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linux-user: Fix GDB complaining about system-supplied DSO string table index

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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:
Ilya Leoshkevich 2024-10-23 22:27:41 +02:00 committed by Richard Henderson
parent c20d09ebee
commit 6e9dcfb906
2 changed files with 87 additions and 51 deletions
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98
linux-user/gen-vdso-elfn.c.inc
View File

@ -68,28 +68,45 @@ static void elfN(search_symtab)(ElfN(Shdr) *shdr, unsigned sym_idx,
void *buf, bool need_bswap) void *buf, bool need_bswap)
{ {
unsigned str_idx = shdr[sym_idx].sh_link; unsigned str_idx = shdr[sym_idx].sh_link;
ElfN(Sym) *sym = buf + shdr[sym_idx].sh_offset; ElfN(Sym) *target_sym = buf + shdr[sym_idx].sh_offset;
unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*sym); unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*target_sym);
const char *str = buf + shdr[str_idx].sh_offset; const char *str = buf + shdr[str_idx].sh_offset;
for (unsigned i = 0; i < sym_n; ++i) { for (unsigned i = 0; i < sym_n; ++i) {
const char *name; const char *name;
ElfN(Sym) sym;
memcpy(&sym, &target_sym[i], sizeof(sym));
if (need_bswap) { if (need_bswap) {
elfN(bswap_sym)(sym + i); elfN(bswap_sym)(&sym);
} }
name = str + sym[i].st_name; name = str + sym.st_name;
if (sigreturn_sym && strcmp(sigreturn_sym, name) == 0) { if (sigreturn_sym && strcmp(sigreturn_sym, name) == 0) {
sigreturn_addr = sym[i].st_value; sigreturn_addr = sym.st_value;
} }
if (rt_sigreturn_sym && strcmp(rt_sigreturn_sym, name) == 0) { if (rt_sigreturn_sym && strcmp(rt_sigreturn_sym, name) == 0) {
rt_sigreturn_addr = sym[i].st_value; rt_sigreturn_addr = sym.st_value;
} }
} }
} }
static void elfN(process)(FILE *outf, void *buf, bool need_bswap) static void elfN(bswap_ps_hdrs)(ElfN(Ehdr) *ehdr)
{
ElfN(Phdr) *phdr = (void *)ehdr + ehdr->e_phoff;
ElfN(Shdr) *shdr = (void *)ehdr + ehdr->e_shoff;
ElfN(Half) i;
for (i = 0; i < ehdr->e_phnum; ++i) {
elfN(bswap_phdr)(&phdr[i]);
}
for (i = 0; i < ehdr->e_shnum; ++i) {
elfN(bswap_shdr)(&shdr[i]);
}
}
static void elfN(process)(FILE *outf, void *buf, long len, bool need_bswap)
{ {
ElfN(Ehdr) *ehdr = buf; ElfN(Ehdr) *ehdr = buf;
ElfN(Phdr) *phdr; ElfN(Phdr) *phdr;
@ -103,24 +120,14 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
int errors = 0; int errors = 0;
if (need_bswap) { if (need_bswap) {
elfN(bswap_ehdr)(ehdr); elfN(bswap_ehdr)(buf);
elfN(bswap_ps_hdrs)(buf);
} }
phnum = ehdr->e_phnum; phnum = ehdr->e_phnum;
phdr = buf + ehdr->e_phoff; phdr = buf + ehdr->e_phoff;
if (need_bswap) {
for (unsigned i = 0; i < phnum; ++i) {
elfN(bswap_phdr)(phdr + i);
}
}
shnum = ehdr->e_shnum; shnum = ehdr->e_shnum;
shdr = buf + ehdr->e_shoff; shdr = buf + ehdr->e_shoff;
if (need_bswap) {
for (unsigned i = 0; i < shnum; ++i) {
elfN(bswap_shdr)(shdr + i);
}
}
for (unsigned i = 0; i < shnum; ++i) { for (unsigned i = 0; i < shnum; ++i) {
switch (shdr[i].sh_type) { switch (shdr[i].sh_type) {
case SHT_SYMTAB: case SHT_SYMTAB:
@ -154,7 +161,24 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
fprintf(stderr, "LOAD segment not loaded at address 0\n"); fprintf(stderr, "LOAD segment not loaded at address 0\n");
errors++; errors++;
} }
first_segsz = phdr[i].p_filesz; /*
* Extend the program header to cover the entire VDSO, so that
* load_elf_vdso() loads everything, including section headers.
