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btrfs-progs: restore, fix page alignment issue for lzo compression

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When runing restore under lzo compression, "bad compress length"
problems are encountered.
It is because there is a page alignment problem with the @decompress_lzo,
as follows:
		|------| |----|-| |------|...|------|
		  page         ^    page       page
			       |
			  3 bytes left

When lzo pages are compressed in memory, we will ensure that the 4 bytes
length header will not cross a page boundary.  There is a situation that
3 (or less) bytes are left at the end of a page, and then the 4 bytes
len is stored at the start of the next page.  But the @decompress_lzo
doesn't go to the start of the next page and continue to read the next 4
bytes which crosses two pages, so a random value is fetched as a "bad
compress length".

So we check page alignment every time before we are going to fetch the
next @len and after the former piece of data is decompressed.  If the
current page that we reach has less than 4 bytes left, then we should
fetch the next @len at the start of next page.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
[simplifed and moved into decompress_lzo]
Signed-off-by: David Sterba <dsterba@suse.cz>
This commit is contained in:
Gui Hecheng 2014-09-22 16:29:28 +08:00 committed by David Sterba
parent 895b772996
commit f18f8b7afc
1 changed files with 13 additions and 0 deletions
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13
cmds-restore.c
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@ -111,6 +111,8 @@ static int decompress_lzo(unsigned char *inbuf, char *outbuf, u64 compress_len,
tot_in = LZO_LEN;
while (tot_in < tot_len) {
size_t mod_page;
size_t rem_page;
in_len = read_compress_length(inbuf);
if ((tot_in + LZO_LEN + in_len) > tot_len) {
@ -134,6 +136,17 @@ static int decompress_lzo(unsigned char *inbuf, char *outbuf, u64 compress_len,
outbuf += new_len;
inbuf += in_len;
tot_in += in_len;
/*
* If the 4 byte header does not fit to the rest of the page we
* have to move to the next one, unless we read some garbage
*/
mod_page = tot_in % PAGE_CACHE_SIZE;
rem_page = PAGE_CACHE_SIZE - mod_page;
if (rem_page < LZO_LEN) {
inbuf += rem_page;
tot_in += rem_page;
}
}
*decompress_len = out_len;