aboutsummaryrefslogtreecommitdiffstats
path: root/lib/decompress_unzstd.c
blob: 790abc472f5bea5ba3186aa199b7ac55ace5edce (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
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 compresed 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 dependeny 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/entropy_common.c"
# include "zstd/fse_decompress.c"
# include "zstd/huf_decompress.c"
# include "zstd/zstd_common.c"
# include "zstd/decompress.c"
#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 int err = ZSTD_getErrorCode(ret);

	if (!ZSTD_isError(ret))
		return 0;

	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_DCtxWorkspaceBound();
	void *wksp = large_malloc(wksp_size);
	ZSTD_DCtx *dctx = ZSTD_initDCtx(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_decompressDCtx() can't handle.
	 */
	ret = ZSTD_findFrameCompressedSize(in_buf, in_len);
	err = handle_zstd_error(ret, error);
	if (err)
		goto out;
	in_len = (long)ret;

	ret = ZSTD_decompressDCtx(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_inBuffer in;
	ZSTD_outBuffer out;
	ZSTD_frameParams params;
	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_getFrameParams(&params, 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 (params.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_DStreamWorkspaceBound(params.windowSize);
	wksp = large_malloc(wksp_size);
	dstream = ZSTD_initDStream(params.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_decompressStream(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

Privacy Policy