2 * vdso2c - A vdso image preparation tool
3 * Copyright (c) 2014 Andy Lutomirski and others
4 * Licensed under the GPL v2
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.
14 * We're keep a section table for a few reasons:
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
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
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
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.
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.
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
62 #include <sys/types.h>
64 #include <tools/le_byteshift.h>
66 #include <linux/elf.h>
67 #include <linux/types.h>
69 const char *outfilename;
71 /* Symbols that we need in vdso2c. */
76 sym_VDSO_FAKE_SECTION_TABLE_START,
77 sym_VDSO_FAKE_SECTION_TABLE_END,
80 const int special_pages[] = {
90 struct vdso_sym required_syms[] = {
91 [sym_vvar_start] = {"vvar_start", true},
92 [sym_vvar_page] = {"vvar_page", true},
93 [sym_hpet_page] = {"hpet_page", true},
94 [sym_VDSO_FAKE_SECTION_TABLE_START] = {
95 "VDSO_FAKE_SECTION_TABLE_START", false
97 [sym_VDSO_FAKE_SECTION_TABLE_END] = {
98 "VDSO_FAKE_SECTION_TABLE_END", false
100 {"VDSO32_NOTE_MASK", true},
101 {"VDSO32_SYSENTER_RETURN", true},
102 {"__kernel_vsyscall", true},
103 {"__kernel_sigreturn", true},
104 {"__kernel_rt_sigreturn", true},
107 __attribute__((format(printf, 1, 2))) __attribute__((noreturn))
108 static void fail(const char *format, ...)
111 va_start(ap, format);
112 fprintf(stderr, "Error: ");
113 vfprintf(stderr, format, ap);
121 * Evil macros for little-endian reads and writes
123 #define GLE(x, bits, ifnot) \
124 __builtin_choose_expr( \
125 (sizeof(*(x)) == bits/8), \
126 (__typeof__(*(x)))get_unaligned_le##bits(x), ifnot)
128 extern void bad_get_le(void);
129 #define LAST_GLE(x) \
130 __builtin_choose_expr(sizeof(*(x)) == 1, *(x), bad_get_le())
133 GLE(x, 64, GLE(x, 32, GLE(x, 16, LAST_GLE(x))))
135 #define PLE(x, val, bits, ifnot) \
136 __builtin_choose_expr( \
137 (sizeof(*(x)) == bits/8), \
138 put_unaligned_le##bits((val), (x)), ifnot)
140 extern void bad_put_le(void);
141 #define LAST_PLE(x, val) \
142 __builtin_choose_expr(sizeof(*(x)) == 1, *(x) = (val), bad_put_le())
144 #define PUT_LE(x, val) \
145 PLE(x, val, 64, PLE(x, val, 32, PLE(x, val, 16, LAST_PLE(x, val))))
148 #define NSYMS (sizeof(required_syms) / sizeof(required_syms[0]))
150 #define BITSFUNC3(name, bits, suffix) name##bits##suffix
151 #define BITSFUNC2(name, bits, suffix) BITSFUNC3(name, bits, suffix)
152 #define BITSFUNC(name) BITSFUNC2(name, ELF_BITS, )
154 #define INT_BITS BITSFUNC2(int, ELF_BITS, _t)
156 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
157 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
158 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
168 static void go(void *raw_addr, size_t raw_len,
169 void *stripped_addr, size_t stripped_len,
170 FILE *outfile, const char *name)
172 Elf64_Ehdr *hdr = (Elf64_Ehdr *)raw_addr;
174 if (hdr->e_ident[EI_CLASS] == ELFCLASS64) {
175 go64(raw_addr, raw_len, stripped_addr, stripped_len,
177 } else if (hdr->e_ident[EI_CLASS] == ELFCLASS32) {
178 go32(raw_addr, raw_len, stripped_addr, stripped_len,
181 fail("unknown ELF class\n");
185 static void map_input(const char *name, void **addr, size_t *len, int prot)
189 int fd = open(name, O_RDONLY);
193 tmp_len = lseek(fd, 0, SEEK_END);
194 if (tmp_len == (off_t)-1)
196 *len = (size_t)tmp_len;
198 *addr = mmap(NULL, tmp_len, prot, MAP_PRIVATE, fd, 0);
199 if (*addr == MAP_FAILED)
205 int main(int argc, char **argv)
207 size_t raw_len, stripped_len;
208 void *raw_addr, *stripped_addr;
214 printf("Usage: vdso2c RAW_INPUT STRIPPED_INPUT OUTPUT\n");
219 * Figure out the struct name. If we're writing to a .so file,
220 * generate raw output insted.
222 name = strdup(argv[3]);
223 namelen = strlen(name);
224 if (namelen >= 3 && !strcmp(name + namelen - 3, ".so")) {
227 tmp = strrchr(name, '/');
230 tmp = strchr(name, '.');
233 for (tmp = name; *tmp; tmp++)
238 map_input(argv[1], &raw_addr, &raw_len, PROT_READ);
239 map_input(argv[2], &stripped_addr, &stripped_len, PROT_READ);
241 outfilename = argv[3];
242 outfile = fopen(outfilename, "w");
244 err(1, "%s", argv[2]);
246 go(raw_addr, raw_len, stripped_addr, stripped_len, outfile, name);
248 munmap(raw_addr, raw_len);
249 munmap(stripped_addr, stripped_len);