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raw-posix: The SEEK_HOLE code is flawed, rewrite it

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On systems where SEEK_HOLE in a trailing hole seeks to EOF (Solaris,
but not Linux), try_seek_hole() reports trailing data instead.

Additionally, unlikely lseek() failures are treated badly:

* When SEEK_HOLE fails, try_seek_hole() reports trailing data.  For
  -ENXIO, there's in fact a trailing hole.  Can happen only when
  something truncated the file since we opened it.

* When SEEK_HOLE succeeds, SEEK_DATA fails, and SEEK_END succeeds,
  then try_seek_hole() reports a trailing hole.  This is okay only
  when SEEK_DATA failed with -ENXIO (which means the non-trailing hole
  found by SEEK_HOLE has since become trailing somehow).  For other
  failures (unlikely), it's wrong.

* When SEEK_HOLE succeeds, SEEK_DATA fails, SEEK_END fails (unlikely),
  then try_seek_hole() reports bogus data [-1,start), which its caller
  raw_co_get_block_status() turns into zero sectors of data.  Could
  theoretically lead to infinite loops in code that attempts to scan
  data vs. hole forward.

Rewrite from scratch, with very careful comments.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Markus Armbruster 2014-11-17 11:18:34 +01:00 committed by Max Reitz
parent c4875e5b22
commit d1f06fe665
1 changed files with 85 additions and 26 deletions
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111
block/raw-posix.c
View file

@ -1475,28 +1475,86 @@ out:
return result;
}
static int try_seek_hole(BlockDriverState *bs, off_t start, off_t *data,
off_t *hole)
/*
* Find allocation range in @bs around offset @start.
* May change underlying file descriptor's file offset.
* If @start is not in a hole, store @start in @data, and the
* beginning of the next hole in @hole, and return 0.
* If @start is in a non-trailing hole, store @start in @hole and the
* beginning of the next non-hole in @data, and return 0.
* If @start is in a trailing hole or beyond EOF, return -ENXIO.
* If we can't find out, return a negative errno other than -ENXIO.
*/
static int find_allocation(BlockDriverState *bs, off_t start,
off_t *data, off_t *hole)
{
#if defined SEEK_HOLE && defined SEEK_DATA
BDRVRawState *s = bs->opaque;
off_t offs;
*hole = lseek(s->fd, start, SEEK_HOLE);
if (*hole == -1) {
return -errno;
/*
* SEEK_DATA cases:
* D1. offs == start: start is in data
* D2. offs > start: start is in a hole, next data at offs
* D3. offs < 0, errno = ENXIO: either start is in a trailing hole
* or start is beyond EOF
* If the latter happens, the file has been truncated behind
* our back since we opened it. All bets are off then.
* Treating like a trailing hole is simplest.
* D4. offs < 0, errno != ENXIO: we learned nothing
*/
offs = lseek(s->fd, start, SEEK_DATA);
if (offs < 0) {
return -errno; /* D3 or D4 */
}
assert(offs >= start);
if (offs > start) {
/* D2: in hole, next data at offs */
*hole = start;
*data = offs;
return 0;
}
if (*hole > start) {
/* D1: in data, end not yet known */
/*
* SEEK_HOLE cases:
* H1. offs == start: start is in a hole
* If this happens here, a hole has been dug behind our back
* since the previous lseek().
* H2. offs > start: either start is in data, next hole at offs,
* or start is in trailing hole, EOF at offs
* Linux treats trailing holes like any other hole: offs ==
* start. Solaris seeks to EOF instead: offs > start (blech).
* If that happens here, a hole has been dug behind our back
* since the previous lseek().
* H3. offs < 0, errno = ENXIO: start is beyond EOF
* If this happens, the file has been truncated behind our
* back since we opened it. Treat it like a trailing hole.
* H4. offs < 0, errno != ENXIO: we learned nothing
* Pretend we know nothing at all, i.e. "forget" about D1.
*/
offs = lseek(s->fd, start, SEEK_HOLE);
if (offs < 0) {
return -errno; /* D1 and (H3 or H4) */
}
assert(offs >= start);
if (offs > start) {
/*
* D1 and H2: either in data, next hole at offs, or it was in
* data but is now in a trailing hole. In the latter case,
* all bets are off. Treating it as if it there was data all
* the way to EOF is safe, so simply do that.
*/
*data = start;
} else {
/* On a hole. We need another syscall to find its end. */
*data = lseek(s->fd, start, SEEK_DATA);
if (*data == -1) {
*data = lseek(s->fd, 0, SEEK_END);
}
*hole = offs;
return 0;
}
return 0;
/* D1 and H1 */
return -EBUSY;
#else
return -ENOTSUP;
#endif
@ -1539,25 +1597,26 @@ static int64_t coroutine_fn raw_co_get_block_status(BlockDriverState *bs,
nb_sectors = DIV_ROUND_UP(total_size - start, BDRV_SECTOR_SIZE);
}
ret = try_seek_hole(bs, start, &data, &hole);
if (ret < 0) {
/* Assume everything is allocated. */
data = 0;
hole = start + nb_sectors * BDRV_SECTOR_SIZE;
ret = 0;
}
assert(ret >= 0);
if (data <= start) {
ret = find_allocation(bs, start, &data, &hole);
if (ret == -ENXIO) {
/* Trailing hole */
*pnum = nb_sectors;
ret = BDRV_BLOCK_ZERO;
} else if (ret < 0) {
/* No info available, so pretend there are no holes */
*pnum = nb_sectors;
ret = BDRV_BLOCK_DATA;
} else if (data == start) {
/* On a data extent, compute sectors to the end of the extent. */
*pnum = MIN(nb_sectors, (hole - start) / BDRV_SECTOR_SIZE);
return ret | BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | start;
ret = BDRV_BLOCK_DATA;
} else {
/* On a hole, compute sectors to the beginning of the next extent. */
assert(hole == start);
*pnum = MIN(nb_sectors, (data - start) / BDRV_SECTOR_SIZE);
return ret | BDRV_BLOCK_ZERO | BDRV_BLOCK_OFFSET_VALID | start;
ret = BDRV_BLOCK_ZERO;
}
return ret | BDRV_BLOCK_OFFSET_VALID | start;
}
static coroutine_fn BlockAIOCB *raw_aio_discard(BlockDriverState *bs,