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block/dmg: improve zeroes handling 

Disk images may contain large all-zeroes gaps (1.66k sectors or 812 MiB
is seen in the real world). These blocks (type 2) do not need to be
extracted into a temporary buffer, there is no need to allocate memory
for these blocks nor to check its length.

(For the test image, the maximum uncompressed size is 1054371 bytes,
probably for a bzip2-compressed block.)

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1420566495-13284-13-git-send-email-peter@lekensteyn.nl
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Peter Wu 2015-01-06 18:48:15 +01:00 committed by Kevin Wolf
parent 6b383c08c4
commit 177b75104d
1 changed files with 15 additions and 3 deletions
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18
block/dmg.c
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@ -137,7 +137,9 @@ static void update_max_chunk_size(BDRVDMGState *s, uint32_t chunk,
uncompressed_sectors = (s->lengths[chunk] + 511) / 512;
break;
case 2: /* zero */
uncompressed_sectors = s->sectorcounts[chunk];
/* as the all-zeroes block may be large, it is treated specially: the
* sector is not copied from a large buffer, a simple memset is used
* instead. Therefore uncompressed_sectors does not need to be set. */
break;
}
@ -267,7 +269,9 @@ static int dmg_read_mish_block(BDRVDMGState *s, DmgHeaderState *ds,
/* sector count */
s->sectorcounts[i] = buff_read_uint64(buffer, offset + 0x10);
if (s->sectorcounts[i] > DMG_SECTORCOUNTS_MAX) {
/* all-zeroes sector (type 2) does not need to be "uncompressed" and can
* therefore be unbounded. */
if (s->types[i] != 2 && s->sectorcounts[i] > DMG_SECTORCOUNTS_MAX) {
error_report("sector count %" PRIu64 " for chunk %" PRIu32
" is larger than max (%u)",
s->sectorcounts[i], i, DMG_SECTORCOUNTS_MAX);
@ -643,7 +647,8 @@ static inline int dmg_read_chunk(BlockDriverState *bs, uint64_t sector_num)
}
break;
case 2: /* zero */
memset(s->uncompressed_chunk, 0, 512 * s->sectorcounts[chunk]);
/* see dmg_read, it is treated specially. No buffer needs to be
* pre-filled, the zeroes can be set directly. */
break;
}
s->current_chunk = chunk;
@ -662,6 +667,13 @@ static int dmg_read(BlockDriverState *bs, int64_t sector_num,
if (dmg_read_chunk(bs, sector_num + i) != 0) {
return -1;
}
/* Special case: current chunk is all zeroes. Do not perform a memcpy as
* s->uncompressed_chunk may be too small to cover the large all-zeroes
* section. dmg_read_chunk is called to find s->current_chunk */
if (s->types[s->current_chunk] == 2) { /* all zeroes block entry */
memset(buf + i * 512, 0, 512);
continue;
}
sector_offset_in_chunk = sector_num + i - s->sectors[s->current_chunk];
memcpy(buf + i * 512,
s->uncompressed_chunk + sector_offset_in_chunk * 512, 512);