linux-user: Fix GDB complaining about system-supplied DSO string table index
When debugging qemu-user processes using gdbstub, the following warning
appears every time:
warning: BFD: warning: system-supplied DSO at 0x7f8253cc3000 has a corrupt string table index
The reason is that QEMU does not map the VDSO's section headers. The
VDSO's ELF header's e_shoff points to zeros, which GDB fails to parse.
The difference with the kernel's VDSO is that the latter is mapped as a
blob, ignoring program headers - which also don't cover the section
table. QEMU, on the other hand, loads it as an ELF file.
There appears to be no way to place section headers inside a section,
and, therefore, no way to refer to them from a linker script. Also, ld
hardcodes section headers to be non-loadable, see
_bfd_elf_assign_file_positions_for_non_load(). In theory ld could be
enhanced by implementing an "SHDRS" keyword in addition to the existing
"FILEHDR" and "PHDRS".
There are multiple ways to resolve the issue:
- Copy VDSO as a blob in load_elf_vdso(). This would require creating
specialized loader logic, that duplicates parts of load_elf_image().
- Fix up VDSO's PHDR size in load_elf_vdso(). This would require either
duplicating the parsing logic, or adding an ugly parameter to
load_elf_image().
- Fix up VDSO's PHDR size in gen-vdso. This is the simplest solution,
so do it.
There are two tricky parts:
- Byte-swaps need to be done either on local copies, or in-place and
then reverted in the end. To preserve the existing code structure, do
the former for Sym and Dyn, and the latter for Ehdr, Phdr, and Shdr.
- There must be no .bss, which is already the case - but having an
explicit check is helpful to ensure correctness.
To verify this change, I diffed the on-disk and the loaded VDSOs; the
result does not show anything unusual, except for what seems to be an
existing oversight (which should probably be fixed separately):
│ Symbol table '.dynsym' contains 8 entries:
│ Num: Value Size Type Bind Vis Ndx Name
│ - 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
│ - 6: 0000000000000000 0 OBJECT GLOBAL DEFAULT ABS LINUX_2.6.29
│ + 0: 00007f61075bf000 0 NOTYPE LOCAL DEFAULT UND
│ + 6: 00007f61075bf000 0 OBJECT GLOBAL DEFAULT ABS LINUX_2.6.29
Fixes: 2fa536d107
("linux-user: Add gen-vdso tool")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20241023202850.55211-1-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
parent
c20d09ebee
commit
6e9dcfb906
@ -68,28 +68,45 @@ static void elfN(search_symtab)(ElfN(Shdr) *shdr, unsigned sym_idx,
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void *buf, bool need_bswap)
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{
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unsigned str_idx = shdr[sym_idx].sh_link;
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ElfN(Sym) *sym = buf + shdr[sym_idx].sh_offset;
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unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*sym);
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ElfN(Sym) *target_sym = buf + shdr[sym_idx].sh_offset;
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unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*target_sym);
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const char *str = buf + shdr[str_idx].sh_offset;
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for (unsigned i = 0; i < sym_n; ++i) {
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const char *name;
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ElfN(Sym) sym;
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memcpy(&sym, &target_sym[i], sizeof(sym));
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if (need_bswap) {
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elfN(bswap_sym)(sym + i);
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elfN(bswap_sym)(&sym);
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}
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name = str + sym[i].st_name;
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name = str + sym.st_name;
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if (sigreturn_sym && strcmp(sigreturn_sym, name) == 0) {
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sigreturn_addr = sym[i].st_value;
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sigreturn_addr = sym.st_value;
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}
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if (rt_sigreturn_sym && strcmp(rt_sigreturn_sym, name) == 0) {
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rt_sigreturn_addr = sym[i].st_value;
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rt_sigreturn_addr = sym.st_value;
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}
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}
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}
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static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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static void elfN(bswap_ps_hdrs)(ElfN(Ehdr) *ehdr)
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{
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ElfN(Phdr) *phdr = (void *)ehdr + ehdr->e_phoff;
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ElfN(Shdr) *shdr = (void *)ehdr + ehdr->e_shoff;
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ElfN(Half) i;
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for (i = 0; i < ehdr->e_phnum; ++i) {
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elfN(bswap_phdr)(&phdr[i]);
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}
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for (i = 0; i < ehdr->e_shnum; ++i) {
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elfN(bswap_shdr)(&shdr[i]);
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}
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}
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static void elfN(process)(FILE *outf, void *buf, long len, bool need_bswap)
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{
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ElfN(Ehdr) *ehdr = buf;
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ElfN(Phdr) *phdr;
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@ -103,24 +120,14 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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int errors = 0;
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if (need_bswap) {
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elfN(bswap_ehdr)(ehdr);
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elfN(bswap_ehdr)(buf);
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elfN(bswap_ps_hdrs)(buf);
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}
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phnum = ehdr->e_phnum;
