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

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: 2fa536d10797 ("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>