Re: [PATCH v1 5/6] objtool: Add skipped member in struct reloc

[Tiezhu Yang]

On 07/25/2023 07:59 PM, Peter Zijlstra wrote:
> On Tue, Jul 25, 2023 at 04:15:09PM +0800, Tiezhu Yang wrote:
> > There exist multiple relocation types in one location, such as a pair of
> > R_LARCH_ADD32 and R_LARCH_SUB32 in section .rela.discard.unreachable and
> > .rela.discard.reachable on LoongArch.
> >
> > Here is an example:
> >
> > $ readelf -rW init/main.o
> >
> > Relocation section '.rela.discard.unreachable' at offset 0x3e20 contains 2 entries:
> >     Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
> > 0000000000000000  0000000a00000032 R_LARCH_ADD32          0000000000000000 .init.text + 230
> > 0000000000000000  0000001a00000037 R_LARCH_SUB32          0000000000000000 L0^A + 0
>
> Please explain; why is this?
>
> How does:
>
> #define __annotate_unreachable(c) ({					\
> 	   asm volatile(__stringify_label(c) ":\n\t"                       \
> 			".pushsection .discard.unreachable\n\t"            \
> 			".long " __stringify_label(c) "b - .\n\t"          \
> 			".popsection\n\t" : : "i" (c));                    \
> })
> #define annotate_unreachable() __annotate_unreachable(__COUNTER__)
>
> Manage to generate this..

Sorry for the late reply, let me try to explain it.

R_LARCH_ADD32 relocation type is 32-bit label addition,
R_LARCH_SUB32 relocation type is 32-bit label subtraction,
they are intended for local labels, the label difference
will be calculated as a constant before linking, preserve
a pair of R_LARCH_ADD32 and R_LARCH_SUB32 to fix the label
difference.

Here is a simple example:
-------------------------------
.L0
 // do something
.L1

.data
.word .L0 - .L1
-------------------------------

NewDiff = 0
R_LARCH_ADD32:
NewDiff = NewDiff + .L0_Addr
R_LARCH_SUB32:
NewDiff = NewDiff - .L1_Addr

I discussed offline with the engineers who are familiar with gnu
assembler of LoongArch, maybe it can modify the gnu assembler
to use R_LARCH_32_PCREL to replace a pair of R_LARCH_ADD32 and
R_LARCH_SUB32, then I will test it again and drop this change if
possible.

Here is LoongArch ELF Relocations spec:
https://github.com/loongson/la-abi-specs/blob/release/laelf.adoc

R_LARCH_ADD32
32-bit in-place addition
*(int32_t *) PC += S + A

R_LARCH_SUB32
32-bit in-place subtraction
*(int32_t *) PC -= S + A

R_LARCH_32_PCREL
32-bit PC relative
(*(uint32_t *) PC) = (S+A-PC) [31 ... 0]

Thanks very much for your review comments, it is helpful to
make progress in the right direction.

Thanks,
Tiezhu