/* * Wrapper for decompressing XZ-compressed kernel, initramfs, and initrd * * Author: Lasse Collin <[email protected]> * * 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 * incompressible. Thus, we must look for worst-case expansion when the * compressor is encoding incompressible data. * * The structure of the .xz file in case of a compressed 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 incompressible 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 * to 128+65536 bytes. */ /* * STATIC is defined to "static" if we are being built for kernel * decompression (pre-boot code). <linux/decompress/mm.h> will define * STATIC to empty if it wasn't already defined. Since we will need to * know later if we are being used for kernel decompression, we define * XZ_PREBOOT here. */ #ifdef STATIC #define XZ_PREBOOT #else #include <linux/decompress/unxz.h> #endif #ifdef __KERNEL__ # include <linux/decompress/mm.h> #endif #define XZ_EXTERN … #ifndef XZ_PREBOOT # include <linux/slab.h> # include <linux/xz.h> #else /* * Use the internal CRC32 code instead of kernel's CRC32 module, which * is not available in early phase of booting. */ #define XZ_INTERNAL_CRC32 … /* * For boot time use, we enable only the BCJ filter of the current * architecture or none if no BCJ filter is available for the architecture. */ #ifdef CONFIG_X86 #define XZ_DEC_X86 #endif #ifdef CONFIG_PPC #define XZ_DEC_POWERPC #endif #ifdef CONFIG_ARM #define XZ_DEC_ARM #endif #ifdef CONFIG_SPARC #define XZ_DEC_SPARC #endif /* * This will get the basic headers so that memeq() and others * can be defined. */ #include "xz/xz_private.h" /* * Replace the normal allocation functions with the versions from * <linux/decompress/mm.h>. vfree() needs to support vfree(NULL) * when XZ_DYNALLOC is used, but the pre-boot free() doesn't support it. * Workaround it here because the other decompressors don't need it. */ #undef kmalloc #undef kfree #undef vmalloc #undef vfree #define kmalloc … #define kfree … #define vmalloc … #define vfree … /* * FIXME: Not all basic memory functions are provided in architecture-specific * files (yet). We define our own versions here for now, but this should be * only a temporary solution. * * memeq and memzero are not used much and any remotely sane implementation * is fast enough. memcpy/memmove speed matters in multi-call mode, but * the kernel image is decompressed in single-call mode, in which only * memmove speed can matter and only if there is a lot of incompressible data * (LZMA2 stores incompressible chunks in uncompressed form). Thus, the * functions below should just be kept small; it's probably not worth * optimizing for speed. */ #ifndef memeq static bool memeq(const void *a, const void *b, size_t size) { const uint8_t *x = a; const uint8_t *y = b; size_t i; for (i = 0; i < size; ++i) if (x[i] != y[i]) return false; return true; } #endif #ifndef memzero static void memzero(void *buf, size_t size) { uint8_t *b = buf; uint8_t *e = b + size; while (b != e) *b++ = '\0'; } #endif #ifndef memmove /* Not static to avoid a conflict with the prototype in the Linux headers. */ void *memmove(void *dest, const void *src, size_t size) { uint8_t *d = dest; const uint8_t *s = src; size_t i; if (d < s) { for (i = 0; i < size; ++i) d[i] = s[i]; } else if (d > s) { i = size; while (i-- > 0) d[i] = s[i]; } return dest; } #endif /* * Since we need memmove anyway, would use it as memcpy too. * Commented out for now to avoid breaking things. */ /* #ifndef memcpy # define memcpy memmove #endif */ #include "xz/xz_crc32.c" #include "xz/xz_dec_stream.c" #include "xz/xz_dec_lzma2.c" #include "xz/xz_dec_bcj.c" #endif /* XZ_PREBOOT */ /* Size of the input and output buffers in multi-call mode */ #define XZ_IOBUF_SIZE … /* * This function implements the API defined in <linux/decompress/generic.h>. * * This wrapper will automatically choose single-call or multi-call mode * of the native XZ decoder API. The single-call mode can be used only when * both input and output buffers are available as a single chunk, i.e. when * fill() and flush() won't be used. */ STATIC int INIT unxz(unsigned char *in, long in_size, long (*fill)(void *dest, unsigned long size), long (*flush)(void *src, unsigned long size), unsigned char *out, long *in_used, void (*error)(char *x)) { … } /* * This macro is used by architecture-specific files to decompress * the kernel image. */ #ifdef XZ_PREBOOT STATIC int INIT __decompress(unsigned char *buf, long len, long (*fill)(void*, unsigned long), long (*flush)(void*, unsigned long), unsigned char *out_buf, long olen, long *pos, void (*error)(char *x)) { return unxz(buf, len, fill, flush, out_buf, pos, error); } #endif
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