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 compresed 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 dependeny 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/entropy_common.c"
  72 # include "zstd/fse_decompress.c"
  73 # include "zstd/huf_decompress.c"
  74 # include "zstd/zstd_common.c"
  75 # include "zstd/decompress.c"
  76 #endif
  77
  78 #include <linux/decompress/mm.h>
  79 #include <linux/kernel.h>
  80 #include <linux/zstd.h>
  81
  82 /* 128MB is the maximum window size supported by zstd. */
  83 #define ZSTD_WINDOWSIZE_MAX     (1 << ZSTD_WINDOWLOG_MAX)
  84 /*
  85  * Size of the input and output buffers in multi-call mode.
  86  * Pick a larger size because it isn't used during kernel decompression,
  87  * since that is single pass, and we have to allocate a large buffer for
  88  * zstd's window anyway. The larger size speeds up initramfs decompression.
  89  */
  90 #define ZSTD_IOBUF_SIZE         (1 << 17)
  91
  92 static int INIT handle_zstd_error(size_t ret, void (*error)(char *x))
  93 {
  94         const int err = ZSTD_getErrorCode(ret);
  95
  96         if (!ZSTD_isError(ret))
  97                 return 0;
  98
  99         switch (err) {
 100         case ZSTD_error_memory_allocation:
 101                 error("ZSTD decompressor ran out of memory");
 102                 break;
 103         case ZSTD_error_prefix_unknown:
 104                 error("Input is not in the ZSTD format (wrong magic bytes)");
 105                 break;
 106         case ZSTD_error_dstSize_tooSmall:
 107         case ZSTD_error_corruption_detected:
 108         case ZSTD_error_checksum_wrong:
 109                 error("ZSTD-compressed data is corrupt");
 110                 break;
 111         default:
 112                 error("ZSTD-compressed data is probably corrupt");
 113                 break;
 114         }
 115         return -1;
 116 }
 117
 118 /*
 119  * Handle the case where we have the entire input and output in one segment.
 120  * We can allocate less memory (no circular buffer for the sliding window),
 121  * and avoid some memcpy() calls.
 122  */
 123 static int INIT decompress_single(const u8 *in_buf, long in_len, u8 *out_buf,
 124                                   long out_len, long *in_pos,
 125                                   void (*error)(char *x))
 126 {
 127         const size_t wksp_size = ZSTD_DCtxWorkspaceBound();
 128         void *wksp = large_malloc(wksp_size);
 129         ZSTD_DCtx *dctx = ZSTD_initDCtx(wksp, wksp_size);
 130         int err;
 131         size_t ret;
 132
 133         if (dctx == NULL) {
 134                 error("Out of memory while allocating ZSTD_DCtx");
 135                 err = -1;
 136                 goto out;
 137         }
 138         /*
 139          * Find out how large the frame actually is, there may be junk at
 140          * the end of the frame that ZSTD_decompressDCtx() can't handle.
 141          */
 142         ret = ZSTD_findFrameCompressedSize(in_buf, in_len);
 143         err = handle_zstd_error(ret, error);
 144         if (err)
 145                 goto out;
 146         in_len = (long)ret;
 147
 148         ret = ZSTD_decompressDCtx(dctx, out_buf, out_len, in_buf, in_len);
 149         err = handle_zstd_error(ret, error);
 150         if (err)
 151                 goto out;
 152
 153         if (in_pos != NULL)
 154                 *in_pos = in_len;
 155
 156         err = 0;
 157 out:
 158         if (wksp != NULL)
 159                 large_free(wksp);
 160         return err;
 161 }
 162
 163 static int INIT __unzstd(unsigned char *in_buf, long in_len,
 164                          long (*fill)(void*, unsigned long),
 165                          long (*flush)(void*, unsigned long),
 166                          unsigned char *out_buf, long out_len,
 167                          long *in_pos,
 168                          void (*error)(char *x))
 169 {
 170         ZSTD_inBuffer in;
 171         ZSTD_outBuffer out;
 172         ZSTD_frameParams params;
 173         void *in_allocated = NULL;
 174         void *out_allocated = NULL;
 175         void *wksp = NULL;
 176         size_t wksp_size;
 177         ZSTD_DStream *dstream;
 178         int err;
 179         size_t ret;
 180
 181         if (out_len == 0)
 182                 out_len = LONG_MAX; /* no limit */
 183
 184         if (fill == NULL && flush == NULL)
 185                 /*
 186                  * We can decompress faster and with less memory when we have a
 187                  * single chunk.
 188                  */
 189                 return decompress_single(in_buf, in_len, out_buf, out_len,
 190                                          in_pos, error);
 191
 192         /*
 193          * If in_buf is not provided, we must be using fill(), so allocate
 194          * a large enough buffer. If it is provided, it must be at least
 195          * ZSTD_IOBUF_SIZE large.