*
* Require that there is no .bss, since it would break this
* approach.
*/
if (phdr[i].p_filesz != phdr[i].p_memsz) {
fprintf(stderr, "LOAD segment's filesz and memsz differ\n");
errors++;
}
if (phdr[i].p_filesz > len) {
fprintf(stderr, "LOAD segment is larger than the whole VDSO\n");
errors++;
}
phdr[i].p_filesz = len;
phdr[i].p_memsz = len;
first_segsz = len;
if (first_segsz < ehdr->e_phoff + phnum * sizeof(*phdr)) { if (first_segsz < ehdr->e_phoff + phnum * sizeof(*phdr)) {
fprintf(stderr, "LOAD segment does not cover PHDRs\n"); fprintf(stderr, "LOAD segment does not cover PHDRs\n");
errors++; errors++;
@ -197,17 +221,24 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
output_reloc(outf, buf, &phdr[i].p_paddr); output_reloc(outf, buf, &phdr[i].p_paddr);
} }
/* Relocate the section headers. */
for (unsigned i = 0; i < shnum; ++i) {
output_reloc(outf, buf, &shdr[i].sh_addr);
}
/* Relocate the DYNAMIC entries. */ /* Relocate the DYNAMIC entries. */
if (dynamic_addr) { if (dynamic_addr) {
ElfN(Dyn) *dyn = buf + dynamic_ofs; ElfN(Dyn) *target_dyn = buf + dynamic_ofs;
__typeof(dyn->d_tag) tag; __typeof(((ElfN(Dyn) *)target_dyn)->d_tag) tag;
do { do {
ElfN(Dyn) dyn;
memcpy(&dyn, target_dyn, sizeof(dyn));
if (need_bswap) { if (need_bswap) {
elfN(bswap_dyn)(dyn); elfN(bswap_dyn)(&dyn);
} }
tag = dyn->d_tag; tag = dyn.d_tag;
switch (tag) { switch (tag) {
case DT_HASH: case DT_HASH:
@ -218,7 +249,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
case DT_PLTGOT: case DT_PLTGOT:
case DT_ADDRRNGLO ... DT_ADDRRNGHI: case DT_ADDRRNGLO ... DT_ADDRRNGHI:
/* These entries store an address in the entry. */ /* These entries store an address in the entry. */
output_reloc(outf, buf, &dyn->d_un.d_val); output_reloc(outf, buf, &target_dyn->d_un.d_val);
break; break;
case DT_NULL: case DT_NULL:
@ -235,7 +266,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
break; break;
case DT_SYMENT: case DT_SYMENT:
if (dyn->d_un.d_val != sizeof(ElfN(Sym))) { if (dyn.d_un.d_val != sizeof(ElfN(Sym))) {
fprintf(stderr, "VDSO has incorrect dynamic symbol size\n"); fprintf(stderr, "VDSO has incorrect dynamic symbol size\n");
errors++; errors++;
} }
@ -251,7 +282,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
* ??? The RISC-V toolchain will emit these even when there * ??? The RISC-V toolchain will emit these even when there
* are no relocations. Validate zeros. * are no relocations. Validate zeros.
