xref: /openbmc/linux/lib/decompress_unxz.c (revision 00444448)
1 /*
2  * Wrapper for decompressing XZ-compressed kernel, initramfs, and initrd
3  *
4  * Author: Lasse Collin <lasse.collin@tukaani.org>
5  *
6  * This file has been put into the public domain.
7  * You can do whatever you want with this file.
8  */
9 
10 /*
11  * Important notes about in-place decompression
12  *
13  * At least on x86, the kernel is decompressed in place: the compressed data
14  * is placed to the end of the output buffer, and the decompressor overwrites
15  * most of the compressed data. There must be enough safety margin to
16  * guarantee that the write position is always behind the read position.
17  *
18  * The safety margin for XZ with LZMA2 or BCJ+LZMA2 is calculated below.
19  * Note that the margin with XZ is bigger than with Deflate (gzip)!
20  *
21  * The worst case for in-place decompression is that the beginning of
22  * the file is compressed extremely well, and the rest of the file is
23  * incompressible. Thus, we must look for worst-case expansion when the
24  * compressor is encoding incompressible data.
25  *
26  * The structure of the .xz file in case of a compressed kernel is as follows.
27  * Sizes (as bytes) of the fields are in parenthesis.
28  *
29  *    Stream Header (12)
30  *    Block Header:
31  *      Block Header (8-12)
32  *      Compressed Data (N)
33  *      Block Padding (0-3)
34  *      CRC32 (4)
35  *    Index (8-20)
36  *    Stream Footer (12)
37  *
38  * Normally there is exactly one Block, but let's assume that there are
39  * 2-4 Blocks just in case. Because Stream Header and also Block Header
40  * of the first Block don't make the decompressor produce any uncompressed
41  * data, we can ignore them from our calculations. Block Headers of possible
42  * additional Blocks have to be taken into account still. With these
43  * assumptions, it is safe to assume that the total header overhead is
44  * less than 128 bytes.
45  *
46  * Compressed Data contains LZMA2 or BCJ+LZMA2 encoded data. Since BCJ
47  * doesn't change the size of the data, it is enough to calculate the
48  * safety margin for LZMA2.
49  *
50  * LZMA2 stores the data in chunks. Each chunk has a header whose size is
51  * a maximum of 6 bytes, but to get round 2^n numbers, let's assume that
52  * the maximum chunk header size is 8 bytes. After the chunk header, there
53  * may be up to 64 KiB of actual payload in the chunk. Often the payload is
54  * quite a bit smaller though; to be safe, let's assume that an average
55  * chunk has only 32 KiB of payload.
56  *
57  * The maximum uncompressed size of the payload is 2 MiB. The minimum
58  * uncompressed size of the payload is in practice never less than the
59  * payload size itself. The LZMA2 format would allow uncompressed size
60  * to be less than the payload size, but no sane compressor creates such
61  * files. LZMA2 supports storing incompressible data in uncompressed form,
62  * so there's never a need to create payloads whose uncompressed size is
63  * smaller than the compressed size.
64  *
65  * The assumption, that the uncompressed size of the payload is never
66  * smaller than the payload itself, is valid only when talking about
67  * the payload as a whole. It is possible that the payload has parts where
68  * the decompressor consumes more input than it produces output. Calculating
69  * the worst case for this would be tricky. Instead of trying to do that,
70  * let's simply make sure that the decompressor never overwrites any bytes
71  * of the payload which it is currently reading.
72  *
73  * Now we have enough information to calculate the safety margin. We need
74  *   - 128 bytes for the .xz file format headers;
75  *   - 8 bytes per every 32 KiB of uncompressed size (one LZMA2 chunk header
76  *     per chunk, each chunk having average payload size of 32 KiB); and
77  *   - 64 KiB (biggest possible LZMA2 chunk payload size) to make sure that
78  *     the decompressor never overwrites anything from the LZMA2 chunk
79  *     payload it is currently reading.
80  *
81  * We get the following formula:
82  *
83  *    safety_margin = 128 + uncompressed_size * 8 / 32768 + 65536
84  *                  = 128 + (uncompressed_size >> 12) + 65536
85  *
86  * For comparison, according to arch/x86/boot/compressed/misc.c, the
87  * equivalent formula for Deflate is this:
88  *
89  *    safety_margin = 18 + (uncompressed_size >> 12) + 32768
90  *
91  * Thus, when updating Deflate-only in-place kernel decompressor to
92  * support XZ, the fixed overhead has to be increased from 18+32768 bytes
93  * to 128+65536 bytes.
94  */
95 
96 /*
97  * STATIC is defined to "static" if we are being built for kernel
98  * decompression (pre-boot code). <linux/decompress/mm.h> will define
99  * STATIC to empty if it wasn't already defined. Since we will need to
100  * know later if we are being used for kernel decompression, we define
101  * XZ_PREBOOT here.
