https://sourceware.org/bugzilla/show_bug.cgi?id=30844
Bug ID: 30844
Summary: ld riscv: --emit-relocs does not retain the original
relocation type
Product: binutils
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: i at maskray dot me
Target Milestone: ---
From
https://maskray.me/blog/2021-03-14-the-dark-side-of-riscv-linker-relaxation#ld---emit-relocs
(with updated instructions)
For GNU ld's AArch64/PPC64/x86-64 ports, the --emit-relocs code retains the
original relocation type even if a linker optimization is applied. This is
partly to communicate more information to the analysis tool and partly because
the transformation may not be described with any existing relocation type.
However, the RISC-V port changes the relocation type, including relocation
types for relaxable instructions (e.g. R_RISCV_CALL_PLT), and markers
(R_RISCV_ALIGN and R_RISCV_RELAX). I believe this was not a deliberate decision
as there is no --emit-relocs test at all in ld/testsuite/ld-riscv-elf/. So far,
the unintentional change seems fine but certain relaxations (such as TLSDESC)
may not have relocation types associated with the relaxed instructions.
cat > aarch64.s <<'eof'
.global _start; _start:
adrp x1, :got:x
ldr x1, [x1, #:got_lo12:x]
.data; .globl x; .hidden x; x: .word 0
eof
cat > ppc64.s <<'eof'
.globl _start; _start:
addis 3, 2, .Lhidden@toc@ha
ld 3, .Lhidden@toc@l(3)
lwa 3, 0(3)
.data; .globl hidden; .hidden hidden; hidden: .long 0
.section .toc,"aw",@progbits
.Lhidden: .tc hidden[TC], hidden
eof
cat > x86-64.s <<'eof'
.globl _start; _start:
movq foo@gotpcrel(%rip), %rax
foo: nop
eof
aarch64-linux-gnu-gcc -fuse-ld=lld -B/tmp/Rel/bin -nostdlib aarch64.s
-Wl,--emit-relocs -o aarch64 && aarch64-linux-gnu-objdump -dr aarch64 | sed -E
'/^\s*$/d'
powerpc64le-linux-gnu-gcc -nostdlib ppc64.s -Wl,--emit-relocs -o ppc64 &&
powerpc64le-linux-gnu-objdump -dr ppc64 | sed -E '/^\s*$/d'
gcc -nostdlib x86-64.s -Wl,--emit-relocs -o x86-64 && objdump -dr x86-64 | sed
-E '/^\s*$/d'
cat > riscv64.s <<'eof'
.global _start; _start:
call f
call f
.balign 8
f: ret
eof
riscv64-linux-gnu-gcc -nostdlib riscv64.s -Wl,--emit-relocs -o riscv64 &&
riscv64-linux-gnu-objdump -dr riscv64 | sed -E '/^\s*$/d'
Output (removed blank lines for compacter output):
```
aarch64: file format elf64-littleaarch64
Disassembly of section .text:
00000000000102f8 <_start>:
102f8: d503201f nop
102f8: R_AARCH64_ADR_GOT_PAGE x
102fc: 10100661 adr x1, 303c8 <x>
102fc: R_AARCH64_LD64_GOT_LO12_NC x
ppc64: file format elf64-powerpcle
Disassembly of section .text:
0000000000000240 <_start>:
240: 00 00 00 60 nop
240: R_PPC64_TOC16_HA hidden
244: 00 81 62 38 addi r3,r2,-32512
244: R_PPC64_TOC16_LO hidden
248: 02 00 63 e8 lwa r3,0(r3)
x86-64: file format elf64-x86-64
Disassembly of section .text:
00000000000012dd <_start>:
12dd: 48 8d 05 00 00 00 00 lea 0x0(%rip),%rax # 12e4
<foo>
12e0: R_X86_64_REX_GOTPCRELX foo-0x4
00000000000012e4 <foo>:
12e4: 90 nop
riscv64: file format elf64-littleriscv
Disassembly of section .text:
00000000000002a0 <_start>:
2a0: 008000ef jal 2a8 <f>
2a0: R_RISCV_JAL f
2a4: 004000ef jal 2a8 <f>
2a4: R_RISCV_NONE *ABS*+0x4
2a4: R_RISCV_JAL f
00000000000002a8 <f>:
2a8: 8082 ret
2a8: R_RISCV_NONE *ABS*+0x4
2a8: R_RISCV_NONE *ABS*+0x6
```
I have a section "ld --emit-relocs" with very brief information on my website
for a while. Recently https://reviews.llvm.org/D159082 motivated me to
elaborate and bring this up:)
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