lib/decompress_unzstd.c

   1 // SPDX-License-Identifier: GPL-2.0
   2
   3 /*
   4  * Important notes about in-place decompression
   5  *
   6  * At least on x86, the kernel is decompressed in place: the compressed data
   7  * is placed to the end of the output buffer, and the decompressor overwrites
   8  * most of the compressed data. There must be enough safety margin to
   9  * guarantee that the write position is always behind the read position.
  10  *
  11  * The safety margin for ZSTD with a 128 KB block size is calculated below.
  12  * Note that the margin with ZSTD is bigger than with GZIP or XZ!
  13  *
  14  * The worst case for in-place decompression is that the beginning of
  15  * the file is compressed extremely well, and the rest of the file is
  16  * uncompressible. Thus, we must look for worst-case expansion when the
  17  * compressor is encoding uncompressible data.
  18  *
  19  * The structure of the .zst file in case of a compressed kernel is as follows.
  20  * Maximum sizes (as bytes) of the fields are in parenthesis.
  21  *
  22  *    Frame Header: (18)
  23  *    Blocks: (N)
  24  *    Checksum: (4)
  25  *
  26  * The frame header and checksum overhead is at most 22 bytes.
  27  *
  28  * ZSTD stores the data in blocks. Each block has a header whose size is
  29  * a 3 bytes. After the block header, there is up to 128 KB of payload.
  30  * The maximum uncompressed size of the payload is 128 KB. The minimum
  31  * uncompressed size of the payload is never less than the payload size
  32  * (excluding the block header).
  33  *
  34  * The assumption, that the uncompressed size of the payload is never
  35  * smaller than the payload itself, is valid only when talking about
  36  * the payload as a whole. It is possible that the payload has parts where
  37  * the decompressor consumes more input than it produces output. Calculating
  38  * the worst case for this would be tricky. Instead of trying to do that,
  39  * let's simply make sure that the decompressor never overwrites any bytes
  40  * of the payload which it is currently reading.
  41  *
  42  * Now we have enough information to calculate the safety margin. We need
  43  *   - 22 bytes for the .zst file format headers;
  44  *   - 3 bytes per every 128 KiB of uncompressed size (one block header per
  45  *     block); and
  46  *   - 128 KiB (biggest possible zstd block size) to make sure that the
  47  *     decompressor never overwrites anything from the block it is currently
  48  *     reading.
  49  *
  50  * We get the following formula:
  51  *
  52  *    safety_margin = 22 + uncompressed_size * 3 / 131072 + 131072
  53  *                 <= 22 + (uncompressed_size >> 15) + 131072
  54  */
  55
  56 /*
  57  * Preboot environments #include "path/to/decompress_unzstd.c".
  58  * All of the source files we depend on must be #included.
  59  * zstd's only source dependency is xxhash, which has no source
  60  * dependencies.
  61  *
  62  * When UNZSTD_PREBOOT is defined we declare __decompress(), which is
  63  * used for kernel decompression, instead of unzstd().
  64  *
  65  * Define __DISABLE_EXPORTS in preboot environments to prevent symbols
  66  * from xxhash and zstd from being exported by the EXPORT_SYMBOL macro.
  67  */
  68 #ifdef STATIC
  69 # define UNZSTD_PREBOOT
  70 # include "xxhash.c"
  71 # include "zstd/decompress_sources.h"
  72 #else
  73 #include <linux/decompress/unzstd.h>
  74 #endif
  75
  76 #include <linux/decompress/mm.h>
  77 #include <linux/kernel.h>
  78 #include <linux/zstd.h>
  79
  80 /* 128MB is the maximum window size supported by zstd. */
  81 #define ZSTD_WINDOWSIZE_MAX     (1 << ZSTD_WINDOWLOG_MAX)
  82 /*
  83  * Size of the input and output buffers in multi-call mode.
  84  * Pick a larger size because it isn't used during kernel decompression,
  85  * since that is single pass, and we have to allocate a large buffer for
  86  * zstd's window anyway. The larger size speeds up initramfs decompression.
  87  */
  88 #define ZSTD_IOBUF_SIZE         (1 << 17)
  89
  90 static int INIT handle_zstd_error(size_t ret, void (*error)(char *x))
  91 {
  92         const zstd_error_code err = zstd_get_error_code(ret);
  93
  94         if (!zstd_is_error(ret))
  95                 return 0;
  96
  97         /*
  98          * zstd_get_error_name() cannot be used because error takes a char *
  99          * not a const char *
 100          */
 101         switch (err) {
 102         case ZSTD_error_memory_allocation:
 103                 error("ZSTD decompressor ran out of memory");
 104                 break;
 105         case ZSTD_error_prefix_unknown:
 106                 error("Input is not in the ZSTD format (wrong magic bytes)");
 107                 break;
 108         case ZSTD_error_dstSize_tooSmall:
 109         case ZSTD_error_corruption_detected:
 110         case ZSTD_error_checksum_wrong:
 111                 error("ZSTD-compressed data is corrupt");
 112                 break;
 113         default:
 114                 error("ZSTD-compressed data is probably corrupt");
 115                 break;
 116         }
 117         return -1;
 118 }
 119
 120 /*
 121  * Handle the case where we have the entire input and output in one segment.