*/ */
if (dyn->d_un.d_val != 0) { if (dyn.d_un.d_val != 0) {
fprintf(stderr, "VDSO has dynamic relocations\n"); fprintf(stderr, "VDSO has dynamic relocations\n");
errors++; errors++;
} }
@ -287,7 +318,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
errors++; errors++;
break; break;
} }
dyn++; target_dyn++;
} while (tag != DT_NULL); } while (tag != DT_NULL);
if (errors) { if (errors) {
exit(EXIT_FAILURE); exit(EXIT_FAILURE);
@ -296,11 +327,11 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
/* Relocate the dynamic symbol table. */ /* Relocate the dynamic symbol table. */
if (dynsym_idx) { if (dynsym_idx) {
ElfN(Sym) *sym = buf + shdr[dynsym_idx].sh_offset; ElfN(Sym) *target_sym = buf + shdr[dynsym_idx].sh_offset;
unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*sym); unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*target_sym);
for (unsigned i = 0; i < sym_n; ++i) { for (unsigned i = 0; i < sym_n; ++i) {
output_reloc(outf, buf, &sym[i].st_value); output_reloc(outf, buf, &target_sym[i].st_value);
} }
} }
@ -311,4 +342,9 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
if (symtab_idx) { if (symtab_idx) {
elfN(search_symtab)(shdr, symtab_idx, buf, need_bswap); elfN(search_symtab)(shdr, symtab_idx, buf, need_bswap);
} }
if (need_bswap) {
elfN(bswap_ps_hdrs)(buf);
elfN(bswap_ehdr)(buf);
}
} }

40
linux-user/gen-vdso.c
View File

@ -131,23 +131,6 @@ int main(int argc, char **argv)
} }
fclose(inf); fclose(inf);
/*
* Write out the vdso image now, before we make local changes.
*/
fprintf(outf,
"/* Automatically generated from linux-user/gen-vdso.c. */\n"
"\n"
"static const uint8_t %s_image[] = {",
prefix);
for (long i = 0; i < total_len; ++i) {
if (i % 12 == 0) {
fputs("\n ", outf);
}
fprintf(outf, " 0x%02x,", buf[i]);
}
fprintf(outf, "\n};\n\n");
/* /*
* Identify which elf flavor we're processing. * Identify which elf flavor we're processing.
* The first 16 bytes of the file are e_ident. * The first 16 bytes of the file are e_ident.
@ -179,14 +162,17 @@ int main(int argc, char **argv)
* Output relocation addresses as we go. * Output relocation addresses as we go.
*/ */
fprintf(outf, "static const unsigned %s_relocs[] = {\n", prefix); fprintf(outf,
"/* Automatically generated by linux-user/gen-vdso.c. */\n"
"\n"
"static const unsigned %s_relocs[] = {\n", prefix);
switch (buf[EI_CLASS]) { switch (buf[EI_CLASS]) {
case ELFCLASS32: case ELFCLASS32:
elf32_process(outf, buf, need_bswap); elf32_process(outf, buf, total_len, need_bswap);
break; break;
case ELFCLASS64: case ELFCLASS64:
elf64_process(outf, buf, need_bswap); elf64_process(outf, buf, total_len, need_bswap);
break; break;
default: default:
fprintf(stderr, "%s: invalid elf EI_CLASS (%u)\n", fprintf(stderr, "%s: invalid elf EI_CLASS (%u)\n",
@ -196,6 +182,20 @@ int main(int argc, char **argv)
fprintf(outf, "};\n\n"); /* end vdso_relocs. */ fprintf(outf, "};\n\n"); /* end vdso_relocs. */
/*
* Write out the vdso image now, after we made local changes.
*/
fprintf(outf,
"static const uint8_t %s_image[] = {",
prefix);
for (long i = 0; i < total_len; ++i) {
if (i % 12 == 0) {
fputs("\n ", outf);
}
fprintf(outf, " 0x%02x,", buf[i]);
}
fprintf(outf, "\n};\n\n");
fprintf(outf, "static const VdsoImageInfo %s_image_info = {\n", prefix); fprintf(outf, "static const VdsoImageInfo %s_image_info = {\n", prefix);
fprintf(outf, " .image = %s_image,\n", prefix); fprintf(outf, " .image = %s_image,\n", prefix);
fprintf(outf, " .relocs = %s_relocs,\n", prefix); fprintf(outf, " .relocs = %s_relocs,\n", prefix);