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Linux/arch/x86/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_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         {"VDSO32_SYSENTER_RETURN", true},
102         {"__kernel_vsyscall", true},
103         {"__kernel_sigreturn", true},
104         {"__kernel_rt_sigreturn", 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 

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