 122  * We can allocate less memory (no circular buffer for the sliding window),
 123  * and avoid some memcpy() calls.
 124  */
 125 static int INIT decompress_single(const u8 *in_buf, long in_len, u8 *out_buf,
 126                                   long out_len, long *in_pos,
 127                                   void (*error)(char *x))
 128 {
 129         const size_t wksp_size = zstd_dctx_workspace_bound();
 130         void *wksp = large_malloc(wksp_size);
 131         zstd_dctx *dctx = zstd_init_dctx(wksp, wksp_size);
 132         int err;
 133         size_t ret;
 134
 135         if (dctx == NULL) {
 136                 error("Out of memory while allocating zstd_dctx");
 137                 err = -1;
 138                 goto out;
 139         }
 140         /*
 141          * Find out how large the frame actually is, there may be junk at
 142          * the end of the frame that zstd_decompress_dctx() can't handle.
 143          */
 144         ret = zstd_find_frame_compressed_size(in_buf, in_len);
 145         err = handle_zstd_error(ret, error);
 146         if (err)
 147                 goto out;
 148         in_len = (long)ret;
 149
 150         ret = zstd_decompress_dctx(dctx, out_buf, out_len, in_buf, in_len);
 151         err = handle_zstd_error(ret, error);
 152         if (err)
 153                 goto out;
 154
 155         if (in_pos != NULL)
 156                 *in_pos = in_len;
 157
 158         err = 0;
 159 out:
 160         if (wksp != NULL)
 161                 large_free(wksp);
 162         return err;
 163 }
 164
 165 static int INIT __unzstd(unsigned char *in_buf, long in_len,
 166                          long (*fill)(void*, unsigned long),
 167                          long (*flush)(void*, unsigned long),
 168                          unsigned char *out_buf, long out_len,
 169                          long *in_pos,
 170                          void (*error)(char *x))
 171 {
 172         zstd_in_buffer in;
 173         zstd_out_buffer out;
 174         zstd_frame_header header;
 175         void *in_allocated = NULL;
 176         void *out_allocated = NULL;
 177         void *wksp = NULL;
 178         size_t wksp_size;
 179         zstd_dstream *dstream;
 180         int err;
 181         size_t ret;
 182
 183         /*
 184          * ZSTD decompression code won't be happy if the buffer size is so big
 185          * that its end address overflows. When the size is not provided, make
 186          * it as big as possible without having the end address overflow.
 187          */
 188         if (out_len == 0)
 189                 out_len = UINTPTR_MAX - (uintptr_t)out_buf;
 190
 191         if (fill == NULL && flush == NULL)
 192                 /*
 193                  * We can decompress faster and with less memory when we have a
 194                  * single chunk.
 195                  */
 196                 return decompress_single(in_buf, in_len, out_buf, out_len,
 197                                          in_pos, error);
 198
 199         /*
 200          * If in_buf is not provided, we must be using fill(), so allocate
 201          * a large enough buffer. If it is provided, it must be at least
 202          * ZSTD_IOBUF_SIZE large.
 203          */
 204         if (in_buf == NULL) {
 205                 in_allocated = large_malloc(ZSTD_IOBUF_SIZE);
 206                 if (in_allocated == NULL) {
 207                         error("Out of memory while allocating input buffer");
 208                         err = -1;
 209                         goto out;
 210                 }
 211                 in_buf = in_allocated;
 212                 in_len = 0;
 213         }
 214         /* Read the first chunk, since we need to decode the frame header. */
 215         if (fill != NULL)
 216                 in_len = fill(in_buf, ZSTD_IOBUF_SIZE);
 217         if (in_len < 0) {
 218                 error("ZSTD-compressed data is truncated");
 219                 err = -1;
 220                 goto out;
 221         }
 222         /* Set the first non-empty input buffer. */
 223         in.src = in_buf;
 224         in.pos = 0;
 225         in.size = in_len;
 226         /* Allocate the output buffer if we are using flush(). */
 227         if (flush != NULL) {
 228                 out_allocated = large_malloc(ZSTD_IOBUF_SIZE);
 229                 if (out_allocated == NULL) {
 230                         error("Out of memory while allocating output buffer");
 231                         err = -1;
 232                         goto out;
 233                 }
 234                 out_buf = out_allocated;
 235                 out_len = ZSTD_IOBUF_SIZE;
 236         }
 237         /* Set the output buffer. */
 238         out.dst = out_buf;
 239         out.pos = 0;
 240         out.size = out_len;
 241
 242         /*
 243          * We need to know the window size to allocate the zstd_dstream.
