vdso2c.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * vdso2c - A vdso image preparation tool
  4 * Copyright (c) 2014 Andy Lutomirski and others
  5 *
  6 * vdso2c requires stripped and unstripped input.  It would be trivial
  7 * to fully strip the input in here, but, for reasons described below,
  8 * we need to write a section table.  Doing this is more or less
  9 * equivalent to dropping all non-allocatable sections, but it's
 10 * easier to let objcopy handle that instead of doing it ourselves.
 11 * If we ever need to do something fancier than what objcopy provides,
 12 * it would be straightforward to add here.
 13 *
 14 * We're keep a section table for a few reasons:
 15 *
 16 * The Go runtime had a couple of bugs: it would read the section
 17 * table to try to figure out how many dynamic symbols there were (it
 18 * shouldn't have looked at the section table at all) and, if there
 19 * were no SHT_SYNDYM section table entry, it would use an
 20 * uninitialized value for the number of symbols.  An empty DYNSYM
 21 * table would work, but I see no reason not to write a valid one (and
 22 * keep full performance for old Go programs).  This hack is only
 23 * needed on x86_64.
 24 *
 25 * The bug was introduced on 2012-08-31 by:
 26 * https://code.google.com/p/go/source/detail?r=56ea40aac72b
 27 * and was fixed on 2014-06-13 by:
 28 * https://code.google.com/p/go/source/detail?r=fc1cd5e12595
 29 *
 30 * Binutils has issues debugging the vDSO: it reads the section table to
 31 * find SHT_NOTE; it won't look at PT_NOTE for the in-memory vDSO, which
 32 * would break build-id if we removed the section table.  Binutils
 33 * also requires that shstrndx != 0.  See:
 34 * https://sourceware.org/bugzilla/show_bug.cgi?id=17064
 35 *
 36 * elfutils might not look for PT_NOTE if there is a section table at
 37 * all.  I don't know whether this matters for any practical purpose.
 38 *
 39 * For simplicity, rather than hacking up a partial section table, we
 40 * just write a mostly complete one.  We omit non-dynamic symbols,
 41 * though, since they're rather large.
 42 *
 43 * Once binutils gets fixed, we might be able to drop this for all but
 44 * the 64-bit vdso, since build-id only works in kernel RPMs, and
 45 * systems that update to new enough kernel RPMs will likely update
 46 * binutils in sync.  build-id has never worked for home-built kernel
 47 * RPMs without manual symlinking, and I suspect that no one ever does
 48 * that.
 49 */
 50
 51#include <inttypes.h>
 52#include <stdint.h>
 53#include <unistd.h>
 54#include <stdarg.h>
 55#include <stdlib.h>
 56#include <stdio.h>
 57#include <string.h>
 58#include <fcntl.h>
 59#include <err.h>
 60
 61#include <sys/mman.h>
 62#include <sys/types.h>
 63
 64#include <tools/le_byteshift.h>
 65
 66#include <linux/elf.h>
 67#include <linux/types.h>
 68#include <linux/kernel.h>
 69
 70const char *outfilename;
 71
 72struct vdso_sym {
 73	const char *name;
 74	bool export;
 75};
 76
 77struct vdso_sym required_syms[] = {
 78	{"VDSO32_NOTE_MASK", true},
 79	{"__kernel_vsyscall", true},
 80	{"__kernel_sigreturn", true},
 81	{"__kernel_rt_sigreturn", true},
 82	{"int80_landing_pad", true},
 83	{"vdso32_rt_sigreturn_landing_pad", true},
 84	{"vdso32_sigreturn_landing_pad", true},
 85};
 86
 87__attribute__((format(printf, 1, 2))) __attribute__((noreturn))
 88static void fail(const char *format, ...)