102  */
103 #ifdef STATIC
104 #	define XZ_PREBOOT
105 #else
106 #include <linux/decompress/unxz.h>
107 #endif
108 #ifdef __KERNEL__
109 #	include <linux/decompress/mm.h>
110 #endif
111 #define XZ_EXTERN STATIC
112 
113 #ifndef XZ_PREBOOT
114 #	include <linux/slab.h>
115 #	include <linux/xz.h>
116 #else
117 /*
118  * Use the internal CRC32 code instead of kernel's CRC32 module, which
119  * is not available in early phase of booting.
120  */
121 #define XZ_INTERNAL_CRC32 1
122 
123 /*
124  * For boot time use, we enable only the BCJ filter of the current
125  * architecture or none if no BCJ filter is available for the architecture.
126  */
127 #ifdef CONFIG_X86
128 #	define XZ_DEC_X86
129 #endif
130 #ifdef CONFIG_PPC
131 #	define XZ_DEC_POWERPC
132 #endif
133 #ifdef CONFIG_ARM
134 #	define XZ_DEC_ARM
135 #endif
136 #ifdef CONFIG_IA64
137 #	define XZ_DEC_IA64
138 #endif
139 #ifdef CONFIG_SPARC
140 #	define XZ_DEC_SPARC
141 #endif
142 
143 /*
144  * This will get the basic headers so that memeq() and others
145  * can be defined.
146  */
147 #include "xz/xz_private.h"
148 
149 /*
150  * Replace the normal allocation functions with the versions from
151  * <linux/decompress/mm.h>. vfree() needs to support vfree(NULL)
152  * when XZ_DYNALLOC is used, but the pre-boot free() doesn't support it.
153  * Workaround it here because the other decompressors don't need it.
154  */
155 #undef kmalloc
156 #undef kfree
157 #undef vmalloc
158 #undef vfree
159 #define kmalloc(size, flags) malloc(size)
160 #define kfree(ptr) free(ptr)
161 #define vmalloc(size) malloc(size)
162 #define vfree(ptr) do { if (ptr != NULL) free(ptr); } while (0)
163 
164 /*
165  * FIXME: Not all basic memory functions are provided in architecture-specific
166  * files (yet). We define our own versions here for now, but this should be
167  * only a temporary solution.
168  *
169  * memeq and memzero are not used much and any remotely sane implementation
170  * is fast enough. memcpy/memmove speed matters in multi-call mode, but
171  * the kernel image is decompressed in single-call mode, in which only
172  * memmove speed can matter and only if there is a lot of incompressible data
173  * (LZMA2 stores incompressible chunks in uncompressed form). Thus, the
174  * functions below should just be kept small; it's probably not worth
175  * optimizing for speed.
176  */
177 
178 #ifndef memeq
memeq(const void * a,const void * b,size_t size)179 static bool memeq(const void *a, const void *b, size_t size)
180 {
181 	const uint8_t *x = a;
182 	const uint8_t *y = b;
183 	size_t i;
184 
185 	for (i = 0; i < size; ++i)
186 		if (x[i] != y[i])
187 			return false;
188 
189 	return true;
190 }
191 #endif
192 
193 #ifndef memzero
memzero(void * buf,size_t size)194 static void memzero(void *buf, size_t size)
195 {
196 	uint8_t *b = buf;
197 	uint8_t *e = b + size;
198 
199 	while (b != e)
200 		*b++ = '\0';
201 }
202 #endif
203 
204 #ifndef memmove
205 /* Not static to avoid a conflict with the prototype in the Linux headers. */
memmove(void * dest,const void * src,size_t size)206 void *memmove(void *dest, const void *src, size_t size)
207 {
208 	uint8_t *d = dest;
209 	const uint8_t *s = src;
210 	size_t i;
211 
212 	if (d < s) {
213 		for (i = 0; i < size; ++i)
214 			d[i] = s[i];
215 	} else if (d > s) {
216 		i = size;
217 		while (i-- > 0)
218 			d[i] = s[i];
219 	}
220 
221 	return dest;
222 }
223 #endif
224 
225 /*
226  * Since we need memmove anyway, would use it as memcpy too.
227  * Commented out for now to avoid breaking things.
228  */
229 /*
230 #ifndef memcpy
231 #	define memcpy memmove
232 #endif
233 */
234 
235 #include "xz/xz_crc32.c"
236 #include "xz/xz_dec_stream.c"
237 #include "xz/xz_dec_lzma2.c"
238 #include "xz/xz_dec_bcj.c"
239 
240 #endif /* XZ_PREBOOT */
241 
242 /* Size of the input and output buffers in multi-call mode */
243 #define XZ_IOBUF_SIZE 4096
244 
245 /*
246  * This function implements the API defined in <linux/decompress/generic.h>.
247  *
248  * This wrapper will automatically choose single-call or multi-call mode
249  * of the native XZ decoder API. The single-call mode can be used only when
250  * both input and output buffers are available as a single chunk, i.e. when
251  * fill() and flush() won't be used.
