linux-imx

/*

 * Wrapper for decompressing XZ-compressed kernel, initramfs, and initrd

 *

 * Author: Lasse Collin <lasse.collin@tukaani.org>

 *

 * This file has been put into the public domain.

 * You can do whatever you want with this file.

 */

​

/*

 * Important notes about in-place decompression

 *

 * At least on x86, the kernel is decompressed in place: the compressed data

 * is placed to the end of the output buffer, and the decompressor overwrites

 * most of the compressed data. There must be enough safety margin to

 * guarantee that the write position is always behind the read position.

 *

 * The safety margin for XZ with LZMA2 or BCJ+LZMA2 is calculated below.

 * Note that the margin with XZ is bigger than with Deflate (gzip)!

 *

 * The worst case for in-place decompression is that the beginning of

 * the file is compressed extremely well, and the rest of the file is

 * uncompressible. Thus, we must look for worst-case expansion when the

 * compressor is encoding uncompressible data.

 *

 * The structure of the .xz file in case of a compresed kernel is as follows.

 * Sizes (as bytes) of the fields are in parenthesis.

 *

 *    Stream Header (12)

 *    Block Header:

 *      Block Header (8-12)

 *      Compressed Data (N)

 *      Block Padding (0-3)

 *      CRC32 (4)

 *    Index (8-20)

 *    Stream Footer (12)

 *

 * Normally there is exactly one Block, but let's assume that there are

 * 2-4 Blocks just in case. Because Stream Header and also Block Header

 * of the first Block don't make the decompressor produce any uncompressed

 * data, we can ignore them from our calculations. Block Headers of possible

 * additional Blocks have to be taken into account still. With these

 * assumptions, it is safe to assume that the total header overhead is

 * less than 128 bytes.

 *

 * Compressed Data contains LZMA2 or BCJ+LZMA2 encoded data. Since BCJ

 * doesn't change the size of the data, it is enough to calculate the

 * safety margin for LZMA2.

 *

 * LZMA2 stores the data in chunks. Each chunk has a header whose size is

 * a maximum of 6 bytes, but to get round 2^n numbers, let's assume that

 * the maximum chunk header size is 8 bytes. After the chunk header, there

 * may be up to 64 KiB of actual payload in the chunk. Often the payload is

 * quite a bit smaller though; to be safe, let's assume that an average

 * chunk has only 32 KiB of payload.

 *

 * The maximum uncompressed size of the payload is 2 MiB. The minimum

 * uncompressed size of the payload is in practice never less than the

 * payload size itself. The LZMA2 format would allow uncompressed size

 * to be less than the payload size, but no sane compressor creates such

 * files. LZMA2 supports storing uncompressible data in uncompressed form,

 * so there's never a need to create payloads whose uncompressed size is

 * smaller than the compressed size.

 *

 * The assumption, that the uncompressed size of the payload is never

 * smaller than the payload itself, is valid only when talking about

 * the payload as a whole. It is possible that the payload has parts where

 * the decompressor consumes more input than it produces output. Calculating

 * the worst case for this would be tricky. Instead of trying to do that,

 * let's simply make sure that the decompressor never overwrites any bytes

 * of the payload which it is currently reading.

 *

 * Now we have enough information to calculate the safety margin. We need

 *   - 128 bytes for the .xz file format headers;

 *   - 8 bytes per every 32 KiB of uncompressed size (one LZMA2 chunk header

 *     per chunk, each chunk having average payload size of 32 KiB); and

 *   - 64 KiB (biggest possible LZMA2 chunk payload size) to make sure that

 *     the decompressor never overwrites anything from the LZMA2 chunk

 *     payload it is currently reading.

 *

 * We get the following formula:

 *

 *    safety_margin = 128 + uncompressed_size * 8 / 32768 + 65536

 *                  = 128 + (uncompressed_size >> 12) + 65536

 *

 * For comparison, according to arch/x86/boot/compressed/misc.c, the

 * equivalent formula for Deflate is this:

 *

 *    safety_margin = 18 + (uncompressed_size >> 12) + 32768

 *

 * Thus, when updating Deflate-only in-place kernel decompressor to

 * support XZ, the fixed overhead has to be increased from 18+32768 bytes