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Linux/arch/x86/entry/vdso/vdso2c.c

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  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_pvclock_page,
 77         sym_VDSO_FAKE_SECTION_TABLE_START,
 78         sym_VDSO_FAKE_SECTION_TABLE_END,
 79 };
 80 
 81 const int special_pages[] = {
 82         sym_vvar_page,
 83         sym_hpet_page,
 84         sym_pvclock_page,
 85 };
 86 
 87 struct vdso_sym {
 88         const char *name;
 89         bool export;
 90 };
 91 
 92 struct vdso_sym required_syms[] = {
 93         [sym_vvar_start] = {"vvar_start", true},
 94         [sym_vvar_page] = {"vvar_page", true},
 95         [sym_hpet_page] = {"hpet_page", true},
 96         [sym_pvclock_page] = {"pvclock_page", true},
 97         [sym_VDSO_FAKE_SECTION_TABLE_START] = {
 98                 "VDSO_FAKE_SECTION_TABLE_START", false
 99         },
100         [sym_VDSO_FAKE_SECTION_TABLE_END] = {
101                 "VDSO_FAKE_SECTION_TABLE_END", false
102         },
103         {"VDSO32_NOTE_MASK", true},
104         {"__kernel_vsyscall", true},
105         {"__kernel_sigreturn", true},
106         {"__kernel_rt_sigreturn", true},
107         {"int80_landing_pad", true},
108 };
109 
110 __attribute__((format(printf, 1, 2))) __attribute__((noreturn))
111 static void fail(const char *format, ...)
112 {
113         va_list ap;
114         va_start(ap, format);
115         fprintf(stderr, "Error: ");
116         vfprintf(stderr, format, ap);
117         if (outfilename)
118                 unlink(outfilename);
119         exit(1);
120         va_end(ap);
121 }
122 
123 /*
124  * Evil macros for little-endian reads and writes
125  */
126 #define GLE(x, bits, ifnot)                                             \
127         __builtin_choose_expr(                                          \
128                 (sizeof(*(x)) == bits/8),                               \
129                 (__typeof__(*(x)))get_unaligned_le##bits(x), ifnot)
130 
131 extern void bad_get_le(void);
132 #define LAST_GLE(x)                                                     \
133         __builtin_choose_expr(sizeof(*(x)) == 1, *(x), bad_get_le())
134 
135 #define GET_LE(x)                                                       \
136         GLE(x, 64, GLE(x, 32, GLE(x, 16, LAST_GLE(x))))
137 
138 #define PLE(x, val, bits, ifnot)                                        \
139         __builtin_choose_expr(                                          \
140                 (sizeof(*(x)) == bits/8),                               \
141                 put_unaligned_le##bits((val), (x)), ifnot)
142 
143 extern void bad_put_le(void);
144 #define LAST_PLE(x, val)                                                \
145         __builtin_choose_expr(sizeof(*(x)) == 1, *(x) = (val), bad_put_le())
146 
147 #define PUT_LE(x, val)                                  \
148         PLE(x, val, 64, PLE(x, val, 32, PLE(x, val, 16, LAST_PLE(x, val))))
149 
150 
151 #define NSYMS (sizeof(required_syms) / sizeof(required_syms[0]))
152 
153 #define BITSFUNC3(name, bits, suffix) name##bits##suffix
154 #define BITSFUNC2(name, bits, suffix) BITSFUNC3(name, bits, suffix)
155 #define BITSFUNC(name) BITSFUNC2(name, ELF_BITS, )
156 
157 #define INT_BITS BITSFUNC2(int, ELF_BITS, _t)
158 
159 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
160 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
161 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
162 
163 #define ELF_BITS 64
164 #include "vdso2c.h"
165 #undef ELF_BITS
166 
167 #define ELF_BITS 32
168 #include "vdso2c.h"
169 #undef ELF_BITS
170 
171 static void go(void *raw_addr, size_t raw_len,
172                void *stripped_addr, size_t stripped_len,
173                FILE *outfile, const char *name)
174 {
175         Elf64_Ehdr *hdr = (Elf64_Ehdr *)raw_addr;
176 
177         if (hdr->e_ident[EI_CLASS] == ELFCLASS64) {
178                 go64(raw_addr, raw_len, stripped_addr, stripped_len,
179                      outfile, name);
180         } else if (hdr->e_ident[EI_CLASS] == ELFCLASS32) {
181                 go32(raw_addr, raw_len, stripped_addr, stripped_len,
182                      outfile, name);
183         } else {
184                 fail("unknown ELF class\n");
185         }
186 }
187 
188 static void map_input(const char *name, void **addr, size_t *len, int prot)
189 {
190         off_t tmp_len;
191 
192         int fd = open(name, O_RDONLY);
193         if (fd == -1)
194                 err(1, "%s", name);
195 
196         tmp_len = lseek(fd, 0, SEEK_END);
197         if (tmp_len == (off_t)-1)
198                 err(1, "lseek");
199         *len = (size_t)tmp_len;
200 
201         *addr = mmap(NULL, tmp_len, prot, MAP_PRIVATE, fd, 0);
202         if (*addr == MAP_FAILED)
203                 err(1, "mmap");
204 
205         close(fd);
206 }
207 
208 int main(int argc, char **argv)
209 {
210         size_t raw_len, stripped_len;
211         void *raw_addr, *stripped_addr;
212         FILE *outfile;
213         char *name, *tmp;
214         int namelen;
215 
216         if (argc != 4) {
217                 printf("Usage: vdso2c RAW_INPUT STRIPPED_INPUT OUTPUT\n");
218                 return 1;
219         }
220 
221         /*
222          * Figure out the struct name.  If we're writing to a .so file,
223          * generate raw output insted.
224          */
225         name = strdup(argv[3]);
226         namelen = strlen(name);
227         if (namelen >= 3 && !strcmp(name + namelen - 3, ".so")) {
228                 name = NULL;
229         } else {
230                 tmp = strrchr(name, '/');
231                 if (tmp)
232                         name = tmp + 1;
233                 tmp = strchr(name, '.');
234                 if (tmp)
235                         *tmp = '\0';
236                 for (tmp = name; *tmp; tmp++)
237                         if (*tmp == '-')
238                                 *tmp = '_';
239         }
240 
241         map_input(argv[1], &raw_addr, &raw_len, PROT_READ);
242         map_input(argv[2], &stripped_addr, &stripped_len, PROT_READ);
243 
244         outfilename = argv[3];
245         outfile = fopen(outfilename, "w");
246         if (!outfile)
247                 err(1, "%s", argv[2]);
248 
249         go(raw_addr, raw_len, stripped_addr, stripped_len, outfile, name);
250 
251         munmap(raw_addr, raw_len);
252         munmap(stripped_addr, stripped_len);
253         fclose(outfile);
254 
255         return 0;
256 }
257 

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