 89{
 90	va_list ap;
 91	va_start(ap, format);
 92	fprintf(stderr, "Error: ");
 93	vfprintf(stderr, format, ap);
 94	if (outfilename)
 95		unlink(outfilename);
 96	exit(1);
 97	va_end(ap);
 98}
 99
100/*
101 * Evil macros for little-endian reads and writes
102 */
103#define GLE(x, bits, ifnot)						\
104	__builtin_choose_expr(						\
105		(sizeof(*(x)) == bits/8),				\
106		(__typeof__(*(x)))get_unaligned_le##bits(x), ifnot)
107
108extern void bad_get_le(void);
109#define LAST_GLE(x)							\
110	__builtin_choose_expr(sizeof(*(x)) == 1, *(x), bad_get_le())
111
112#define GET_LE(x)							\
113	GLE(x, 64, GLE(x, 32, GLE(x, 16, LAST_GLE(x))))
114
115#define PLE(x, val, bits, ifnot)					\
116	__builtin_choose_expr(						\
117		(sizeof(*(x)) == bits/8),				\
118		put_unaligned_le##bits((val), (x)), ifnot)
119
120extern void bad_put_le(void);
121#define LAST_PLE(x, val)						\
122	__builtin_choose_expr(sizeof(*(x)) == 1, *(x) = (val), bad_put_le())
123
124#define PUT_LE(x, val)					\
125	PLE(x, val, 64, PLE(x, val, 32, PLE(x, val, 16, LAST_PLE(x, val))))
126
127
128#define NSYMS ARRAY_SIZE(required_syms)
129
130#define BITSFUNC3(name, bits, suffix) name##bits##suffix
131#define BITSFUNC2(name, bits, suffix) BITSFUNC3(name, bits, suffix)
132#define BITSFUNC(name) BITSFUNC2(name, ELF_BITS, )
133
134#define INT_BITS BITSFUNC2(int, ELF_BITS, _t)
135
136#define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
137#define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
138#define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
139
140#define ELF_BITS 64
141#include "vdso2c.h"
142#undef ELF_BITS
143
144#define ELF_BITS 32
145#include "vdso2c.h"
146#undef ELF_BITS
147
148static void go(void *raw_addr, size_t raw_len,
149	       void *stripped_addr, size_t stripped_len,
150	       FILE *outfile, const char *name)
151{
152	Elf64_Ehdr *hdr = (Elf64_Ehdr *)raw_addr;
153
154	if (hdr->e_ident[EI_CLASS] == ELFCLASS64) {
155		go64(raw_addr, raw_len, stripped_addr, stripped_len,
156		     outfile, name);
157	} else if (hdr->e_ident[EI_CLASS] == ELFCLASS32) {
158		go32(raw_addr, raw_len, stripped_addr, stripped_len,
159		     outfile, name);
160	} else {
161		fail("unknown ELF class\n");
162	}
163}
164
165static void map_input(const char *name, void **addr, size_t *len, int prot)
166{
167	off_t tmp_len;
168
169	int fd = open(name, O_RDONLY);
170	if (fd == -1)
171		err(1, "open(%s)", name);
172
173	tmp_len = lseek(fd, 0, SEEK_END);
174	if (tmp_len == (off_t)-1)
175		err(1, "lseek");
176	*len = (size_t)tmp_len;
177
178	*addr = mmap(NULL, tmp_len, prot, MAP_PRIVATE, fd, 0);
179	if (*addr == MAP_FAILED)
180		err(1, "mmap");
181
182	close(fd);
183}
184
185int main(int argc, char **argv)
186{
187	size_t raw_len, stripped_len;
188	void *raw_addr, *stripped_addr;
189	FILE *outfile;
190	char *name, *tmp;
191	int namelen;
192
193	if (argc != 4) {
194		printf("Usage: vdso2c RAW_INPUT STRIPPED_INPUT OUTPUT\n");
195		return 1;
196	}
197
198	/*
199	 * Figure out the struct name.  If we're writing to a .so file,
200	 * generate raw output instead.
201	 */
202	name = strdup(argv[3]);
203	namelen = strlen(name);
204	if (namelen >= 3 && !strcmp(name + namelen - 3, ".so")) {
205		name = NULL;
206	} else {
207		tmp = strrchr(name, '/');
208		if (tmp)
209			name = tmp + 1;
210		tmp = strchr(name, '.');
211		if (tmp)
212			*tmp = '\0';
213		for (tmp = name; *tmp; tmp++)
214			if (*tmp == '-')
215				*tmp = '_';
216	}
217
218	map_input(argv[1], &raw_addr, &raw_len, PROT_READ);
219	map_input(argv[2], &stripped_addr, &stripped_len, PROT_READ);
220
221	outfilename = argv[3];
222	outfile = fopen(outfilename, "w");
223	if (!outfile)
224		err(1, "fopen(%s)", outfilename);
225
226	go(raw_addr, raw_len, stripped_addr, stripped_len, outfile, name);
227
228	munmap(raw_addr, raw_len);
229	munmap(stripped_addr, stripped_len);
230	fclose(outfile);
231
232	return 0;
233}