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Diffstat (limited to 'lib/decompress_unzstd.c')
| -rw-r--r-- | lib/decompress_unzstd.c | 350 |
1 files changed, 350 insertions, 0 deletions
diff --git a/lib/decompress_unzstd.c b/lib/decompress_unzstd.c new file mode 100644 index 000000000..a512b99ae --- /dev/null +++ b/lib/decompress_unzstd.c @@ -0,0 +1,350 @@ +// SPDX-License-Identifier: GPL-2.0 + +/* + * 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 ZSTD with a 128 KB block size is calculated below. + * Note that the margin with ZSTD is bigger than with GZIP or XZ! + * + * 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 .zst file in case of a compressed kernel is as follows. + * Maximum sizes (as bytes) of the fields are in parenthesis. + * + * Frame Header: (18) + * Blocks: (N) + * Checksum: (4) + * + * The frame header and checksum overhead is at most 22 bytes. + * + * ZSTD stores the data in blocks. Each block has a header whose size is + * a 3 bytes. After the block header, there is up to 128 KB of payload. + * The maximum uncompressed size of the payload is 128 KB. The minimum + * uncompressed size of the payload is never less than the payload size + * (excluding the block header). + * + * 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 + * - 22 bytes for the .zst file format headers; + * - 3 bytes per every 128 KiB of uncompressed size (one block header per + * block); and + * - 128 KiB (biggest possible zstd block size) to make sure that the + * decompressor never overwrites anything from the block it is currently + * reading. + * + * We get the following formula: + * + * safety_margin = 22 + uncompressed_size * 3 / 131072 + 131072 + * <= 22 + (uncompressed_size >> 15) + 131072 + */ + +/* + * Preboot environments #include "path/to/decompress_unzstd.c". + * All of the source files we depend on must be #included. + * zstd's only source dependency is xxhash, which has no source + * dependencies. + * + * When UNZSTD_PREBOOT is defined we declare __decompress(), which is + * used for kernel decompression, instead of unzstd(). + * + * Define __DISABLE_EXPORTS in preboot environments to prevent symbols + * from xxhash and zstd from being exported by the EXPORT_SYMBOL macro. + */ +#ifdef STATIC +# define UNZSTD_PREBOOT +# include "xxhash.c" +# include "zstd/decompress_sources.h" +#endif + +#include <linux/decompress/mm.h> +#include <linux/kernel.h> +#include <linux/zstd.h> + +/* 128MB is the maximum window size supported by zstd. */ +#define ZSTD_WINDOWSIZE_MAX (1 << ZSTD_WINDOWLOG_MAX) +/* + * Size of the input and output buffers in multi-call mode. + * Pick a larger size because it isn't used during kernel decompression, + * since that is single pass, and we have to allocate a large buffer for + * zstd's window anyway. The larger size speeds up initramfs decompression. + */ +#define ZSTD_IOBUF_SIZE (1 << 17) + +static int INIT handle_zstd_error(size_t ret, void (*error)(char *x)) +{ + const zstd_error_code err = zstd_get_error_code(ret); + + if (!zstd_is_error(ret)) + return 0; + + /* + * zstd_get_error_name() cannot be used because error takes a char * + * not a const char * + */ + switch (err) { + case ZSTD_error_memory_allocation: + error("ZSTD decompressor ran out of memory"); + break; + case ZSTD_error_prefix_unknown: + error("Input is not in the ZSTD format (wrong magic bytes)"); + break; + case ZSTD_error_dstSize_tooSmall: + case ZSTD_error_corruption_detected: + case ZSTD_error_checksum_wrong: + error("ZSTD-compressed data is corrupt"); + break; + default: + error("ZSTD-compressed data is probably corrupt"); + break; + } + return -1; +} + +/* + * Handle the case where we have the entire input and output in one segment. + * We can allocate less memory (no circular buffer for the sliding window), + * and avoid some memcpy() calls. + */ +static int INIT decompress_single(const u8 *in_buf, long in_len, u8 *out_buf, + long out_len, long *in_pos, + void (*error)(char *x)) +{ + const size_t wksp_size = zstd_dctx_workspace_bound(); + void *wksp = large_malloc(wksp_size); + zstd_dctx *dctx = zstd_init_dctx(wksp, wksp_size); + int err; + size_t ret; + + if (dctx == NULL) { + error("Out of memory while allocating zstd_dctx"); + err = -1; + goto out; + } + /* + * Find out how large the frame actually is, there may be junk at + * the end of the frame that zstd_decompress_dctx() can't handle. + */ + ret = zstd_find_frame_compressed_size(in_buf, in_len); + err = handle_zstd_error(ret, error); + if (err) + goto out; + in_len = (long)ret; + + ret = zstd_decompress_dctx(dctx, out_buf, out_len, in_buf, in_len); + err = handle_zstd_error(ret, error); + if (err) + goto out; + + if (in_pos != NULL) + *in_pos = in_len; + + err = 0; +out: + if (wksp != NULL) + large_free(wksp); + return err; +} + +static int INIT __unzstd(unsigned char *in_buf, long in_len, + long (*fill)(void*, unsigned long), + long (*flush)(void*, unsigned long), + unsigned char *out_buf, long out_len, + long *in_pos, + void (*error)(char *x)) +{ + zstd_in_buffer in; + zstd_out_buffer out; + zstd_frame_header header; + void *in_allocated = NULL; + void *out_allocated = NULL; + void *wksp = NULL; + size_t wksp_size; + zstd_dstream *dstream; + int err; + size_t ret; + + /* + * ZSTD decompression code won't be happy if the buffer size is so big + * that its end address overflows. When the size is not provided, make + * it as big as possible without having the end address overflow. + */ + if (out_len == 0) + out_len = UINTPTR_MAX - (uintptr_t)out_buf; + + if (fill == NULL && flush == NULL) + /* + * We can decompress faster and with less memory when we have a + * single chunk. + */ + return decompress_single(in_buf, in_len, out_buf, out_len, + in_pos, error); + + /* + * If in_buf is not provided, we must be using fill(), so allocate + * a large enough buffer. If it is provided, it must be at least + * ZSTD_IOBUF_SIZE large. + */ + if (in_buf == NULL) { + in_allocated = large_malloc(ZSTD_IOBUF_SIZE); + if (in_allocated == NULL) { + error("Out of memory while allocating input buffer"); + err = -1; + goto out; + } + in_buf = in_allocated; + in_len = 0; + } + /* Read the first chunk, since we need to decode the frame header. */ + if (fill != NULL) + in_len = fill(in_buf, ZSTD_IOBUF_SIZE); + if (in_len < 0) { + error("ZSTD-compressed data is truncated"); + err = -1; + goto out; + } + /* Set the first non-empty input buffer. */ + in.src = in_buf; + in.pos = 0; + in.size = in_len; + /* Allocate the output buffer if we are using flush(). */ + if (flush != NULL) { + out_allocated = large_malloc(ZSTD_IOBUF_SIZE); + if (out_allocated == NULL) { + error("Out of memory while allocating output buffer"); + err = -1; + goto out; + } + out_buf = out_allocated; + out_len = ZSTD_IOBUF_SIZE; + } + /* Set the output buffer. */ + out.dst = out_buf; + out.pos = 0; + out.size = out_len; + + /* + * We need to know the window size to allocate the zstd_dstream. + * Since we are streaming, we need to allocate a buffer for the sliding + * window. The window size varies from 1 KB to ZSTD_WINDOWSIZE_MAX + * (8 MB), so it is important to use the actual value so as not to + * waste memory when it is smaller. + */ + ret = zstd_get_frame_header(&header, in.src, in.size); + err = handle_zstd_error(ret, error); + if (err) + goto out; + if (ret != 0) { + error("ZSTD-compressed data has an incomplete frame header"); + err = -1; + goto out; + } + if (header.windowSize > ZSTD_WINDOWSIZE_MAX) { + error("ZSTD-compressed data has too large a window size"); + err = -1; + goto out; + } + + /* + * Allocate the zstd_dstream now that we know how much memory is + * required. + */ + wksp_size = zstd_dstream_workspace_bound(header.windowSize); + wksp = large_malloc(wksp_size); + dstream = zstd_init_dstream(header.windowSize, wksp, wksp_size); + if (dstream == NULL) { + error("Out of memory while allocating ZSTD_DStream"); + err = -1; + goto out; + } + + /* + * Decompression loop: + * Read more data if necessary (error if no more data can be read). + * Call the decompression function, which returns 0 when finished. + * Flush any data produced if using flush(). + */ + if (in_pos != NULL) + *in_pos = 0; + do { + /* + * If we need to reload data, either we have fill() and can + * try to get more data, or we don't and the input is truncated. + */ + if (in.pos == in.size) { + if (in_pos != NULL) + *in_pos += in.pos; + in_len = fill ? fill(in_buf, ZSTD_IOBUF_SIZE) : -1; + if (in_len < 0) { + error("ZSTD-compressed data is truncated"); + err = -1; + goto out; + } + in.pos = 0; + in.size = in_len; + } + /* Returns zero when the frame is complete. */ + ret = zstd_decompress_stream(dstream, &out, &in); + err = handle_zstd_error(ret, error); + if (err) + goto out; + /* Flush all of the data produced if using flush(). */ + if (flush != NULL && out.pos > 0) { + if (out.pos != flush(out.dst, out.pos)) { + error("Failed to flush()"); + err = -1; + goto out; + } + out.pos = 0; + } + } while (ret != 0); + + if (in_pos != NULL) + *in_pos += in.pos; + + err = 0; +out: + if (in_allocated != NULL) + large_free(in_allocated); + if (out_allocated != NULL) + large_free(out_allocated); + if (wksp != NULL) + large_free(wksp); + return err; +} + +#ifndef UNZSTD_PREBOOT +STATIC int INIT unzstd(unsigned char *buf, long len, + long (*fill)(void*, unsigned long), + long (*flush)(void*, unsigned long), + unsigned char *out_buf, + long *pos, + void (*error)(char *x)) +{ + return __unzstd(buf, len, fill, flush, out_buf, 0, pos, error); +} +#else +STATIC int INIT __decompress(unsigned char *buf, long len, + long (*fill)(void*, unsigned long), + long (*flush)(void*, unsigned long), + unsigned char *out_buf, long out_len, + long *pos, + void (*error)(char *x)) +{ + return __unzstd(buf, len, fill, flush, out_buf, out_len, pos, error); +} +#endif |