Re: [RFC PATCH 04/28] x86/boot: Permit GOTPCREL relocations for x86_64 builds

Josh Poimboeuf
2024-10-01T05:33:22Z

On Wed, Sep 25, 2024 at 05:01:04PM +0200, Ard Biesheuvel wrote:
> + if (r_type == R_X86_64_GOTPCREL) {
> + Elf_Shdr *s = &secs[sec->shdr.sh_info].shdr;
> + unsigned file_off = offset - s->sh_addr + s->sh_offset;
> +
> + /*
> + * GOTPCREL relocations refer to instructions that load
> + * a 64-bit address via a 32-bit relative reference to
> + * the GOT. In this case, it is the GOT entry that
> + * needs to be fixed up, not the immediate offset in
> + * the opcode. Note that the linker will have applied an
> + * addend of -4 to compensate for the delta between the
> + * relocation offset and the value of RIP when the
> + * instruction executes, and this needs to be backed out
> + * again. (Addends other than -4 are permitted in
> + * principle, but make no sense in practice so they are
> + * not supported.)
> + */
> + if (rel->r_addend != -4) {
> + die("invalid addend (%ld) for %s relocation: %s\n",
> + rel->r_addend, rel_type(r_type), symname);
> + break;
> + }

For x86 PC-relative addressing, the addend is <reloc offset> -
<subsequent insn offset>. So a PC-relative addend can be something
other than -4 when the relocation applies to the middle of an
instruction, e.g.:

5b381: 66 81 3d 00 00 00 00 01 06 cmpw $0x601,0x0(%rip) # 5b38a <generic_validate_add_page+0x4a> 5b384: R_X86_64_PC32 boot_cpu_data-0x6

5f283: 81 3d 00 00 00 00 ff ff ff 00 cmpl $0xffffff,0x0(%rip) # 5f28d <x86_acpi_suspend_lowlevel+0x9d> 5f285: R_X86_64_PC32 smpboot_control-0x8

72f67: c6 05 00 00 00 00 01 movb $0x1,0x0(%rip) # 72f6e <sched_itmt_update_handler+0x6e> 72f69: R_X86_64_PC32 x86_topology_update-0x5

Presumably that could also happen with R_X86_64_GOTPCREL?

--
Josh