The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/lib/decompress_unxz.c

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    1 /*
    2  * Wrapper for decompressing XZ-compressed kernel, initramfs, and initrd
    3  *
    4  * Author: Lasse Collin <lasse.collin@tukaani.org>
    5  *
    6  * This file has been put into the public domain.
    7  * You can do whatever you want with this file.
    8  */
    9 
   10 /*
   11  * Important notes about in-place decompression
   12  *
   13  * At least on x86, the kernel is decompressed in place: the compressed data
   14  * is placed to the end of the output buffer, and the decompressor overwrites
   15  * most of the compressed data. There must be enough safety margin to
   16  * guarantee that the write position is always behind the read position.
   17  *
   18  * The safety margin for XZ with LZMA2 or BCJ+LZMA2 is calculated below.
   19  * Note that the margin with XZ is bigger than with Deflate (gzip)!
   20  *
   21  * The worst case for in-place decompression is that the beginning of
   22  * the file is compressed extremely well, and the rest of the file is
   23  * uncompressible. Thus, we must look for worst-case expansion when the
   24  * compressor is encoding uncompressible data.
   25  *
   26  * The structure of the .xz file in case of a compresed kernel is as follows.
   27  * Sizes (as bytes) of the fields are in parenthesis.
   28  *
   29  *    Stream Header (12)
   30  *    Block Header:
   31  *      Block Header (8-12)
   32  *      Compressed Data (N)
   33  *      Block Padding (0-3)
   34  *      CRC32 (4)
   35  *    Index (8-20)
   36  *    Stream Footer (12)
   37  *
   38  * Normally there is exactly one Block, but let's assume that there are
   39  * 2-4 Blocks just in case. Because Stream Header and also Block Header
   40  * of the first Block don't make the decompressor produce any uncompressed
   41  * data, we can ignore them from our calculations. Block Headers of possible
   42  * additional Blocks have to be taken into account still. With these
   43  * assumptions, it is safe to assume that the total header overhead is
   44  * less than 128 bytes.
   45  *
   46  * Compressed Data contains LZMA2 or BCJ+LZMA2 encoded data. Since BCJ
   47  * doesn't change the size of the data, it is enough to calculate the
   48  * safety margin for LZMA2.
   49  *
   50  * LZMA2 stores the data in chunks. Each chunk has a header whose size is
   51  * a maximum of 6 bytes, but to get round 2^n numbers, let's assume that
   52  * the maximum chunk header size is 8 bytes. After the chunk header, there
   53  * may be up to 64 KiB of actual payload in the chunk. Often the payload is
   54  * quite a bit smaller though; to be safe, let's assume that an average
   55  * chunk has only 32 KiB of payload.
   56  *
   57  * The maximum uncompressed size of the payload is 2 MiB. The minimum
   58  * uncompressed size of the payload is in practice never less than the
   59  * payload size itself. The LZMA2 format would allow uncompressed size
   60  * to be less than the payload size, but no sane compressor creates such
   61  * files. LZMA2 supports storing uncompressible data in uncompressed form,
   62  * so there's never a need to create payloads whose uncompressed size is
   63  * smaller than the compressed size.
   64  *
   65  * The assumption, that the uncompressed size of the payload is never
   66  * smaller than the payload itself, is valid only when talking about
   67  * the payload as a whole. It is possible that the payload has parts where
   68  * the decompressor consumes more input than it produces output. Calculating
   69  * the worst case for this would be tricky. Instead of trying to do that,
   70  * let's simply make sure that the decompressor never overwrites any bytes
   71  * of the payload which it is currently reading.
   72  *
   73  * Now we have enough information to calculate the safety margin. We need
   74  *   - 128 bytes for the .xz file format headers;
   75  *   - 8 bytes per every 32 KiB of uncompressed size (one LZMA2 chunk header
   76  *     per chunk, each chunk having average payload size of 32 KiB); and
   77  *   - 64 KiB (biggest possible LZMA2 chunk payload size) to make sure that
   78  *     the decompressor never overwrites anything from the LZMA2 chunk
   79  *     payload it is currently reading.
   80  *
   81  * We get the following formula:
   82  *
   83  *    safety_margin = 128 + uncompressed_size * 8 / 32768 + 65536
   84  *                  = 128 + (uncompressed_size >> 12) + 65536
   85  *
   86  * For comparison, according to arch/x86/boot/compressed/misc.c, the
   87  * equivalent formula for Deflate is this:
   88  *
   89  *    safety_margin = 18 + (uncompressed_size >> 12) + 32768
   90  *
   91  * Thus, when updating Deflate-only in-place kernel decompressor to
   92  * support XZ, the fixed overhead has to be increased from 18+32768 bytes
   93  * to 128+65536 bytes.
   94  */
   95 
   96 /*
   97  * STATIC is defined to "static" if we are being built for kernel
   98  * decompression (pre-boot code). <linux/decompress/mm.h> will define
   99  * STATIC to empty if it wasn't already defined. Since we will need to
  100  * know later if we are being used for kernel decompression, we define
  101  * XZ_PREBOOT here.
