Re: [Qemu-devel] [PATCH 1/1] block/vpc.c: Detect too-large vpc file

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Kevin Wolf (kwolf@redhat.com):
> Am 25.07.2011 20:34, schrieb Serge E. Hallyn:
> > VHD files technically can be up to 2Tb, but virtual pc is limited
> > to 127G.  Currently qemu-img refused to create vpc files > 127G,
> > but it is failing to return error when converting from a non-vpc
> > VHD file which is >127G.  It returns success, but creates a truncated
> > converted image.  Also, qemu-img info claims the vpc file is 127G
> > (and clean).
> > 
> > This patch detects a too-large vpc file and returns -EFBIG.  Without
> > this patch,
> > 
> > =============================================================
> > root@ip-10-38-123-242:~/qemu-fixed# qemu-img info /mnt/140g-dynamic.vhd 
> > image: /mnt/140g-dynamic.vhd
> > file format: vpc
> > virtual size: 127G (136899993600 bytes)
> > disk size: 284K
> > root@ip-10-38-123-242:~/qemu-fixed# qemu-img convert -f vpc -O raw /mnt/140g-dynamic.vhd /mnt/y
> > root@ip-10-38-123-242:~/qemu-fixed# echo $?
> > 0
> > root@ip-10-38-123-242:~/qemu-fixed# qemu-img info /mnt/y
> > image: /mnt/y
> > file format: raw
> > virtual size: 127G (136899993600 bytes)
> > disk size: 0
> > =============================================================
> > 
> > (The 140G image was truncated with no warning or error.)
> > 
> > With the patch, I get:
> > 
> > =============================================================
> > root@ip-10-38-123-242:~/qemu-fixed# ./qemu-img info /mnt/140g-dynamic.vhd 
> > qemu-img: Could not open '/mnt/140g-dynamic.vhd': File too large
> > root@ip-10-38-123-242:~/qemu-fixed# ./qemu-img convert -f vpc -O raw /mnt/140g-dynamic.vhd /mnt/y
> > qemu-img: Could not open '/mnt/140g-dynamic.vhd': File too large
> > qemu-img: Could not open '/mnt/140g-dynamic.vhd'
> > =============================================================
> > 
> > See https://bugs.launchpad.net/qemu/+bug/814222 for details.
> > 
> > Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
> > ---
> >  block/vpc.c |    8 +++++++-
> >  1 files changed, 7 insertions(+), 1 deletions(-)
> > 
> > diff --git a/block/vpc.c b/block/vpc.c
> > index 56865da..fdd5236 100644
> > --- a/block/vpc.c
> > +++ b/block/vpc.c
> > @@ -156,6 +156,7 @@ static int vpc_open(BlockDriverState *bs, int flags)
> >      struct vhd_dyndisk_header* dyndisk_header;
> >      uint8_t buf[HEADER_SIZE];
> >      uint32_t checksum;
> > +    int err = -1;
> >  
> >      if (bdrv_pread(bs->file, 0, s->footer_buf, HEADER_SIZE) != HEADER_SIZE)
> >          goto fail;
> > @@ -176,6 +177,11 @@ static int vpc_open(BlockDriverState *bs, int flags)
> >      bs->total_sectors = (int64_t)
> >          be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;
> >  
> > +    if (bs->total_sectors >= 65535 * 16 * 255) {
> > +        err = -EFBIG;
> > +        goto fail;
> > +    }
> 
> I wonder why this works. If bs->total_sectors was right, shouldn't it

bs->total_sectors is exactly 65535 * 16 * 255.  It never gets bigger.
So really the check could be

    if (bs->total_sectors == 65535 * 16 * 255) {

and it would still work.

> have converted the full 140 GB? I can't see where else we would limit it
> to 127 GB, so what I had expected is that the CHS geometry stored in the
> image header is already too small.

If I understand you correctly, what you're saying is exactly what I'm
finding.  total_sectors is not reflective of the true size.

I assume the true size is somewhere to be found :)  But I haven't found a
nice spec for these.

thanks,
-serge