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Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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/* * 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"#else#include <linux/decompress/unzstd.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_PREBOOTSTATIC 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);}#elseSTATIC 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