On Thu, 2014-09-18 at 10:25 +0200, Marc Dietrich wrote:
> Hello Gui,
>
> Am Donnerstag, 18. September 2014, 11:34:43 schrieb Gui Hecheng:
> > When runing restore under lzo compression, "bad compress length"
> > problems are encountered.
> > It is because there is a page align problem with the @decompress_lzo,
> > as follows:
> > |------| |----|-| |------|...|------|
> > page ^ page page
> > |
> > 3 bytes left
> >
> > When lzo compress pages im RAM, lzo will ensure that
> > the 4 bytes len will be in one page as a whole.
> > 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 goto the start of
> > the next page and continue to read the next 4 bytes
> > which is across two pages, so a random value is fetched
> > as a "bad compress length".
> >
> > So we just switch to the page-aligned start position to read
> > the len of next piece of data when "bad compress length" is encounterd.
> > If we still get bad compress length in this case, then there is a
> > real "bad compress length", and we shall report error.
> >
> > Signed-off-by: Gui Hecheng <guihc.fnst@xxxxxxxxxxxxxx>
> > ---
> > cmds-restore.c | 20 ++++++++++++++++++++
> > 1 file changed, 20 insertions(+)
> >
> > diff --git a/cmds-restore.c b/cmds-restore.c
> > index 38a131e..8b230ab 100644
> > --- a/cmds-restore.c
> > +++ b/cmds-restore.c
> > @@ -57,6 +57,9 @@ static int dry_run = 0;
> >
> > #define LZO_LEN 4
> > #define PAGE_CACHE_SIZE 4096
> > +#define PAGE_CACHE_MASK (~(PAGE_CACHE_SIZE - 1))
> > +#define PAGE_CACHE_ALIGN(addr) (((addr) + PAGE_CACHE_SIZE - 1) \
> > + & PAGE_CACHE_MASK)
> > #define lzo1x_worst_compress(x) ((x) + ((x) / 16) + 64 + 3)
> >
> > static int decompress_zlib(char *inbuf, char *outbuf, u64 compress_len,
> > @@ -101,6 +104,8 @@ static int decompress_lzo(unsigned char *inbuf, char *outbuf, u64 compress_len,
> > size_t out_len = 0;
> > size_t tot_len;
> > size_t tot_in;
> > + size_t tot_in_aligned;
> > + int aligned = 0;
> > int ret;
> >
> > ret = lzo_init();
> > @@ -117,6 +122,20 @@ static int decompress_lzo(unsigned char *inbuf, char *outbuf, u64 compress_len,
> > in_len = read_compress_length(inbuf);
> >
> > if ((tot_in + LZO_LEN + in_len) > tot_len) {
> > + /*
> > + * The LZO_LEN bytes is guaranteed to be
> > + * in one page as a whole, so if a page
> > + * has fewer than LZO_LEN bytes left,
> > + * the LZO_LEN bytes should be fetched
> > + * at the start of the next page
> > + */
> > + if (!aligned) {
> > + tot_in_aligned = PAGE_CACHE_ALIGN(tot_in);
> > + inbuf += (tot_in_aligned - tot_in);
> > + tot_in = tot_in_aligned;
> > + aligned = 1;
> > + continue;
> > + }
>
> Small question, shouldn't the aligned check be moved out of the if block?
> First, we could have a bad length caused by the alignment which could result
> in a stream length less than tot_len.
Ah, you have reminded me of a missing case to be covered.
> Second, if we know that the length record never crosses a page, why not
> always check for proper alignment. I think the overhead should be minimal.
I don't think alignment should be checked always, because in the
"normal" case the lzo stuff is "compact":
[len][----data----][len][----data----]...
It is never aligned to anything and we never knows where next @len
starts before we read the former one. The alignement-related issue is a
rare case.
Thanks for your comments.
-Gui
> Marc
>
>
> > fprintf(stderr, "bad compress length %lu\n",
> > (unsigned long)in_len);
> > return -1;
> > @@ -137,6 +156,7 @@ static int decompress_lzo(unsigned char *inbuf, char *outbuf, u64 compress_len,
> > outbuf += new_len;
> > inbuf += in_len;
> > tot_in += in_len;
> > + aligned = 0;
> > }
> >
> > *decompress_len = out_len;
> >
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