[RFC] RISC-V: Go PLT for CALL/JUMP/RVC_JUMP if `h->plt.offset' isn't -1

[Fangrui Song]

On Sun, Mar 9, 2025 at 7:59 PM Nelson Chu <nelson@rivosinc.com> wrote:
>
>
>
> On Mon, Mar 10, 2025 at 10:04 AM Fangrui Song <i@maskray.me> wrote:
>>
>> I am intrigued by the problem but I have trouble understanding the
>> description. What behavior does this patch change?
>>
>> > riscv64-unknown-linux-gnu-gcc test.c -Wl,--unresolved-symbols=ignore-in-object-files
>>
>> There is no -nostdlib, therefore I assume that this command creates a
>> dynamically linked executable (both -fpie -pie and -fno-pie -no-pie
>> are possible, depending on how users configure the GCC).
>
>
> Oops, that's true.  I think the original problem is for -no-pie.
>
>>
>> I've tested a few cases but do not observe a behavior difference (in
>> terms of whether `foo` is in .dynsym and whether there is a JUMP_SLOT
>> relocation).
>>
>> ld-new --unresolved-symbols=ignore-in-object-files -o a a.o b.so
>> ld-new --unresolved-symbols=ignore-in-object-files -o a a.o
>
>
> % cat a.s
> .text
> .weak c
> .globl _start
> _start:
> call a
> call b
> call c
> % cat b.s
> .globl b
> b:
> nop
> % riscv64-unknown-linux-gnu-as a.s -o a.o
> % riscv64-unknown-linux-gnu-as b.s -o b.o
> % riscv64-unknown-linux-gnu-ld -shared b.o -o b.so
> % riscv64-unknown-linux-gnu-ld -unresolved-symbols=ignore-in-object-files a.o b.o
> % riscv64-unknown-linux-gnu-objdump -d a.out
> ...
> Disassembly of section .text:
>
> 00000000000100b0 <_start>:
>    100b0: ffff0097           auipc ra,0xffff0
>    100b4: f50080e7           jalr -176(ra) # 0 <_start-0x100b0>
>    100b8: 00c000ef           jal 100c4 <b>
>    100bc: 00000097           auipc ra,0x0
>    100c0: 000000e7           jalr zero # 0 <_start-0x100b0>
>
> 00000000000100c4 <b>:
>    100c4: 00000013           nop
> % riscv64-unknown-linux-gnu-ld -unresolved-symbols=ignore-in-object-files a.o b.so
> % riscv64-unknown-linux-gnu-objdump -d a.out
> ...
> Disassembly of section .plt:
> ...
> 0000000000010330 <b@plt>:
>    10330: 00002e17           auipc t3,0x2
>    10334: cd0e3e03           ld t3,-816(t3) # 12000 <b>
>    10338: 000e0367           jalr t1,t3
>    1033c: 00000013           nop
>
> 0000000000010340 <c@plt>:
>    10340: 00002e17           auipc t3,0x2
>    10344: cc8e3e03           ld t3,-824(t3) # 12008 <c>
>    10348: 000e0367           jalr t1,t3
>    1034c: 00000013           nop
>
> 0000000000010350 <a@plt>:
>    10350: 00002e17           auipc t3,0x2
>    10354: cc0e3e03           ld t3,-832(t3) # 12010 <a>
>    10358: 000e0367           jalr t1,t3
>    1035c: 00000013           nop
>
> Disassembly of section .text:
>
> 0000000000010360 <_start>:
>    10360: ff1ff0ef           jal 10350 <a@plt>
>    10364: fcdff0ef           jal 10330 <b@plt>
>    10368: fd9ff0ef           jal 10340 <c@plt>
>
> But before this patch, all undefined weak and non-weak symbols will be forced to zero,
>
> % riscv64-unknown-linux-gnu-ld-old -unresolved-symbols=ignore-in-object-files a.o b.so
> % riscv64-unknown-linux-gnu-objdump -d a.out
> ...
> Disassembly of section .plt:
> ...
> 0000000000010330 <b@plt>:
>    10330: 00002e17           auipc t3,0x2
>    10334: cd0e3e03           ld t3,-816(t3) # 12000 <b>
>    10338: 000e0367           jalr t1,t3
>    1033c: 00000013           nop
>
> 0000000000010340 <c@plt>:
>    10340: 00002e17           auipc t3,0x2
>    10344: cc8e3e03           ld t3,-824(t3) # 12008 <c>
>    10348: 000e0367           jalr t1,t3
>    1034c: 00000013           nop
>
> 0000000000010350 <a@plt>:
>    10350: 00002e17           auipc t3,0x2
>    10354: cc0e3e03           ld t3,-832(t3) # 12010 <a>
>    10358: 000e0367           jalr t1,t3
>    1035c: 00000013           nop
>
> Disassembly of section .text:
>
> 0000000000010360 <_start>:
>    10360: ffff0097           auipc ra,0xffff0
>    10364: ca0080e7           jalr -864(ra) # 0 <_PROCEDURE_LINKAGE_TABLE_-0x10310>
>    10368: fc9ff0ef           jal 10330 <b@plt>
>    1036c: 00000097           auipc ra,0x0
>    10370: 000000e7           jalr zero # 0 <_PROCEDURE_LINKAGE_TABLE_-0x10310>
>
>> (lld has a quite simple model where undefined non-weak and undefined
>> weak symbols are handled in a unified way.
>> A symbol is preemptible if:
>>
>> * -shared or at least one input file is DSO, and
>> * the symbol is undefined or exported (to .dynsym due to
>> --export-dynamic/--dynamic-list/referenced by DSO/etc), and
>> * other conditions that the symbol is preemptible
>>
>> Then, a preemptible symbol might need a PLT and associated JUMP_SLOT
>> relocation.)

Thanks for the example! I see that the branch target is now more
meaningful after the patch.

>
> Yeah I think now the behavior is more similar to lld.  Well, except the special handling for a call to undefined weak function, according to this page,
> https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/126
> I suddenly feel like we should also apply this special handling to the calls which jump to an undefined non-weak function...
>
> Thanks
> Nelson