometry that is the maximum value.
+ *
+ * If the CHS geometry is the maximum CHS geometry, then we assume that
+ * the size is the footer->current_size to avoid truncation.  Otherwise,
+ * we follow the table based on footer->creator_app:
+ *
+ *  Currently known creator apps:
+ *      'vpc '  :  CHS              Virtual PC (uses disk geometry)
+ *      'qemu'  :  CHS              QEMU (uses disk geometry)
+ *      'qem2'  :  current_size     QEMU (uses current_size)
+ *      'win '  :  current_size     Hyper-V
+ *      'd2v '  :  current_size     Disk2vhd
+ *      'tap\0' :  current_size     XenServer
+ *      'CTXS'  :  current_size     XenConverter
+ *      'wa\0\0':  current_size     Azure
+ *
+ *  The user can override the table values via drive options, however
+ *  even with an override we will still use current_size for images
+ *  that have CHS geometry of the maximum size.
+ */
+static bool vpc_ignore_current_size(VHDFooter *footer)
+{
+    return !strncmp(footer->creator_app, "vpc ", 4) ||
+        !strncmp(footer->creator_app, "qemu", 4);
+}
+
 static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
                     Error **errp)
 {
@@ -304,36 +337,8 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
     bs->total_sectors = (int64_t)
         be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;
 
-    /* Microsoft Virtual PC and Microsoft Hyper-V produce and read
-     * VHD image sizes differently.  VPC will rely on CHS geometry,
-     * while Hyper-V and disk2vhd use the size specified in the footer.
-     *
-     * We use a couple of approaches to try and determine the correct method:
-     * look at the Creator App field, and look for images that have CHS
-     * geometry that is the maximum value.
-     *
-     * If the CHS geometry is the maximum CHS geometry, then we assume that
-     * the size is the footer->current_size to avoid truncation.  Otherwise,
-     * we follow the table based on footer->creator_app:
-     *
-     *  Known creator apps:
-     *      'vpc '  :  CHS              Virtual PC (uses disk geometry)
-     *      'qemu'  :  CHS              QEMU (uses disk geometry)
-     *      'qem2'  :  current_size     QEMU (uses current_size)
-     *      'win '  :  current_size     Hyper-V
-     *      'd2v '  :  current_size     Disk2vhd
-     *      'tap\0' :  current_size     XenServer
-     *      'CTXS'  :  current_size     XenConverter
-     *
-     *  The user can override the table values via drive options, however
-     *  even with an override we will still use current_size for images
-     *  that have CHS geometry of the maximum size.
-     */
-    use_chs = (!!strncmp(footer->creator_app, "win ", 4) &&
-               !!strncmp(footer->creator_app, "qem2", 4) &&
-               !!strncmp(footer->creator_app, "d2v ", 4) &&
-               !!strncmp(footer->creator_app, "CTXS", 4) &&
-               !!memcmp(footer->creator_app, "tap", 4)) || s->force_use_chs;
+    /* Use CHS or current_size to determine the image size */
+    use_chs = vpc_ignore_current_size(footer) || s->force_use_chs;
 
     if (!use_chs || bs->total_sectors == VHD_MAX_GEOMETRY || s->force_use_sz) {
         bs->total_sectors = be64_to_cpu(footer->current_size) /
-- 
2.47.0

