libarchive: treat one "FIXME: avoid seek" patch issue

[Denys Vlasenko]

On Sun, May 26, 2019 at 12:48 AM Rostyslav Kurylo
<rostawesomegd at gmail.com> wrote:
>
> Hi everyone,
>
> I was playing around with the busybox master branch and found an
> issue with the "libarchive: treat one "FIXME: avoid seek"" patch
> (commit hash dff2bd733fc2dac08d34f2cfad0e68aeb8e7a7a2).
>
> The xmalloc_read_with_inital_buf() accepts xstate->signature_skipped as the
> buffer offset/initial buffer size. In case of uncompressed files
> (no known 2-byte signature was found at the start), the signature_skipped is 0
> (see open_transformer.c:355).
> However, the src_fd file position is already shifted, and the first two bytes
> have been already read into the xstate by open_transformer() function.
> Passing 0 as offset/buffer size leads to the first two bytes being lost.

you are right, it's misplaced:

--- a/archival/libarchive/open_transformer.c
+++ b/archival/libarchive/open_transformer.c
@@ -352,13 +352,13 @@ void* FAST_FUNC
xmalloc_open_zipped_read_close(const char *fname, size_t *maxsz_
                 *   read(4, "LF\2\1\1\0\0\0\0"...
                 * ...and we avoided seeking on the fd! :)
                 */
-               xstate->signature_skipped = 0;
                image = xmalloc_read_with_initial_buf(
                        xstate->src_fd,
                        maxsz_p,
                        xmemdup(&xstate->magic, xstate->signature_skipped),
                        xstate->signature_skipped
                );
+               xstate->signature_skipped = 0;
        }