linux-user/gen-vdso-elfn.c.inc | 98 +++++++++++++++++++++++----------- linux-user/gen-vdso.c | 40 +++++++------- 2 files changed, 87 insertions(+), 51 deletions(-)
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: 2fa536d10797 ("linux-user: Add gen-vdso tool")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
v1: https://lore.kernel.org/qemu-devel/20241023144744.50440-1-iii@linux.ibm.com/
v1 -> v2: Check that there is no bss, mention linker scripts in the
commit message (Richard).
linux-user/gen-vdso-elfn.c.inc | 98 +++++++++++++++++++++++-----------
linux-user/gen-vdso.c | 40 +++++++-------
2 files changed, 87 insertions(+), 51 deletions(-)
diff --git a/linux-user/gen-vdso-elfn.c.inc b/linux-user/gen-vdso-elfn.c.inc
index 95856eb839b..b47019e136c 100644
--- a/linux-user/gen-vdso-elfn.c.inc
+++ b/linux-user/gen-vdso-elfn.c.inc
@@ -68,28 +68,45 @@ static void elfN(search_symtab)(ElfN(Shdr) *shdr, unsigned sym_idx,
void *buf, bool need_bswap)
{
unsigned str_idx = shdr[sym_idx].sh_link;
- ElfN(Sym) *sym = buf + shdr[sym_idx].sh_offset;
- unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*sym);
+ ElfN(Sym) *target_sym = buf + shdr[sym_idx].sh_offset;
+ unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*target_sym);
const char *str = buf + shdr[str_idx].sh_offset;
for (unsigned i = 0; i < sym_n; ++i) {
const char *name;
+ ElfN(Sym) sym;
+ memcpy(&sym, &target_sym[i], sizeof(sym));
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) {
- sigreturn_addr = sym[i].st_value;
+ sigreturn_addr = sym.st_value;
}
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(Phdr) *phdr;
@@ -103,24 +120,14 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
int errors = 0;
if (need_bswap) {
- elfN(bswap_ehdr)(ehdr);
+ elfN(bswap_ehdr)(buf);
+ elfN(bswap_ps_hdrs)(buf);
}
phnum = ehdr->e_phnum;
phdr = buf + ehdr->e_phoff;
- if (need_bswap) {
- for (unsigned i = 0; i < phnum; ++i) {
- elfN(bswap_phdr)(phdr + i);
- }
- }
-
shnum = ehdr->e_shnum;
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) {
switch (shdr[i].sh_type) {
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");
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)) {
fprintf(stderr, "LOAD segment does not cover PHDRs\n");
errors++;
@@ -197,17 +221,24 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
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. */
if (dynamic_addr) {
- ElfN(Dyn) *dyn = buf + dynamic_ofs;
- __typeof(dyn->d_tag) tag;
+ ElfN(Dyn) *target_dyn = buf + dynamic_ofs;
+ __typeof(((ElfN(Dyn) *)target_dyn)->d_tag) tag;
do {
+ ElfN(Dyn) dyn;
+ memcpy(&dyn, target_dyn, sizeof(dyn));
if (need_bswap) {
- elfN(bswap_dyn)(dyn);
+ elfN(bswap_dyn)(&dyn);
}
- tag = dyn->d_tag;
+ tag = dyn.d_tag;
switch (tag) {
case DT_HASH:
@@ -218,7 +249,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
case DT_PLTGOT:
case DT_ADDRRNGLO ... DT_ADDRRNGHI:
/* 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;
case DT_NULL:
@@ -235,7 +266,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
break;
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");
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
* 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");
errors++;
}
@@ -287,7 +318,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
errors++;
break;
}
- dyn++;
+ target_dyn++;
} while (tag != DT_NULL);
if (errors) {
exit(EXIT_FAILURE);
@@ -296,11 +327,11 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap)
/* Relocate the dynamic symbol table. */
if (dynsym_idx) {
- ElfN(Sym) *sym = buf + shdr[dynsym_idx].sh_offset;
- unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*sym);
+ ElfN(Sym) *target_sym = buf + shdr[dynsym_idx].sh_offset;
+ unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*target_sym);
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) {
elfN(search_symtab)(shdr, symtab_idx, buf, need_bswap);
}
+
+ if (need_bswap) {
+ elfN(bswap_ps_hdrs)(buf);
+ elfN(bswap_ehdr)(buf);
+ }
}
diff --git a/linux-user/gen-vdso.c b/linux-user/gen-vdso.c
index 31e333be802..721f38d5a3b 100644
--- a/linux-user/gen-vdso.c
+++ b/linux-user/gen-vdso.c
@@ -131,23 +131,6 @@ int main(int argc, char **argv)
}
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.
* 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.
*/
- 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]) {
case ELFCLASS32:
- elf32_process(outf, buf, need_bswap);
+ elf32_process(outf, buf, total_len, need_bswap);
break;
case ELFCLASS64:
- elf64_process(outf, buf, need_bswap);
+ elf64_process(outf, buf, total_len, need_bswap);
break;
default:
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. */
+ /*
+ * 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, " .image = %s_image,\n", prefix);
fprintf(outf, " .relocs = %s_relocs,\n", prefix);
--
2.47.0
On 10/23/24 21:27, Ilya Leoshkevich wrote:
> 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: 2fa536d10797 ("linux-user: Add gen-vdso tool")
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>
> v1: https://lore.kernel.org/qemu-devel/20241023144744.50440-1-iii@linux.ibm.com/
> v1 -> v2: Check that there is no bss, mention linker scripts in the
> commit message (Richard).
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
r~
On 10/25/24 12:41, Richard Henderson wrote:
> On 10/23/24 21:27, Ilya Leoshkevich wrote:
>> 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: 2fa536d10797 ("linux-user: Add gen-vdso tool")
>> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
>> ---
>>
>> v1: https://lore.kernel.org/qemu-devel/20241023144744.50440-1-iii@linux.ibm.com/
>> v1 -> v2: Check that there is no bss, mention linker scripts in the
>> commit message (Richard).
>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
>
> r~
Queued, thanks.
r~
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