Hi Vitaly,

On 12/12/24 13:25, Vitaly Kuznetsov wrote:
> It was found that 'qemu-nbd' is not able to work with some disk images
> exported from Azure. Looking at the 512b footer (which contains VPC
> metadata):
> 
> 00000000  63 6f 6e 65 63 74 69 78  00 00 00 02 00 01 00 00  |conectix........|
> 00000010  ff ff ff ff ff ff ff ff  2e c7 9b 96 77 61 00 00  |............wa..|
> 00000020  00 07 00 00 57 69 32 6b  00 00 00 01 40 00 00 00  |....Wi2k....@...|
> 00000030  00 00 00 01 40 00 00 00  28 a2 10 3f 00 00 00 02  |....@...(..?....|
> 00000040  ff ff e7 47 8c 54 df 94  bd 35 71 4c 94 5f e5 44  |...G.T...5qL._.D|
> 00000050  44 53 92 1a 00 00 00 00  00 00 00 00 00 00 00 00  |DS..............|
> 00000060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> 
> we can see that Azure uses a different 'Creator application' --
> 'wa\0\0' (offset 0x1c, likely reads as 'Windows Azure') and QEMU uses this
> field to determine how it can get image size. Apparently, Azure uses 'new'
> method, just like Hyper-V.
> 
> Overall, it seems that only VPC and old QEMUs need to be ignored as all new
> creator apps seem to have reliable current_size. Invert the logic and make
> 'current_size' method the default to avoid adding every new creator app to
> the list.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> Changes since v1/v2: invert the logic and make 'vpc' and 'qemu' use CHS
> while defaulting to current_size.
> ---
>   block/vpc.c | 65 ++++++++++++++++++++++++++++-------------------------
>   1 file changed, 35 insertions(+), 30 deletions(-)
> 
> diff --git a/block/vpc.c b/block/vpc.c
> index d95a204612b7..e22d4bfe3fc1 100644
> --- a/block/vpc.c
> +++ b/block/vpc.c
> @@ -216,6 +216,39 @@ static void vpc_parse_options(BlockDriverState *bs, QemuOpts *opts,
>       }
>   }
>   
> +/*
> + * Microsoft Virtual PC and Microsoft Hyper-V produce and read
> + * VHD image sizes differently.  VPC will rely on CHS geometry,
> + * while Hyper-V and disk2vhd use the size specified in the footer.
> + *
> + * We use a couple of approaches to try and determine the correct method:
> + * look at the Creator App field, and look for images that have CHS
> + * geometry that is the maximum value.
> + *
> + * If the CHS geometry is the maximum CHS geometry, then we assume that
> + * the size is the footer->current_size to avoid truncation.  Otherwise,
> + * we follow the table based on footer->creator_app:
> + *
> + *  Currently known creator apps:
> + *      'vpc '  :  CHS              Virtual PC (uses disk geometry)
> + *      'qemu'  :  CHS              QEMU (uses disk geometry)
> + *      'qem2'  :  current_size     QEMU (uses current_size)
> + *      'win '  :  current_size     Hyper-V
> + *      'd2v '  :  current_size     Disk2vhd
> + *      'tap\0' :  current_size     XenServer
> + *      'CTXS'  :  current_size     XenConverter
> + *      'wa\0\0':  current_size     Azure
> + *
> + *  The user can override the table values via drive options, however
> + *  even with an override we will still use current_size for images
> + *  that have CHS geometry of the maximum size.
> + */
> +static bool vpc_ignore_current_size(VHDFooter *footer)
> +{
> +    return !strncmp(footer->creator_app, "vpc ", 4) ||
> +        !strncmp(footer->creator_app, "qemu", 4);
> +}
> +
>   static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>                       Error **errp)
>   {
> @@ -304,36 +337,8 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>       bs->total_sectors = (int64_t)
>           be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;
>   
> -    /* Microsoft Virtual PC and Microsoft Hyper-V produce and read
> -     * VHD image sizes differently.  VPC will rely on CHS geometry,
> -     * while Hyper-V and disk2vhd use the size specified in the footer.
> -     *
> -     * We use a couple of approaches to try and determine the correct method:
> -     * look at the Creator App field, and look for images that have CHS
> -     * geometry that is the maximum value.
> -     *
> -     * If the CHS geometry is the maximum CHS geometry, then we assume that
> -     * the size is the footer->current_size to avoid truncation.  Otherwise,
> -     * we follow the table based on footer->creator_app:
> -     *
> -     *  Known creator apps:
> -     *      'vpc '  :  CHS              Virtual PC (uses disk geometry)
> -     *      'qemu'  :  CHS              QEMU (uses disk geometry)
> -     *      'qem2'  :  current_size     QEMU (uses current_size)
> -     *      'win '  :  current_size     Hyper-V
> -     *      'd2v '  :  current_size     Disk2vhd
> -     *      'tap\0' :  current_size     XenServer
> -     *      'CTXS'  :  current_size     XenConverter
> -     *
> -     *  The user can override the table values via drive options, however
> -     *  even with an override we will still use current_size for images
> -     *  that have CHS geometry of the maximum size.
> -     */
> -    use_chs = (!!strncmp(footer->creator_app, "win ", 4) &&
> -               !!strncmp(footer->creator_app, "qem2", 4) &&
> -               !!strncmp(footer->creator_app, "d2v ", 4) &&
> -               !!strncmp(footer->creator_app, "CTXS", 4) &&
> -               !!memcmp(footer->creator_app, "tap", 4)) || s->force_use_chs;
> +    /* Use CHS or current_size to determine the image size */
> +    use_chs = vpc_ignore_current_size(footer) || s->force_use_chs;
>   
>       if (!use_chs || bs->total_sectors == VHD_MAX_GEOMETRY || s->force_use_sz) {
>           bs->total_sectors = be64_to_cpu(footer->current_size) /

Easier to review in 2 commits.

1/ Extract vpc_ignore_current_size(), no logical change.

