[PATCH] Improve IFUNC check

[Fangrui Song]

On 2020-02-06, Aurelien Jarno wrote:
>Hi,
>
>On 2020-02-04 22:09, Fangrui Song wrote:
>> GNU ld's RISCV port does not support IFUNC. ld -no-pie produces no
>> relocation and the test passed incorrectly. Be more rigid by testing
>> IRELATIVE explicitly.
>
>Thanks for your patch and sorry for the time to answer, I was testing it
>on many architectures. First of all I confirm it fixes the issue when
>building glibc with PIE on mips* and riscv64. I also confirm it
>introduces no regression on aarch64, armv5, armv7, hppa, m68k, powerpc,
>ppc64, ppc64le, s390x and x86_64.
>
>However it wrongly detects that IFUNC is not supported on sparc64 when
>PIE is not in use. Here is the output of of readelf for the non-PIE
>case:
>
>| Relocation section '.rela.dyn' at offset 0x110 contains 1 entry:
>|   Offset          Info           Type           Sym. Value    Sym. Name + Addend
>| 000000200280  0000000000f8 R_SPARC_JMP_IREL                     100128

sparc64 -no-pie does not produce an R_SPARC_IRELATIVE. This looks weird.
All other archs emit an R_*_IRELATIVE.

Can you dump the assembly (including .text and .[i]plt)?
Or can someone with sparc64 experience answer why sparc64 is different
here?

As my lld R_RISCV_IRELATIVE patch (https://reviews.llvm.org/D74022) shows

   # DIS64:      _start:
   # DIS64-NEXT:     1264: auipc a0, 0
   # DIS64-NEXT:           addi a0, a0, 12
   # DIS64:      Disassembly of section .iplt:
   # DIS64:      00001270 func:
   ## 64-bit: &.got.plt[func]-. = 0x3370-0x1270 = 4096*2+256
   # DIS64-NEXT:          auipc t3, 2
   # DIS64-NEXT:          ld t3, 256(t3)
   # DIS64-NEXT:          jalr t1, t3
   # DIS64-NEXT:          nop

The idea is to create an IPLT entry and bind absolute/PC-relative relocations to that address.
The IPLT entry needs a .got.plt slot, which is relocated by an R_*_IRELATIVE at runtime.
It is R_*_IRELATIVE because no symbol index is needed.


R_SPARC_JMP_IREL looks strange. I hope someone can explain.

>And for the PIE case:
>
>| Relocation section '.rela.dyn' at offset 0x290 contains 1 entry:
>|   Offset          Info           Type           Sym. Value    Sym. Name + Addend
>| 0000001021a0  0000000000f9 R_SPARC_IRELATIVE                    2c0
>|
>| Relocation section '.rela.plt' at offset 0x2a8 contains 1 entry:
>|   Offset          Info           Type           Sym. Value    Sym. Name + Addend
>| 000000102180  0000000000f8 R_SPARC_JMP_IREL                     2c0
>
>Looks like you might want to slightly adjust the regex.
>
>Also note that this is BZ#25506, to add to the commit message.
>
>-- 
>Aurelien Jarno                          GPG: 4096R/1DDD8C9B
>aurelien@aurel32.net                 http://www.aurel32.net