 244          * Since we are streaming, we need to allocate a buffer for the sliding
 245          * window. The window size varies from 1 KB to ZSTD_WINDOWSIZE_MAX
 246          * (8 MB), so it is important to use the actual value so as not to
 247          * waste memory when it is smaller.
 248          */
 249         ret = zstd_get_frame_header(&header, in.src, in.size);
 250         err = handle_zstd_error(ret, error);
 251         if (err)
 252                 goto out;
 253         if (ret != 0) {
 254                 error("ZSTD-compressed data has an incomplete frame header");
 255                 err = -1;
 256                 goto out;
 257         }
 258         if (header.windowSize > ZSTD_WINDOWSIZE_MAX) {
 259                 error("ZSTD-compressed data has too large a window size");
 260                 err = -1;
 261                 goto out;
 262         }
 263
 264         /*
 265          * Allocate the zstd_dstream now that we know how much memory is
 266          * required.
 267          */
 268         wksp_size = zstd_dstream_workspace_bound(header.windowSize);
 269         wksp = large_malloc(wksp_size);
 270         dstream = zstd_init_dstream(header.windowSize, wksp, wksp_size);
 271         if (dstream == NULL) {
 272                 error("Out of memory while allocating ZSTD_DStream");
 273                 err = -1;
 274                 goto out;
 275         }
 276
 277         /*
 278          * Decompression loop:
 279          * Read more data if necessary (error if no more data can be read).
 280          * Call the decompression function, which returns 0 when finished.
 281          * Flush any data produced if using flush().
 282          */
 283         if (in_pos != NULL)
 284                 *in_pos = 0;
 285         do {
 286                 /*
 287                  * If we need to reload data, either we have fill() and can
 288                  * try to get more data, or we don't and the input is truncated.
 289                  */
 290                 if (in.pos == in.size) {
 291                         if (in_pos != NULL)
 292                                 *in_pos += in.pos;
 293                         in_len = fill ? fill(in_buf, ZSTD_IOBUF_SIZE) : -1;
 294                         if (in_len < 0) {
 295                                 error("ZSTD-compressed data is truncated");
 296                                 err = -1;
 297                                 goto out;
 298                         }
 299                         in.pos = 0;
 300                         in.size = in_len;
 301                 }
 302                 /* Returns zero when the frame is complete. */
 303                 ret = zstd_decompress_stream(dstream, &out, &in);
 304                 err = handle_zstd_error(ret, error);
 305                 if (err)
 306                         goto out;
 307                 /* Flush all of the data produced if using flush(). */
 308                 if (flush != NULL && out.pos > 0) {
 309                         if (out.pos != flush(out.dst, out.pos)) {
 310                                 error("Failed to flush()");
 311                                 err = -1;
 312                                 goto out;
 313                         }
 314                         out.pos = 0;
 315                 }
 316         } while (ret != 0);
 317
 318         if (in_pos != NULL)
 319                 *in_pos += in.pos;
 320
 321         err = 0;
 322 out:
 323         if (in_allocated != NULL)
 324                 large_free(in_allocated);
 325         if (out_allocated != NULL)
 326                 large_free(out_allocated);
 327         if (wksp != NULL)
 328                 large_free(wksp);
 329         return err;
 330 }
 331
 332 #ifndef UNZSTD_PREBOOT
 333 STATIC int INIT unzstd(unsigned char *buf, long len,
 334                        long (*fill)(void*, unsigned long),
 335                        long (*flush)(void*, unsigned long),
 336                        unsigned char *out_buf,
 337                        long *pos,
 338                        void (*error)(char *x))
 339 {
 340         return __unzstd(buf, len, fill, flush, out_buf, 0, pos, error);
 341 }
 342 #else
 343 STATIC int INIT __decompress(unsigned char *buf, long len,
 344                              long (*fill)(void*, unsigned long),
 345                              long (*flush)(void*, unsigned long),
 346                              unsigned char *out_buf, long out_len,
 347                              long *pos,
 348                              void (*error)(char *x))
 349 {
 350         return __unzstd(buf, len, fill, flush, out_buf, out_len, pos, error);
 351 }
 352 #endif