Re: [Qemu-devel] [PATCH v2 2/9] block: vhdx - add header update capability.

[Jeff Cody]

On Thu, Aug 01, 2013 at 03:44:41PM +0200, Stefan Hajnoczi wrote:
> On Wed, Jul 31, 2013 at 11:23:47PM -0400, Jeff Cody wrote:
> > @@ -212,6 +242,24 @@ bool vhdx_checksum_is_valid(uint8_t *buf, size_t size, 
> > int crc_offset)
> >  
> >  
> >  /*
> > + * This generates a UUID that is compliant with the MS GUIDs used
> > + * in the VHDX spec (and elsewhere).
> > + *
> > + * We can do this with uuid_generate if uuid.h is present,
> > + * however not all systems have uuid and the generation is
> > + * pretty straightforward for the DCE + random usage case
> 
> The talk of uuid.h not being available confuses me.  The code has no
> #ifdef and we do not define uuid_generate() ourselves.
> 
> Is this an outdated comment?
> 
> > +/* Update the VHDX headers
> > + *
> > + * This follows the VHDX spec procedures for header updates.
> > + *
> > + *  - non-current header is updated with largest sequence number
> > + */
> > +static int vhdx_update_header(BlockDriverState *bs, BDRVVHDXState *s, bool 
> > rw)
> 
> "rw" should be named "generate_file_write_guid".  If you call
> vhdx_update_header() again later you do not need to update FileWriteGuid
> again according to the spec.  There's probably no harm in doing so but
> the spec explicitly says "If this is the first header update within the
> session, use a new value for the FileWriteGuid".
> 
> This means that future calls to vhdx_update_header() in the same session
> should set "generate_file_write_guid" to false.  Therefore "rw" is not
> the right name.
> 

Reading through your comments again made me realize there is a
misunderstanding (a good sign in itself that 'rw' should be renamed).
The 'rw' is not to generate the FileWriteGuid, but rather the
DataWriteGuid.  FileWriteGuid is copied from our session_guid, so it
is only generated once and used through the entire open session.  The
s->session_guid field is generated upon vhdx_open(), for usage if the
file is opened BDRV_O_RDWR.

The DataWriteGuid is generated before any user-visible writes
(anything observable via a virtual disk read).  And that DataWriteGuid
is what the 'rw' field is used for, and the function
vhdx_user_visible_write() handles calling vhdx_update_headers() with
that field set to true.

I'll rename 'rw' to 'generate_data_write_guid'.


> > +{
> > +    int ret = 0;
> > +    int hdr_idx = 0;
> > +    uint64_t header_offset = VHDX_HEADER1_OFFSET;
> > +
> > +    VHDXHeader *active_header;
> > +    VHDXHeader *inactive_header;
> > +    VHDXHeader header_le;
> > +    uint8_t *buffer;
> > +
> > +    /* operate on the non-current header */
> > +    if (s->curr_header == 0) {
> > +        hdr_idx = 1;
> > +        header_offset = VHDX_HEADER2_OFFSET;
> > +    }
> > +
> > +    active_header   = s->headers[s->curr_header];
> > +    inactive_header = s->headers[hdr_idx];
> > +
> > +    inactive_header->sequence_number = active_header->sequence_number + 1;
> > +
> > +    /* a new file guid must be generate before any file write, including
> 
> s/generate/generated/
> 
> > +     * headers */
> > +    memcpy(&inactive_header->file_write_guid, &s->session_guid,
> > +           sizeof(MSGUID));
> > +
> > +    /* a new data guid only needs to be generate before any guest-visible
> > +     * writes, so update it if the image is opened r/w. */
> > +    if (rw) {
> > +        vhdx_guid_generate(&inactive_header->data_write_guid);
> > +    }
> > +
> > +    /* the header checksum is not over just the packed size of VHDXHeader,
> > +     * but rather over the entire 'reserved' range for the header, which is
> > +     * 4KB (VHDX_HEADER_SIZE). */
> > +
> > +    buffer = qemu_blockalign(bs, VHDX_HEADER_SIZE);
> > +    /* we can't assume the extra reserved bytes are 0 */
> > +    ret = bdrv_pread(bs->file, header_offset, buffer, VHDX_HEADER_SIZE);
> > +    if (ret < 0) {
> > +        goto exit;
> > +    }
> > +    /* overwrite the actual VHDXHeader portion */
> > +    memcpy(buffer, inactive_header, sizeof(VHDXHeader));
> > +    inactive_header->checksum = vhdx_update_checksum(buffer,
> > +                                                     VHDX_HEADER_SIZE, 4);
> > +    vhdx_header_le_export(inactive_header, &header_le);
> > +    bdrv_pwrite_sync(bs->file, header_offset, &header_le, 
> > sizeof(VHDXHeader));
> 
> This function can fail and it's a good indicator that future I/Os will
> also be in trouble.  Please propagate the error return.
> 
> > @@ -801,8 +955,7 @@ static int vhdx_open(BlockDriverState *bs, QDict 
> > *options, int flags)
> >      }
> >  
> >      if (flags & BDRV_O_RDWR) {
> > -        ret = -ENOTSUP;
> > -        goto fail;
> > +        vhdx_update_headers(bs, s, false);
> 
> Error handling is missing.  At this point it looks like we cannot write
> to the file.  Propagating the error seems reasonable.