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phdr = buf + ehdr->e_phoff;
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if (need_bswap) {
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for (unsigned i = 0; i < phnum; ++i) {
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elfN(bswap_phdr)(phdr + i);
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}
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}
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shnum = ehdr->e_shnum;
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shdr = buf + ehdr->e_shoff;
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if (need_bswap) {
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for (unsigned i = 0; i < shnum; ++i) {
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elfN(bswap_shdr)(shdr + i);
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}
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}
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for (unsigned i = 0; i < shnum; ++i) {
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switch (shdr[i].sh_type) {
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case SHT_SYMTAB:
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@ -154,7 +161,24 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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fprintf(stderr, "LOAD segment not loaded at address 0\n");
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errors++;
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}
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first_segsz = phdr[i].p_filesz;
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/*
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* Extend the program header to cover the entire VDSO, so that
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* load_elf_vdso() loads everything, including section headers.
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*
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* Require that there is no .bss, since it would break this
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* approach.
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*/
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if (phdr[i].p_filesz != phdr[i].p_memsz) {
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fprintf(stderr, "LOAD segment's filesz and memsz differ\n");
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errors++;
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}
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if (phdr[i].p_filesz > len) {
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fprintf(stderr, "LOAD segment is larger than the whole VDSO\n");
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errors++;
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}
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phdr[i].p_filesz = len;
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phdr[i].p_memsz = len;
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first_segsz = len;
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if (first_segsz < ehdr->e_phoff + phnum * sizeof(*phdr)) {
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fprintf(stderr, "LOAD segment does not cover PHDRs\n");
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errors++;
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@ -197,17 +221,24 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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output_reloc(outf, buf, &phdr[i].p_paddr);
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}
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/* Relocate the section headers. */
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for (unsigned i = 0; i < shnum; ++i) {
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output_reloc(outf, buf, &shdr[i].sh_addr);
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}
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/* Relocate the DYNAMIC entries. */
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if (dynamic_addr) {
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ElfN(Dyn) *dyn = buf + dynamic_ofs;
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__typeof(dyn->d_tag) tag;
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ElfN(Dyn) *target_dyn = buf + dynamic_ofs;
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__typeof(((ElfN(Dyn) *)target_dyn)->d_tag) tag;
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do {
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ElfN(Dyn) dyn;
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memcpy(&dyn, target_dyn, sizeof(dyn));
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if (need_bswap) {
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elfN(bswap_dyn)(dyn);
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elfN(bswap_dyn)(&dyn);
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}
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tag = dyn->d_tag;
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tag = dyn.d_tag;
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switch (tag) {
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case DT_HASH:
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@ -218,7 +249,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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case DT_PLTGOT:
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case DT_ADDRRNGLO ... DT_ADDRRNGHI:
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/* These entries store an address in the entry. */
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output_reloc(outf, buf, &dyn->d_un.d_val);
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output_reloc(outf, buf, &target_dyn->d_un.d_val);
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break;
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case DT_NULL:
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@ -235,7 +266,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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break;
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case DT_SYMENT:
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if (dyn->d_un.d_val != sizeof(ElfN(Sym))) {
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if (dyn.d_un.d_val != sizeof(ElfN(Sym))) {
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fprintf(stderr, "VDSO has incorrect dynamic symbol size\n");
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errors++;
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}
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@ -251,7 +282,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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* ??? The RISC-V toolchain will emit these even when there
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* are no relocations. Validate zeros.