 196          */
 197         if (in_buf == NULL) {
 198                 in_allocated = large_malloc(ZSTD_IOBUF_SIZE);
 199                 if (in_allocated == NULL) {
 200                         error("Out of memory while allocating input buffer");
 201                         err = -1;
 202                         goto out;
 203                 }
 204                 in_buf = in_allocated;
 205                 in_len = 0;
 206         }
 207         /* Read the first chunk, since we need to decode the frame header. */
 208         if (fill != NULL)
 209                 in_len = fill(in_buf, ZSTD_IOBUF_SIZE);
 210         if (in_len < 0) {
 211                 error("ZSTD-compressed data is truncated");
 212                 err = -1;
 213                 goto out;
 214         }
 215         /* Set the first non-empty input buffer. */
 216         in.src = in_buf;
 217         in.pos = 0;
 218         in.size = in_len;
 219         /* Allocate the output buffer if we are using flush(). */
 220         if (flush != NULL) {
 221                 out_allocated = large_malloc(ZSTD_IOBUF_SIZE);
 222                 if (out_allocated == NULL) {
 223                         error("Out of memory while allocating output buffer");
 224                         err = -1;
 225                         goto out;
 226                 }
 227                 out_buf = out_allocated;
 228                 out_len = ZSTD_IOBUF_SIZE;
 229         }
 230         /* Set the output buffer. */
 231         out.dst = out_buf;
 232         out.pos = 0;
 233         out.size = out_len;
 234
 235         /*
 236          * We need to know the window size to allocate the ZSTD_DStream.
 237          * Since we are streaming, we need to allocate a buffer for the sliding
 238          * window. The window size varies from 1 KB to ZSTD_WINDOWSIZE_MAX
 239          * (8 MB), so it is important to use the actual value so as not to
 240          * waste memory when it is smaller.
 241          */
 242         ret = ZSTD_getFrameParams(&params, in.src, in.size);
 243         err = handle_zstd_error(ret, error);
 244         if (err)
 245                 goto out;
 246         if (ret != 0) {
 247                 error("ZSTD-compressed data has an incomplete frame header");
 248                 err = -1;
 249                 goto out;
 250         }
 251         if (params.windowSize > ZSTD_WINDOWSIZE_MAX) {
 252                 error("ZSTD-compressed data has too large a window size");
 253                 err = -1;
 254                 goto out;
 255         }
 256
 257         /*
 258          * Allocate the ZSTD_DStream now that we know how much memory is
 259          * required.
 260          */
 261         wksp_size = ZSTD_DStreamWorkspaceBound(params.windowSize);
 262         wksp = large_malloc(wksp_size);
 263         dstream = ZSTD_initDStream(params.windowSize, wksp, wksp_size);
 264         if (dstream == NULL) {
 265                 error("Out of memory while allocating ZSTD_DStream");
 266                 err = -1;
 267                 goto out;
 268         }
 269
 270         /*
 271          * Decompression loop:
 272          * Read more data if necessary (error if no more data can be read).
 273          * Call the decompression function, which returns 0 when finished.
 274          * Flush any data produced if using flush().
 275          */
 276         if (in_pos != NULL)
 277                 *in_pos = 0;
 278         do {
 279                 /*
 280                  * If we need to reload data, either we have fill() and can
 281                  * try to get more data, or we don't and the input is truncated.
 282                  */
 283                 if (in.pos == in.size) {
 284                         if (in_pos != NULL)
 285                                 *in_pos += in.pos;
 286                         in_len = fill ? fill(in_buf, ZSTD_IOBUF_SIZE) : -1;
 287                         if (in_len < 0) {
 288                                 error("ZSTD-compressed data is truncated");
 289                                 err = -1;
 290                                 goto out;
 291                         }
 292                         in.pos = 0;
 293                         in.size = in_len;
 294                 }
 295                 /* Returns zero when the frame is complete. */
 296                 ret = ZSTD_decompressStream(dstream, &out, &in);
 297                 err = handle_zstd_error(ret, error);
 298                 if (err)
 299                         goto out;
 300                 /* Flush all of the data produced if using flush(). */
 301                 if (flush != NULL && out.pos > 0) {
 302                         if (out.pos != flush(out.dst, out.pos)) {
 303                                 error("Failed to flush()");
 304                                 err = -1;
 305                                 goto out;
 306                         }
 307                         out.pos = 0;
 308                 }
 309         } while (ret != 0);
 310
 311         if (in_pos != NULL)
 312                 *in_pos += in.pos;
 313
 314         err = 0;
 315 out:
 316         if (in_allocated != NULL)
 317                 large_free(in_allocated);
 318         if (out_allocated != NULL)
 319                 large_free(out_allocated);
 320         if (wksp != NULL)
 321                 large_free(wksp);
 322         return err;
 323 }
 324
 325 #ifndef UNZSTD_PREBOOT
 326 STATIC int INIT unzstd(unsigned char *buf, long len,
 327                        long (*fill)(void*, unsigned long),
 328                        long (*flush)(void*, unsigned long),
 329                        unsigned char *out_buf,
 330                        long *pos,
 331                        void (*error)(char *x))
 332 {
 333         return __unzstd(buf, len, fill, flush, out_buf, 0, pos, error);
 334 }
 335 #else
 336 STATIC int INIT __decompress(unsigned char *buf, long len,
 337                              long (*fill)(void*, unsigned long),
 338                              long (*flush)(void*, unsigned long),
 339                              unsigned char *out_buf, long out_len,
 340                              long *pos,
 341                              void (*error)(char *x))
 342 {
 343         return __unzstd(buf, len, fill, flush, out_buf, out_len, pos, error);
 344 }
 345 #endif