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         /*
 182          * ZSTD decompression code won't be happy if the buffer size is so big
 183          * that its end address overflows. When the size is not provided, make
 184          * it as big as possible without having the end address overflow.
 185          */
 186         if (out_len == 0)
 187                 out_len = UINTPTR_MAX - (uintptr_t)out_buf;
 188
 189         if (fill == NULL && flush == NULL)
 190                 /*
 191                  * We can decompress faster and with less memory when we have a
 192                  * single chunk.
 193                  */
 194                 return decompress_single(in_buf, in_len, out_buf, out_len,
 195                                          in_pos, error);
 196
 197         /*
 198          * If in_buf is not provided, we must be using fill(), so allocate
 199          * a large enough buffer. If it is provided, it must be at least
 200          * ZSTD_IOBUF_SIZE large.
 201          */
 202         if (in_buf == NULL) {
 203                 in_allocated = large_malloc(ZSTD_IOBUF_SIZE);
 204                 if (in_allocated == NULL) {
 205                         error("Out of memory while allocating input buffer");
 206                         err = -1;
 207                         goto out;
 208                 }
 209                 in_buf = in_allocated;
 210                 in_len = 0;
 211         }
 212         /* Read the first chunk, since we need to decode the frame header. */
 213         if (fill != NULL)
 214                 in_len = fill(in_buf, ZSTD_IOBUF_SIZE);
 215         if (in_len < 0) {
 216                 error("ZSTD-compressed data is truncated");
 217                 err = -1;
 218                 goto out;
 219         }
 220         /* Set the first non-empty input buffer. */
 221         in.src = in_buf;
 222         in.pos = 0;
 223         in.size = in_len;
 224         /* Allocate the output buffer if we are using flush(). */
 225         if (flush != NULL) {
 226                 out_allocated = large_malloc(ZSTD_IOBUF_SIZE);
 227                 if (out_allocated == NULL) {
 228                         error("Out of memory while allocating output buffer");
 229                         err = -1;
 230                         goto out;
 231                 }
 232                 out_buf = out_allocated;
 233                 out_len = ZSTD_IOBUF_SIZE;
 234         }
 235         /* Set the output buffer. */
 236         out.dst = out_buf;
 237         out.pos = 0;
 238         out.size = out_len;
 239
 240         /*
 241          * We need to know the window size to allocate the ZSTD_DStream.
 242          * Since we are streaming, we need to allocate a buffer for the sliding
 243          * window. The window size varies from 1 KB to ZSTD_WINDOWSIZE_MAX
 244          * (8 MB), so it is important to use the actual value so as not to
 245          * waste memory when it is smaller.
 246          */
 247         ret = ZSTD_getFrameParams(&params, in.src, in.size);
 248         err = handle_zstd_error(ret, error);
 249         if (err)
 250                 goto out;
 251         if (ret != 0) {
 252                 error("ZSTD-compressed data has an incomplete frame header");
 253                 err = -1;
 254                 goto out;
 255         }
 256         if (params.windowSize > ZSTD_WINDOWSIZE_MAX) {
 257                 error("ZSTD-compressed data has too large a window size");
 258                 err = -1;
 259                 goto out;
 260         }
 261
 262         /*
 263          * Allocate the ZSTD_DStream now that we know how much memory is
 264          * required.
 265          */
 266         wksp_size = ZSTD_DStreamWorkspaceBound(params.windowSize);
 267         wksp = large_malloc(wksp_size);
 268         dstream = ZSTD_initDStream(params.windowSize, wksp, wksp_size);
 269         if (dstream == NULL) {
 270                 error("Out of memory while allocating ZSTD_DStream");
 271                 err = -1;
 272                 goto out;
 273         }
 274
 275         /*
 276          * Decompression loop:
 277          * Read more data if necessary (error if no more data can be read).
 278          * Call the decompression function, which returns 0 when finished.
 279          * Flush any data produced if using flush().
 280          */
 281         if (in_pos != NULL)
 282                 *in_pos = 0;
 283         do {
 284                 /*
 285                  * If we need to reload data, either we have fill() and can
 286                  * try to get more data, or we don't and the input is truncated.
 287                  */
 288                 if (in.pos == in.size) {
 289                         if (in_pos != NULL)
 290                                 *in_pos += in.pos;
 291                         in_len = fill ? fill(in_buf, ZSTD_IOBUF_SIZE) : -1;
 292                         if (in_len < 0) {
 293                                 error("ZSTD-compressed data is truncated");
 294                                 err = -1;
 295                                 goto out;
 296                         }
 297                         in.pos = 0;
 298                         in.size = in_len;
 299                 }
 300                 /* Returns zero when the frame is complete. */
 301                 ret = ZSTD_decompressStream(dstream, &out, &in);
 302                 err = handle_zstd_error(ret, error);
 303                 if (err)
 304                         goto out;
 305                 /* Flush all of the data produced if using flush(). */
 306                 if (flush != NULL && out.pos > 0) {
 307                         if (out.pos != flush(out.dst, out.pos)) {
 308                                 error("Failed to flush()");
 309                                 err = -1;
 310                                 goto out;
 311                         }
 312                         out.pos = 0;
 313                 }
 314         } while (ret != 0);
 315
 316         if (in_pos != NULL)
 317                 *in_pos += in.pos;
 318
 319         err = 0;
 320 out:
 321         if (in_allocated != NULL)
 322                 large_free(in_allocated);
 323         if (out_allocated != NULL)
 324                 large_free(out_allocated);
 325         if (wksp != NULL)
 326                 large_free(wksp);
 327         return err;
 328 }
 329
 330 #ifndef UNZSTD_PREBOOT
 331 STATIC int INIT unzstd(unsigned char *buf, long len,
 332                        long (*fill)(void*, unsigned long),
 333                        long (*flush)(void*, unsigned long),
 334                        unsigned char *out_buf,
 335                        long *pos,
 336                        void (*error)(char *x))
 337 {
 338         return __unzstd(buf, len, fill, flush, out_buf, 0, pos, error);
 339 }
 340 #else
 341 STATIC int INIT __decompress(unsigned char *buf, long len,
 342                              long (*fill)(void*, unsigned long),
 343                              long (*flush)(void*, unsigned long),
 344                              unsigned char *out_buf, long out_len,
 345                              long *pos,
 346                              void (*error)(char *x))
 347 {
 348         return __unzstd(buf, len, fill, flush, out_buf, out_len, pos, error);
 349 }
 350 #endif