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

[Josh Poimboeuf <jpoimboe@xxxxxxxxxx>]

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