252  */
unxz(unsigned char * in,long in_size,long (* fill)(void * dest,unsigned long size),long (* flush)(void * src,unsigned long size),unsigned char * out,long * in_used,void (* error)(char * x))253 STATIC int INIT unxz(unsigned char *in, long in_size,
254 		     long (*fill)(void *dest, unsigned long size),
255 		     long (*flush)(void *src, unsigned long size),
256 		     unsigned char *out, long *in_used,
257 		     void (*error)(char *x))
258 {
259 	struct xz_buf b;
260 	struct xz_dec *s;
261 	enum xz_ret ret;
262 	bool must_free_in = false;
263 
264 #if XZ_INTERNAL_CRC32
265 	xz_crc32_init();
266 #endif
267 
268 	if (in_used != NULL)
269 		*in_used = 0;
270 
271 	if (fill == NULL && flush == NULL)
272 		s = xz_dec_init(XZ_SINGLE, 0);
273 	else
274 		s = xz_dec_init(XZ_DYNALLOC, (uint32_t)-1);
275 
276 	if (s == NULL)
277 		goto error_alloc_state;
278 
279 	if (flush == NULL) {
280 		b.out = out;
281 		b.out_size = (size_t)-1;
282 	} else {
283 		b.out_size = XZ_IOBUF_SIZE;
284 		b.out = malloc(XZ_IOBUF_SIZE);
285 		if (b.out == NULL)
286 			goto error_alloc_out;
287 	}
288 
289 	if (in == NULL) {
290 		must_free_in = true;
291 		in = malloc(XZ_IOBUF_SIZE);
292 		if (in == NULL)
293 			goto error_alloc_in;
294 	}
295 
296 	b.in = in;
297 	b.in_pos = 0;
298 	b.in_size = in_size;
299 	b.out_pos = 0;
300 
301 	if (fill == NULL && flush == NULL) {
302 		ret = xz_dec_run(s, &b);
303 	} else {
304 		do {
305 			if (b.in_pos == b.in_size && fill != NULL) {
306 				if (in_used != NULL)
307 					*in_used += b.in_pos;
308 
309 				b.in_pos = 0;
310 
311 				in_size = fill(in, XZ_IOBUF_SIZE);
312 				if (in_size < 0) {
313 					/*
314 					 * This isn't an optimal error code
315 					 * but it probably isn't worth making
316 					 * a new one either.
317 					 */
318 					ret = XZ_BUF_ERROR;
319 					break;
320 				}
321 
322 				b.in_size = in_size;
323 			}
324 
325 			ret = xz_dec_run(s, &b);
326 
327 			if (flush != NULL && (b.out_pos == b.out_size
328 					|| (ret != XZ_OK && b.out_pos > 0))) {
329 				/*
330 				 * Setting ret here may hide an error
331 				 * returned by xz_dec_run(), but probably
332 				 * it's not too bad.
333 				 */
334 				if (flush(b.out, b.out_pos) != (long)b.out_pos)
335 					ret = XZ_BUF_ERROR;
336 
337 				b.out_pos = 0;
338 			}
339 		} while (ret == XZ_OK);
340 
341 		if (must_free_in)
342 			free(in);
343 
344 		if (flush != NULL)
345 			free(b.out);
346 	}
347 
348 	if (in_used != NULL)
349 		*in_used += b.in_pos;
350 
351 	xz_dec_end(s);
352 
353 	switch (ret) {
354 	case XZ_STREAM_END:
355 		return 0;
356 
357 	case XZ_MEM_ERROR:
358 		/* This can occur only in multi-call mode. */
359 		error("XZ decompressor ran out of memory");
360 		break;
361 
362 	case XZ_FORMAT_ERROR:
363 		error("Input is not in the XZ format (wrong magic bytes)");
364 		break;
365 
366 	case XZ_OPTIONS_ERROR:
367 		error("Input was encoded with settings that are not "
368 				"supported by this XZ decoder");
369 		break;
370 
371 	case XZ_DATA_ERROR:
372 	case XZ_BUF_ERROR:
373 		error("XZ-compressed data is corrupt");
374 		break;
375 
376 	default:
377 		error("Bug in the XZ decompressor");
378 		break;
379 	}
380 
381 	return -1;
382 
383 error_alloc_in:
384 	if (flush != NULL)
385 		free(b.out);
386 
387 error_alloc_out:
388 	xz_dec_end(s);
389 
390 error_alloc_state:
391 	error("XZ decompressor ran out of memory");
392 	return -1;
393 }
394 
395 /*
396  * This macro is used by architecture-specific files to decompress
397  * the kernel image.
398  */
399 #ifdef XZ_PREBOOT
__decompress(unsigned char * buf,long len,long (* fill)(void *,unsigned long),long (* flush)(void *,unsigned long),unsigned char * out_buf,long olen,long * pos,void (* error)(char * x))400 STATIC int INIT __decompress(unsigned char *buf, long len,
401 			   long (*fill)(void*, unsigned long),
402 			   long (*flush)(void*, unsigned long),
403 			   unsigned char *out_buf, long olen,
404 			   long *pos,
405 			   void (*error)(char *x))
406 {
407 	return unxz(buf, len, fill, flush, out_buf, pos, error);
408 }
409 #endif
410