On Friday 02 January 2015 18:58:00 John Snow wrote: > > On 12/27/2014 10:01 AM, Peter Wu wrote: > > DMG files have a variable length with a UDIF trailer at the end of a > > file. This UDIF trailer is essential as it describes the contents of > > the image. At the moment however, the start of this trailer is almost > > always incorrect as bdrv_getlength() returns a multiple of the block > > size (rounded up). This results in a failure to recognize DMG files, > > resulting in Invalid argument (EINVAL) errors. > > > > As there is no API to retrieve the real file size, look for the magic > > header in the last two sectors to find the start of this 512-byte UDIF > > trailer (the "koly" block). > > > > The resource fork offset ("info_begin") has its offset adjusted as the > > initial value of offset does not mean "end of file" anymore, but "begin > > of UDIF trailer". > > > > Signed-off-by: Peter Wu <pe...@lekensteyn.nl> > > --- > > block/dmg.c | 40 ++++++++++++++++++++++++++++++++++++---- > > 1 file changed, 36 insertions(+), 4 deletions(-) > > > > diff --git a/block/dmg.c b/block/dmg.c > > index e455886..df274f9 100644 > > --- a/block/dmg.c > > +++ b/block/dmg.c > > @@ -131,6 +131,39 @@ static void update_max_chunk_size(BDRVDMGState *s, > > uint32_t chunk, > > } > > } > > > > +static int64_t dmg_find_koly_offset(BlockDriverState *file_bs) > > +{ > > + int64_t length; > > + int64_t offset = 0; > > + uint8_t buffer[515]; > > + int i, ret; > > + > > + /* bdrv_getlength returns a multiple of block size (512), rounded up. > > Since > > + * dmg images can have odd sizes, try to look for the "koly" magic > > which > > + * marks the begin of the UDIF trailer (512 bytes). This magic can be > > found > > + * in the last 511 bytes of the second-last sector or the first 4 > > bytes of > > + * the last sector (search space: 515 bytes) */ > > + length = bdrv_getlength(file_bs); > > + if (length < 512) { > > + return length < 0 ? length : -EINVAL; > > + } > > + if (length > 511 + 512) { > > + offset = length - 511 - 512; > > + } > > + length = length < 515 ? length : 515; > > + ret = bdrv_pread(file_bs, offset, buffer, length); > > + if (ret < 4) { > > + return ret < 0 ? ret : -EINVAL; > > + } > > + for (i = 0; i < length - 3; i++) { > > + if (buffer[i] == 'k' && buffer[i+1] == 'o' && > > + buffer[i+2] == 'l' && buffer[i+3] == 'y') { > > + return offset + i; > > + } > > + } > > + return -EINVAL; > > +} > > + > > static int dmg_open(BlockDriverState *bs, QDict *options, int flags, > > Error **errp) > > { > > @@ -145,15 +178,14 @@ static int dmg_open(BlockDriverState *bs, QDict > > *options, int flags, > > s->n_chunks = 0; > > s->offsets = s->lengths = s->sectors = s->sectorcounts = NULL; > > > > - /* read offset of info blocks */ > > - offset = bdrv_getlength(bs->file); > > + /* locate the UDIF trailer */ > > + offset = dmg_find_koly_offset(bs->file); > > if (offset < 0) { > > ret = offset; > > goto fail; > > } > > - offset -= 0x1d8; > > > > - ret = read_uint64(bs, offset, &info_begin); > > + ret = read_uint64(bs, offset + 0x28, &info_begin); > > if (ret < 0) { > > goto fail; > > } else if (info_begin == 0) { > > > > If there really is no convenient way to retrieve the real length ... > (Stefan: Would that be difficult to add?) > > Reviewed-by: John Snow <js...@redhat.com>
The real length can be stored, but it takes more effort to do so. See the stalled work on this bdrv-getlength-conversion branch[1] and in particular "block: do not directly set total_sectors"[2]. -- Kind regards, Peter https://lekensteyn.nl [1]: https://git.lekensteyn.nl/peter/qemu/log/?h=bdrv-getlength-conversion [2]: https://git.lekensteyn.nl/peter/qemu/commit/?h=bdrv-getlength-conversion&id=e5164735e5b674a10134894589a060a0f5f32ccc