  102  */
  103 #ifdef STATIC
  104 #       define XZ_PREBOOT
  105 #endif
  106 #ifdef __KERNEL__
  107 #       include <linux/decompress/mm.h>
  108 #endif
  109 #define XZ_EXTERN STATIC
  110 
  111 #ifndef XZ_PREBOOT
  112 #       include <linux/slab.h>
  113 #       include <linux/xz.h>
  114 #else
  115 /*
  116  * Use the internal CRC32 code instead of kernel's CRC32 module, which
  117  * is not available in early phase of booting.
  118  */
  119 #define XZ_INTERNAL_CRC32 1
  120 
  121 /*
  122  * For boot time use, we enable only the BCJ filter of the current
  123  * architecture or none if no BCJ filter is available for the architecture.
  124  */
  125 #ifdef CONFIG_X86
  126 #       define XZ_DEC_X86
  127 #endif
  128 #ifdef CONFIG_PPC
  129 #       define XZ_DEC_POWERPC
  130 #endif
  131 #ifdef CONFIG_ARM
  132 #       define XZ_DEC_ARM
  133 #endif
  134 #ifdef CONFIG_IA64
  135 #       define XZ_DEC_IA64
  136 #endif
  137 #ifdef CONFIG_SPARC
  138 #       define XZ_DEC_SPARC
  139 #endif
  140 
  141 /*
  142  * This will get the basic headers so that memeq() and others
  143  * can be defined.
  144  */
  145 #include "xz/xz_private.h"
  146 
  147 /*
  148  * Replace the normal allocation functions with the versions from
  149  * <linux/decompress/mm.h>. vfree() needs to support vfree(NULL)
  150  * when XZ_DYNALLOC is used, but the pre-boot free() doesn't support it.
  151  * Workaround it here because the other decompressors don't need it.
  152  */
  153 #undef kmalloc
  154 #undef kfree
  155 #undef vmalloc
  156 #undef vfree
  157 #define kmalloc(size, flags) malloc(size)
  158 #define kfree(ptr) free(ptr)
  159 #define vmalloc(size) malloc(size)
  160 #define vfree(ptr) do { if (ptr != NULL) free(ptr); } while (0)
  161 
  162 /*
  163  * FIXME: Not all basic memory functions are provided in architecture-specific
  164  * files (yet). We define our own versions here for now, but this should be
  165  * only a temporary solution.
  166  *
  167  * memeq and memzero are not used much and any remotely sane implementation
  168  * is fast enough. memcpy/memmove speed matters in multi-call mode, but
  169  * the kernel image is decompressed in single-call mode, in which only
  170  * memcpy speed can matter and only if there is a lot of uncompressible data
  171  * (LZMA2 stores uncompressible chunks in uncompressed form). Thus, the
  172  * functions below should just be kept small; it's probably not worth
  173  * optimizing for speed.
  174  */
  175 
  176 #ifndef memeq
  177 static bool memeq(const void *a, const void *b, size_t size)
  178 {
  179         const uint8_t *x = a;
  180         const uint8_t *y = b;
  181         size_t i;
  182 
  183         for (i = 0; i < size; ++i)
  184                 if (x[i] != y[i])
  185                         return false;
  186 
  187         return true;
  188 }
  189 #endif
  190 
  191 #ifndef memzero
  192 static void memzero(void *buf, size_t size)
  193 {
  194         uint8_t *b = buf;
  195         uint8_t *e = b + size;
  196 
  197         while (b != e)
  198                 *b++ = '\0';
  199 }
  200 #endif
  201 
  202 #ifndef memmove
  203 /* Not static to avoid a conflict with the prototype in the Linux headers. */
  204 void *memmove(void *dest, const void *src, size_t size)
  205 {
  206         uint8_t *d = dest;
  207         const uint8_t *s = src;
  208         size_t i;
  209 
  210         if (d < s) {
  211                 for (i = 0; i < size; ++i)
  212                         d[i] = s[i];
  213         } else if (d > s) {
  214                 i = size;
  215                 while (i-- > 0)
  216                         d[i] = s[i];
  217         }
  218 
  219         return dest;
  220 }
  221 #endif
  222 
  223 /*
  224  * Since we need memmove anyway, would use it as memcpy too.
  225  * Commented out for now to avoid breaking things.
  226  */
  227 /*
  228 #ifndef memcpy
  229 #       define memcpy memmove
  230 #endif
  231 */
  232 
  233 #include "xz/xz_crc32.c"
  234 #include "xz/xz_dec_stream.c"
  235 #include "xz/xz_dec_lzma2.c"
  236 #include "xz/xz_dec_bcj.c"
  237 
  238 #endif /* XZ_PREBOOT */
  239 
  240 /* Size of the input and output buffers in multi-call mode */
  241 #define XZ_IOBUF_SIZE 4096
  242 
  243 /*
  244  * This function implements the API defined in <linux/decompress/generic.h>.