-- >8 --
diff --git a/block/vpc.c b/block/vpc.c
index d95a204612b..7ee31aaa810 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -218,2 +218,37 @@ static void vpc_parse_options(BlockDriverState *bs, 
QemuOpts *opts,

+/*
+ * Microsoft Virtual PC and Microsoft Hyper-V produce and read
+ * VHD image sizes differently.  VPC will rely on CHS geometry,
+ * while Hyper-V and disk2vhd use the size specified in the footer.
+ *
+ * We use a couple of approaches to try and determine the correct method:
+ * look at the Creator App field, and look for images that have CHS
+ * geometry that is the maximum value.
+ *
+ * If the CHS geometry is the maximum CHS geometry, then we assume that
+ * the size is the footer->current_size to avoid truncation.  Otherwise,
+ * we follow the table based on footer->creator_app:
+ *
+ *  Currently known creator apps:
+ *      'vpc '  :  CHS              Virtual PC (uses disk geometry)
+ *      'qemu'  :  CHS              QEMU (uses disk geometry)
+ *      'qem2'  :  current_size     QEMU (uses current_size)
+ *      'win '  :  current_size     Hyper-V
+ *      'd2v '  :  current_size     Disk2vhd
+ *      'tap\0' :  current_size     XenServer
+ *      'CTXS'  :  current_size     XenConverter
+ *
+ *  The user can override the table values via drive options, however
+ *  even with an override we will still use current_size for images
+ *  that have CHS geometry of the maximum size.
+ */
+static bool vpc_ignore_current_size(VHDFooter *footer)
+{
+    return !!strncmp(footer->creator_app, "win ", 4) &&
+           !!strncmp(footer->creator_app, "qem2", 4) &&
+           !!strncmp(footer->creator_app, "d2v ", 4) &&
+           !!strncmp(footer->creator_app, "CTXS", 4) &&
+           !!memcmp(footer->creator_app, "tap", 4);
+}
+
  static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
@@ -306,32 +341,4 @@ static int vpc_open(BlockDriverState *bs, QDict 
*options, int flags,

-    /* Microsoft Virtual PC and Microsoft Hyper-V produce and read
-     * VHD image sizes differently.  VPC will rely on CHS geometry,
-     * while Hyper-V and disk2vhd use the size specified in the footer.
-     *
-     * We use a couple of approaches to try and determine the correct 
method:
-     * look at the Creator App field, and look for images that have CHS
-     * geometry that is the maximum value.
-     *
-     * If the CHS geometry is the maximum CHS geometry, then we assume that
-     * the size is the footer->current_size to avoid truncation. 
Otherwise,
-     * we follow the table based on footer->creator_app:
-     *
-     *  Known creator apps:
-     *      'vpc '  :  CHS              Virtual PC (uses disk geometry)
-     *      'qemu'  :  CHS              QEMU (uses disk geometry)
-     *      'qem2'  :  current_size     QEMU (uses current_size)
-     *      'win '  :  current_size     Hyper-V
-     *      'd2v '  :  current_size     Disk2vhd
-     *      'tap\0' :  current_size     XenServer
-     *      'CTXS'  :  current_size     XenConverter
-     *
-     *  The user can override the table values via drive options, however
-     *  even with an override we will still use current_size for images
-     *  that have CHS geometry of the maximum size.
-     */
-    use_chs = (!!strncmp(footer->creator_app, "win ", 4) &&
-               !!strncmp(footer->creator_app, "qem2", 4) &&
-               !!strncmp(footer->creator_app, "d2v ", 4) &&
-               !!strncmp(footer->creator_app, "CTXS", 4) &&
-               !!memcmp(footer->creator_app, "tap", 4)) || 
s->force_use_chs;
+    /* Use CHS or current_size to determine the image size */
+    use_chs = vpc_ignore_current_size(footer) || s->force_use_chs;

---

2/ Support Azure.

-- >8 --
diff --git a/block/vpc.c b/block/vpc.c
index 7ee31aaa810..febf7061491 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -239,2 +239,3 @@ static void vpc_parse_options(BlockDriverState *bs, 
QemuOpts *opts,
   *      'CTXS'  :  current_size     XenConverter
+ *      'wa\0\0':  current_size     Azure
   *
@@ -246,7 +247,4 @@ static bool vpc_ignore_current_size(VHDFooter *footer)
  {
-    return !!strncmp(footer->creator_app, "win ", 4) &&
-           !!strncmp(footer->creator_app, "qem2", 4) &&
-           !!strncmp(footer->creator_app, "d2v ", 4) &&
-           !!strncmp(footer->creator_app, "CTXS", 4) &&
-           !!memcmp(footer->creator_app, "tap", 4);
+    return !strncmp(footer->creator_app, "vpc ", 4)
+        || !strncmp(footer->creator_app, "qemu", 4);
  }
---