[PATCH v2 0/8] RISC-V: Support GNU indirect functions

[Fangrui Song]

On 2020-08-07, Nelson Chu wrote:
>Consider the ld's behavior,
>$ riscv64-unknown-linux-gnu-as ifunc-plt-02.s -o ifunc-plt-02.o
>$ riscv64-unknown-linux-gnu-ld -pie ifunc-plt-02.o -o ld.pie
>$ riscv64-unknown-linux-gnu-readelf -Ws ld.pie
>Symbol table '.symtab' contains 30 entries:
>...
>14: 0000000000000270     4 FUNC    LOCAL  DEFAULT    9 foo_resolver
>...
>25: 0000000000000270     4 IFUNC   GLOBAL DEFAULT    9 foo
>...
>
>$ riscv64-unknown-linux-gnu-readelf -Wr ld.pie
>Relocation section '.rela.dyn' at offset 0x210 contains 1 entry:
>    Offset             Info             Type               Symbol's
>Value  Symbol's Name + Addend
>0000000000002000  000000000000003a R_RISCV_IRELATIVE                         270
>
>Relocation section '.rela.plt' at offset 0x228 contains 1 entry:
>    Offset             Info             Type               Symbol's
>Value  Symbol's Name + Addend
>0000000000002018  000000000000003a R_RISCV_IRELATIVE                         270
>
>$ riscv64-unknown-linux-gnu-objdump -D ld.pie | less
>Disassembly of section .plt:
>
>0000000000000240 <.plt>:
> 240:   00002397                auipc   t2,0x2
> 244:   41c30333                sub     t1,t1,t3
> 248:   dc83be03                ld      t3,-568(t2) # 2008 <.got>
> 24c:   fd430313                addi    t1,t1,-44
> 250:   dc838293                addi    t0,t2,-568
> 254:   00135313                srli    t1,t1,0x1
> 258:   0082b283                ld      t0,8(t0)
> 25c:   000e0067                jr      t3
>
>0000000000000260 <*ABS*+0x270@plt>:
> 260:   00002e17                auipc   t3,0x2
> 264:   db8e3e03                ld      t3,-584(t3) # 2018 <.got+0x10>
> 268:   000e0367                jalr    t1,t3
> 26c:   00000013                nop
>
>Disassembly of section .text:
>
>0000000000000270 <foo_resolver>:
> 270:   00008067                ret
>
>0000000000000274 <bar>:
> 274:   00002097                auipc   ra,0x2
> 278:   da40a083                lw      ra,-604(ra) # 2018 <.got+0x10>
> 27c:   00002117                auipc   sp,0x2
> 280:   d8412103                lw      sp,-636(sp) # 2000 <__DATA_BEGIN__>
> 284:   00000197                auipc   gp,0x0
> 288:   fdc18193                addi    gp,gp,-36 # 260 <*ABS*+0x270@plt>
> 28c:   00000097                auipc   ra,0x0
> 290:   fd4080e7                jalr    -44(ra) # 260 <*ABS*+0x270@plt>
> 294:   00000097                auipc   ra,0x0
> 298:   fcc080e7                jalr    -52(ra) # 260 <*ABS*+0x270@plt>
> 29c:   00008067                ret
>...
>Disassembly of section .data:
>
>0000000000002000 <__DATA_BEGIN__>:
>    2000:       0000                    unimp
>        ...
>
>0x2018 is a got.plt entry, the GOT and PLT share the same .got.plt in this case.
>And 0x2000 is the data section.  The GOT refers to `foo` will get the address
>of the resolved IFUNC address after ld.so resolved the R_RISCV_IRELATIVE
>reloc.  Also, ld.so will fill the resolved IFUNC address to the data section.
>However, the function pointer equality looks fine, too.  Since we should always
>get the resolved IFUNC address as the function address.

Do ra (after .L1), sp (after .l2) and gp (after .L3) in <bar> have the same
value?  IIUC gp is 0x260. If the got entry (relocated by R_RISCV_IRELATIVE) does
not always return 0x260, there will be an inequality.