cregit-Linux how code gets into the kernel

Release 4.7 lib/decompress_unxz.c

Directory: lib
 * Wrapper for decompressing XZ-compressed kernel, initramfs, and initrd
 * Author: Lasse Collin <>
 * 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
 * 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
#ifdef __KERNEL__
#	include <linux/decompress/mm.h>


#ifndef XZ_PREBOOT
#	include <linux/slab.h>
#	include <linux/xz.h>
 * Use the internal CRC32 code instead of kernel's CRC32 module, which
 * is not available in early phase of booting.

#define XZ_INTERNAL_CRC32 1

 * 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


#	define XZ_DEC_ARM
#ifdef CONFIG_IA64

#	define XZ_DEC_IA64

#	define XZ_DEC_SPARC

 * 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(size, flags) malloc(size)

#define kfree(ptr) free(ptr)

#define vmalloc(size) malloc(size)

#define vfree(ptr) do { if (ptr != NULL) free(ptr); } while (0)

 * 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
 * memcpy speed can matter and only if there is a lot of uncompressible data
 * (LZMA2 stores uncompressible 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; }


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#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'; }


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#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; }


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#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 4096 /* * 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)) { struct xz_buf b; struct xz_dec *s; enum xz_ret ret; bool must_free_in = false; #if XZ_INTERNAL_CRC32 xz_crc32_init(); #endif if (in_used != NULL) *in_used = 0; if (fill == NULL && flush == NULL) s = xz_dec_init(XZ_SINGLE, 0); else s = xz_dec_init(XZ_DYNALLOC, (uint32_t)-1); if (s == NULL) goto error_alloc_state; if (flush == NULL) { b.out = out; b.out_size = (size_t)-1; } else { b.out_size = XZ_IOBUF_SIZE; b.out = malloc(XZ_IOBUF_SIZE); if (b.out == NULL) goto error_alloc_out; } if (in == NULL) { must_free_in = true; in = malloc(XZ_IOBUF_SIZE); if (in == NULL) goto error_alloc_in; } = in; b.in_pos = 0; b.in_size = in_size; b.out_pos = 0; if (fill == NULL && flush == NULL) { ret = xz_dec_run(s, &b); } else { do { if (b.in_pos == b.in_size && fill != NULL) { if (in_used != NULL) *in_used += b.in_pos; b.in_pos = 0; in_size = fill(in, XZ_IOBUF_SIZE); if (in_size < 0) { /* * This isn't an optimal error code * but it probably isn't worth making * a new one either. */ ret = XZ_BUF_ERROR; break; } b.in_size = in_size; } ret = xz_dec_run(s, &b); if (flush != NULL && (b.out_pos == b.out_size || (ret != XZ_OK && b.out_pos > 0))) { /* * Setting ret here may hide an error * returned by xz_dec_run(), but probably * it's not too bad. */ if (flush(b.out, b.out_pos) != (long)b.out_pos) ret = XZ_BUF_ERROR; b.out_pos = 0; } } while (ret == XZ_OK); if (must_free_in) free(in); if (flush != NULL) free(b.out); } if (in_used != NULL) *in_used += b.in_pos; xz_dec_end(s); switch (ret) { case XZ_STREAM_END: return 0; case XZ_MEM_ERROR: /* This can occur only in multi-call mode. */ error("XZ decompressor ran out of memory"); break; case XZ_FORMAT_ERROR: error("Input is not in the XZ format (wrong magic bytes)"); break; case XZ_OPTIONS_ERROR: error("Input was encoded with settings that are not " "supported by this XZ decoder"); break; case XZ_DATA_ERROR: case XZ_BUF_ERROR: error("XZ-compressed data is corrupt"); break; default: error("Bug in the XZ decompressor"); break; } return -1; error_alloc_in: if (flush != NULL) free(b.out); error_alloc_out: xz_dec_end(s); error_alloc_state: error("XZ decompressor ran out of memory"); return -1; }


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/* * 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); }


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Directory: lib
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