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*/
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if (dyn->d_un.d_val != 0) {
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if (dyn.d_un.d_val != 0) {
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fprintf(stderr, "VDSO has dynamic relocations\n");
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errors++;
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}
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@ -287,7 +318,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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errors++;
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break;
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}
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dyn++;
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target_dyn++;
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} while (tag != DT_NULL);
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if (errors) {
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exit(EXIT_FAILURE);
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@ -296,11 +327,11 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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/* Relocate the dynamic symbol table. */
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if (dynsym_idx) {
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ElfN(Sym) *sym = buf + shdr[dynsym_idx].sh_offset;
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unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*sym);
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ElfN(Sym) *target_sym = buf + shdr[dynsym_idx].sh_offset;
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unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*target_sym);
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for (unsigned i = 0; i < sym_n; ++i) {
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output_reloc(outf, buf, &sym[i].st_value);
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output_reloc(outf, buf, &target_sym[i].st_value);
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}
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}
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@ -311,4 +342,9 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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if (symtab_idx) {
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elfN(search_symtab)(shdr, symtab_idx, buf, need_bswap);
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}
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if (need_bswap) {
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elfN(bswap_ps_hdrs)(buf);
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elfN(bswap_ehdr)(buf);
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}
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}
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@ -131,23 +131,6 @@ int main(int argc, char **argv)
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}
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fclose(inf);
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/*
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* Write out the vdso image now, before we make local changes.
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*/
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fprintf(outf,
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"/* Automatically generated from linux-user/gen-vdso.c. */\n"
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"\n"
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"static const uint8_t %s_image[] = {",
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prefix);
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for (long i = 0; i < total_len; ++i) {
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if (i % 12 == 0) {
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fputs("\n ", outf);
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}
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fprintf(outf, " 0x%02x,", buf[i]);
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}
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fprintf(outf, "\n};\n\n");
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/*
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* Identify which elf flavor we're processing.
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* The first 16 bytes of the file are e_ident.
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@ -179,14 +162,17 @@ int main(int argc, char **argv)
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* Output relocation addresses as we go.
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*/
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fprintf(outf, "static const unsigned %s_relocs[] = {\n", prefix);
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fprintf(outf,
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"/* Automatically generated by linux-user/gen-vdso.c. */\n"
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"\n"
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"static const unsigned %s_relocs[] = {\n", prefix);
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switch (buf[EI_CLASS]) {
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case ELFCLASS32:
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elf32_process(outf, buf, need_bswap);
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elf32_process(outf, buf, total_len, need_bswap);
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break;
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case ELFCLASS64:
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elf64_process(outf, buf, need_bswap);
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elf64_process(outf, buf, total_len, need_bswap);
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break;
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default:
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fprintf(stderr, "%s: invalid elf EI_CLASS (%u)\n",
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@ -196,6 +182,20 @@ int main(int argc, char **argv)
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fprintf(outf, "};\n\n"); /* end vdso_relocs. */
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/*
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* Write out the vdso image now, after we made local changes.
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*/
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fprintf(outf,
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"static const uint8_t %s_image[] = {",
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prefix);
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for (long i = 0; i < total_len; ++i) {
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if (i % 12 == 0) {
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fputs("\n ", outf);
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}
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fprintf(outf, " 0x%02x,", buf[i]);
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}
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fprintf(outf, "\n};\n\n");
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fprintf(outf, "static const VdsoImageInfo %s_image_info = {\n", prefix);
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fprintf(outf, " .image = %s_image,\n", prefix);
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fprintf(outf, " .relocs = %s_relocs,\n", prefix);
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