Re: [Qemu-devel] [PATCH 4/4] block/parallels: 2TB+ parallels images support

[Jeff Cody]

On Fri, Jul 25, 2014 at 07:51:47AM +0400, Denis V. Lunev wrote:
> On 24/07/14 23:25, Jeff Cody wrote:
> >On Tue, Jul 22, 2014 at 05:19:37PM +0400, Denis V. Lunev wrote:
> >>Parallels has released in the recent updates of Parallels Server 5/6
> >>new addition to his image format. Images with signature WithouFreSpacExt
> >>have offsets in the catalog coded not as offsets in sectors (multiple
> >>of 512 bytes) but offsets coded in blocks (i.e. header->tracks * 512)
> >>
> >>In this case all 64 bits of header->nb_sectors are used for image size.
> >>
> >>This patch implements support of this for qemu-img.
> >>
> >>Signed-off-by: Denis V. Lunev <address@hidden>
> >>CC: Kevin Wolf <address@hidden>
> >>CC: Stefan Hajnoczi <address@hidden>
> >>---
> >>  block/parallels.c | 20 +++++++++++++++-----
> >>  1 file changed, 15 insertions(+), 5 deletions(-)
> >>
> >>diff --git a/block/parallels.c b/block/parallels.c
> >>index 02739cf..d9cb04f 100644
> >>--- a/block/parallels.c
> >>+++ b/block/parallels.c
> >>@@ -30,6 +30,7 @@
> >>  /**************************************************************/
> >>  #define HEADER_MAGIC "WithoutFreeSpace"
> >>+#define HEADER_MAGIC2 "WithouFreSpacExt"
> >>  #define HEADER_VERSION 2
> >>  #define HEADER_SIZE 64
> >>@@ -54,6 +55,8 @@ typedef struct BDRVParallelsState {
> >>      unsigned int catalog_size;
> >>      unsigned int tracks;
> >>+
> >>+    unsigned int off_multiplier;
> >>  } BDRVParallelsState;
> >>  static int parallels_probe(const uint8_t *buf, int buf_size, const char 
> >> *filename)
> >>@@ -63,7 +66,8 @@ static int parallels_probe(const uint8_t *buf, int 
> >>buf_size, const char *filenam
> >>      if (buf_size < HEADER_SIZE)
> >>          return 0;
> >>-    if (!memcmp(ph->magic, HEADER_MAGIC, 16) &&
> >>+    if ((!memcmp(ph->magic, HEADER_MAGIC, 16) ||
> >>+        !memcmp(ph->magic, HEADER_MAGIC2, 16)) &&
> >>          (le32_to_cpu(ph->version) == HEADER_VERSION))
> >>          return 100;
> >>@@ -85,13 +89,18 @@ static int parallels_open(BlockDriverState *bs, QDict 
> >>*options, int flags,
> >>          goto fail;
> >>      }
> >>+    bs->total_sectors = le64_to_cpu(ph.nb_sectors);
> >>+
> >bs->total_sectors is a int64_t, and ph.nb_sectors is a uint64_t.
> >Should we do a sanity check here on the max number of sectors?
> in reality such image will not be usable as we will later have to multiply
> this count with 512 to calculate offset in the file
> >>      if (le32_to_cpu(ph.version) != HEADER_VERSION)
> >>          goto fail_format;
> >>-    if (memcmp(ph.magic, HEADER_MAGIC, 16))
> >>+    if (!memcmp(ph.magic, HEADER_MAGIC, 16)) {
> >>+        s->off_multiplier = 1;
> >>+        bs->total_sectors = (uint32_t)bs->total_sectors;
> >(same comment as in patch 1, w.r.t. cast vs. bitmask)
> ok
> >>+    } else if (!memcmp(ph.magic, HEADER_MAGIC2, 16))
> >>+        s->off_multiplier = le32_to_cpu(ph.tracks);
> >Is there a maximum size in the specification for ph.tracks?
> no, there is no limitation. Though in reality I have seen the
> following images only:
>   252 sectors, 63 sectors, 256 sectors, 2048 sectors
> and only 2048 was used with this new header.
> 
> This is just an observation.
> 
> >>+    else
> >>          goto fail_format;
> >(same comment as the last patch, w.r.t. braces)
> >
> >>-    bs->total_sectors = (uint32_t)le64_to_cpu(ph.nb_sectors);
> >>-
> >>      s->tracks = le32_to_cpu(ph.tracks);
> >>      if (s->tracks == 0) {
> >>          error_setg(errp, "Invalid image: Zero sectors per track");
> >>@@ -137,7 +146,8 @@ static int64_t seek_to_sector(BlockDriverState *bs, 
> >>int64_t sector_num)
> >>      /* not allocated */
> >>      if ((index > s->catalog_size) || (s->catalog_bitmap[index] == 0))
> >>          return -1;
> >>-    return (uint64_t)(s->catalog_bitmap[index] + offset) * 512;
> >>+    return
> >>+        ((uint64_t)s->catalog_bitmap[index] * s->off_multiplier + offset) 
> >>* 512;
> >Do we need to worry about overflow here, depending on the value of
> >off_multiplier?
> >
> >Also, (and this existed prior to this patch), - we are casting to
> >uint64_t, although the function returns int64_t.
> good catch. I'll change the declaration to uint64_t and will return 0
> from previous if aka
>

Thanks.

I'm not sure that is the best option though - the only place the
return value from this function is used is in parallels_read(), which
passes the output into bdrv_pread() as the offset.  The offset
parameter for bdrv_pread/pwrite is an int64_t, not a uint64_t.


> if ((index > s->catalog_size) || (s->catalog_bitmap[index] == 0))
> 
> It is not possible to obtain 0 as a real offset of any sector as
> this is an offset of the header.
> 
> Regarding overflow check. Do you think that we should return
> error to the upper layer or we could silently fill result data
> with zeroes as was done originally for the following case
> 'index > s->catalog_size' ?

I think it would be best to return error (anything < 0 will also cause
bdrv_pread to return -EIO, and it is also checked for in
parallels_read).

I also don't mean to over-complicate things here for you, which is why
I asked about the max value of ph.tracks.  If that has a reasonable
bounds check, then we don't need to worry about overflow at all, and
can just change the cast to an int64_t instead of uint64_t.  

I think we are safe so long as ph.tracks <= (INT32_MAX/513), and it
that seems like it would be a reasonably flexible bounds limit check
when first opening the image (since QEMU can't really support higher
than that, anyway).

Thanks,
Jeff

> >>  }
> >>  static int parallels_read(BlockDriverState *bs, int64_t sector_num,
> >>-- 
> >>1.9.1
> >>
> >>
>