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
Relocation section '.rela.discard.reachable' at offset 0x4a00 contains 6 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
0000000000000000 0000000a00000032 R_LARCH_ADD32 0000000000000000 .init.text + ae0
0000000000000000 0000002800000037 R_LARCH_SUB32 0000000000000000 L0^A + 0
0000000000000004 0000000a00000032 R_LARCH_ADD32 0000000000000000 .init.text + b88
0000000000000004 0000002b00000037 R_LARCH_SUB32 0000000000000004 L0^A + 0
0000000000000008 0000000200000032 R_LARCH_ADD32 0000000000000000 .text + 23c
0000000000000008 0000002e00000037 R_LARCH_SUB32 0000000000000008 L0^A + 0
In order to silence the related objtool warnings, add skipped member
in struct reloc to record and skip the latter relocation.
Co-developed-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Co-developed-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---
tools/objtool/check.c | 4 ++++
tools/objtool/elf.c | 6 ++++++
tools/objtool/include/objtool/elf.h | 1 +
3 files changed, 11 insertions(+)
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 9e5e462..602318e 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -594,6 +594,8 @@ static int add_dead_ends(struct objtool_file *file)
goto reachable;
for_each_reloc(rsec, reloc) {
+ if (reloc->skipped)
+ continue;
if (reloc->sym->type != STT_SECTION) {
WARN("unexpected relocation symbol type in %s", rsec->name);
@@ -633,6 +635,8 @@ static int add_dead_ends(struct objtool_file *file)
return 0;
for_each_reloc(rsec, reloc) {
+ if (reloc->skipped)
+ continue;
if (reloc->sym->type != STT_SECTION) {
WARN("unexpected relocation symbol type in %s", rsec->name);
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index d420b5d..bd950d4 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -975,6 +975,12 @@ static int read_relocs(struct elf *elf)
return -1;
}
+ if (!(strcmp(rsec->name, ".rela.discard.unreachable")) ||
+ !(strcmp(rsec->name, ".rela.discard.reachable"))) {
+ if (reloc->sym->type != STT_SECTION)
+ reloc->skipped = true;
+ }
+
elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
reloc->sym_next_reloc = sym->relocs;
sym->relocs = reloc;
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index c532d70..4141837 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -75,6 +75,7 @@ struct reloc {
struct section *sec;
struct symbol *sym;
struct reloc *sym_next_reloc;
+ bool skipped;
};
struct elf {
--
2.1.0On 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..
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
On Thu, Aug 03, 2023 at 07:36:24PM +0800, Tiezhu Yang wrote: > > ".long " __stringify_label(c) "b - .\n\t" \ > 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. Yes, this is exactly what PC relative relocations are for.
On Thu, 2023-08-03 at 14:11 +0200, Peter Zijlstra wrote: > On Thu, Aug 03, 2023 at 07:36:24PM +0800, Tiezhu Yang wrote: > > > > ".long " __stringify_label(c) "b - .\n\t" \ > > > 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. Hmm, but I don't like the idea to disallow users having GAS <= 2.41 from using ORC unwinder. Can we emit the R_LARCH_32_PCREL reloc with an explicit .reloc directive? > Yes, this is exactly what PC relative relocations are for. -- Xi Ruoyao <xry111@xry111.site> School of Aerospace Science and Technology, Xidian University
On Wed, 2023-08-09 at 19:51 +0800, Xi Ruoyao wrote: > On Thu, 2023-08-03 at 14:11 +0200, Peter Zijlstra wrote: > > On Thu, Aug 03, 2023 at 07:36:24PM +0800, Tiezhu Yang wrote: > > > > > > ".long " __stringify_label(c) "b - .\n\t" \ > > > > > 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. > > Hmm, but I don't like the idea to disallow users having GAS <= 2.41 from > using ORC unwinder. > > Can we emit the R_LARCH_32_PCREL reloc with an explicit .reloc > directive? Answering myself: "maybe", I've written a simple PoC: $ cat t.s .global main main: la.pcrel $t0, f ld.w $t1, $t0, 0 ldx.d $a0, $t1, $t0 jr $ra t: .dword 42 .zero 64 f: .reloc f, R_LARCH_32_PCREL, t .word 0 $ ./a.out $ echo $? 42 -- Xi Ruoyao <xry111@xry111.site> School of Aerospace Science and Technology, Xidian University
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