  245  *
  246  * This wrapper will automatically choose single-call or multi-call mode
  247  * of the native XZ decoder API. The single-call mode can be used only when
  248  * both input and output buffers are available as a single chunk, i.e. when
  249  * fill() and flush() won't be used.
  250  */
  251 STATIC int INIT unxz(unsigned char *in, int in_size,
  252                      int (*fill)(void *dest, unsigned int size),
  253                      int (*flush)(void *src, unsigned int size),
  254                      unsigned char *out, int *in_used,
  255                      void (*error)(char *x))
  256 {
  257         struct xz_buf b;
  258         struct xz_dec *s;
  259         enum xz_ret ret;
  260         bool must_free_in = false;
  261 
  262 #if XZ_INTERNAL_CRC32
  263         xz_crc32_init();
  264 #endif
  265 
  266         if (in_used != NULL)
  267                 *in_used = 0;
  268 
  269         if (fill == NULL && flush == NULL)
  270                 s = xz_dec_init(XZ_SINGLE, 0);
  271         else
  272                 s = xz_dec_init(XZ_DYNALLOC, (uint32_t)-1);
  273 
  274         if (s == NULL)
  275                 goto error_alloc_state;
  276 
  277         if (flush == NULL) {
  278                 b.out = out;
  279                 b.out_size = (size_t)-1;
  280         } else {
  281                 b.out_size = XZ_IOBUF_SIZE;
  282                 b.out = malloc(XZ_IOBUF_SIZE);
  283                 if (b.out == NULL)
  284                         goto error_alloc_out;
  285         }
  286 
  287         if (in == NULL) {
  288                 must_free_in = true;
  289                 in = malloc(XZ_IOBUF_SIZE);
  290                 if (in == NULL)
  291                         goto error_alloc_in;
  292         }
  293 
  294         b.in = in;
  295         b.in_pos = 0;
  296         b.in_size = in_size;
  297         b.out_pos = 0;
  298 
  299         if (fill == NULL && flush == NULL) {
  300                 ret = xz_dec_run(s, &b);
  301         } else {
  302                 do {
  303                         if (b.in_pos == b.in_size && fill != NULL) {
  304                                 if (in_used != NULL)
  305                                         *in_used += b.in_pos;
  306 
  307                                 b.in_pos = 0;
  308 
  309                                 in_size = fill(in, XZ_IOBUF_SIZE);
  310                                 if (in_size < 0) {
  311                                         /*
  312                                          * This isn't an optimal error code
  313                                          * but it probably isn't worth making
  314                                          * a new one either.
  315                                          */
  316                                         ret = XZ_BUF_ERROR;
  317                                         break;
  318                                 }
  319 
  320                                 b.in_size = in_size;
  321                         }
  322 
  323                         ret = xz_dec_run(s, &b);
  324 
  325                         if (flush != NULL && (b.out_pos == b.out_size
  326                                         || (ret != XZ_OK && b.out_pos > 0))) {
  327                                 /*
  328                                  * Setting ret here may hide an error
  329                                  * returned by xz_dec_run(), but probably
  330                                  * it's not too bad.
  331                                  */
  332                                 if (flush(b.out, b.out_pos) != (int)b.out_pos)
  333                                         ret = XZ_BUF_ERROR;
  334 
  335                                 b.out_pos = 0;
  336                         }
  337                 } while (ret == XZ_OK);
  338 
  339                 if (must_free_in)
  340                         free(in);
  341 
  342                 if (flush != NULL)
  343                         free(b.out);
  344         }
  345 
  346         if (in_used != NULL)
  347                 *in_used += b.in_pos;
  348 
  349         xz_dec_end(s);
  350 
  351         switch (ret) {
  352         case XZ_STREAM_END:
  353                 return 0;
  354 
  355         case XZ_MEM_ERROR:
  356                 /* This can occur only in multi-call mode. */
  357                 error("XZ decompressor ran out of memory");
  358                 break;
  359 
  360         case XZ_FORMAT_ERROR:
  361                 error("Input is not in the XZ format (wrong magic bytes)");
  362                 break;
  363 
  364         case XZ_OPTIONS_ERROR:
  365                 error("Input was encoded with settings that are not "
  366                                 "supported by this XZ decoder");
  367                 break;
  368 
  369         case XZ_DATA_ERROR:
  370         case XZ_BUF_ERROR:
  371                 error("XZ-compressed data is corrupt");
  372                 break;
  373 
  374         default:
  375                 error("Bug in the XZ decompressor");
  376                 break;
  377         }
  378 
  379         return -1;
  380 
  381 error_alloc_in:
  382         if (flush != NULL)
  383                 free(b.out);
  384 
  385 error_alloc_out:
  386         xz_dec_end(s);
  387 
  388 error_alloc_state:
  389         error("XZ decompressor ran out of memory");
  390         return -1;
  391 }
  392 
  393 /*
  394  * This macro is used by architecture-specific files to decompress
  395  * the kernel image.
  396  */
  397 #define decompress unxz

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