1 /* 2 * vdso2c - A vdso image preparation tool 3 * Copyright (c) 2014 Andy Lutomirski and others 4 * Licensed under the GPL v2 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 69 const char *outfilename; 70 71 /* Symbols that we need in vdso2c. */ 72 enum { 73 sym_vvar_start, 74 sym_vvar_page, 75 sym_hpet_page, 76 sym_VDSO_FAKE_SECTION_TABLE_START, 77 sym_VDSO_FAKE_SECTION_TABLE_END, 78 }; 79 80 const int special_pages[] = { 81 sym_vvar_page, 82 sym_hpet_page, 83 }; 84 85 struct vdso_sym { 86 const char *name; 87 bool export; 88 }; 89 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 96 }, 97 [sym_VDSO_FAKE_SECTION_TABLE_END] = { 98 "VDSO_FAKE_SECTION_TABLE_END", false 99 }, 100 {"VDSO32_NOTE_MASK", true}, 101 {"__kernel_vsyscall", true}, 102 {"__kernel_sigreturn", true}, 103 {"__kernel_rt_sigreturn", true}, 104 {"int80_landing_pad", true}, 105 }; 106 107 __attribute__((format(printf, 1, 2))) __attribute__((noreturn)) 108 static void fail(const char *format, ...) 109 { 110 va_list ap; 111 va_start(ap, format); 112 fprintf(stderr, "Error: "); 113 vfprintf(stderr, format, ap); 114 if (outfilename) 115 unlink(outfilename); 116 exit(1); 117 va_end(ap); 118 } 119 120 /* 121 * Evil macros for little-endian reads and writes 122 */ 123 #define GLE(x, bits, ifnot) \ 124 __builtin_choose_expr( \ 125 (sizeof(*(x)) == bits/8), \ 126 (__typeof__(*(x)))get_unaligned_le##bits(x), ifnot) 127 128 extern void bad_get_le(void); 129 #define LAST_GLE(x) \ 130 __builtin_choose_expr(sizeof(*(x)) == 1, *(x), bad_get_le()) 131 132 #define GET_LE(x) \ 133 GLE(x, 64, GLE(x, 32, GLE(x, 16, LAST_GLE(x)))) 134 135 #define PLE(x, val, bits, ifnot) \ 136 __builtin_choose_expr( \ 137 (sizeof(*(x)) == bits/8), \ 138 put_unaligned_le##bits((val), (x)), ifnot) 139 140 extern void bad_put_le(void); 141 #define LAST_PLE(x, val) \ 142 __builtin_choose_expr(sizeof(*(x)) == 1, *(x) = (val), bad_put_le()) 143 144 #define PUT_LE(x, val) \ 145 PLE(x, val, 64, PLE(x, val, 32, PLE(x, val, 16, LAST_PLE(x, val)))) 146 147 148 #define NSYMS (sizeof(required_syms) / sizeof(required_syms[0])) 149 150 #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, ) 153 154 #define INT_BITS BITSFUNC2(int, ELF_BITS, _t) 155 156 #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) 159 160 #define ELF_BITS 64 161 #include "vdso2c.h" 162 #undef ELF_BITS 163 164 #define ELF_BITS 32 165 #include "vdso2c.h" 166 #undef ELF_BITS 167 168 static void go(void *raw_addr, size_t raw_len, 169 void *stripped_addr, size_t stripped_len, 170 FILE *outfile, const char *name) 171 { 172 Elf64_Ehdr *hdr = (Elf64_Ehdr *)raw_addr; 173 174 if (hdr->e_ident[EI_CLASS] == ELFCLASS64) { 175 go64(raw_addr, raw_len, stripped_addr, stripped_len, 176 outfile, name); 177 } else if (hdr->e_ident[EI_CLASS] == ELFCLASS32) { 178 go32(raw_addr, raw_len, stripped_addr, stripped_len, 179 outfile, name); 180 } else { 181 fail("unknown ELF class\n"); 182 } 183 } 184 185 static void map_input(const char *name, void **addr, size_t *len, int prot) 186 { 187 off_t tmp_len; 188 189 int fd = open(name, O_RDONLY); 190 if (fd == -1) 191 err(1, "%s", name); 192 193 tmp_len = lseek(fd, 0, SEEK_END); 194 if (tmp_len == (off_t)-1) 195 err(1, "lseek"); 196 *len = (size_t)tmp_len; 197 198 *addr = mmap(NULL, tmp_len, prot, MAP_PRIVATE, fd, 0); 199 if (*addr == MAP_FAILED) 200 err(1, "mmap"); 201 202 close(fd); 203 } 204 205 int main(int argc, char **argv) 206 { 207 size_t raw_len, stripped_len; 208 void *raw_addr, *stripped_addr; 209 FILE *outfile; 210 char *name, *tmp; 211 int namelen; 212 213 if (argc != 4) { 214 printf("Usage: vdso2c RAW_INPUT STRIPPED_INPUT OUTPUT\n"); 215 return 1; 216 } 217 218 /* 219 * Figure out the struct name. If we're writing to a .so file, 220 * generate raw output insted. 221 */ 222 name = strdup(argv[3]); 223 namelen = strlen(name); 224 if (namelen >= 3 && !strcmp(name + namelen - 3, ".so")) { 225 name = NULL; 226 } else { 227 tmp = strrchr(name, '/'); 228 if (tmp) 229 name = tmp + 1; 230 tmp = strchr(name, '.'); 231 if (tmp) 232 *tmp = '\0'; 233 for (tmp = name; *tmp; tmp++) 234 if (*tmp == '-') 235 *tmp = '_'; 236 } 237 238 map_input(argv[1], &raw_addr, &raw_len, PROT_READ); 239 map_input(argv[2], &stripped_addr, &stripped_len, PROT_READ); 240 241 outfilename = argv[3]; 242 outfile = fopen(outfilename, "w"); 243 if (!outfile) 244 err(1, "%s", argv[2]); 245 246 go(raw_addr, raw_len, stripped_addr, stripped_len, outfile, name); 247 248 munmap(raw_addr, raw_len); 249 munmap(stripped_addr, stripped_len); 250 fclose(outfile); 251 252 return 0; 253 } 254