[PATCH v4] MIPS/Gas: Disallow branch to absolute address for PIC

[YunQiang Su]

Maciej W. Rozycki <macro@orcam.me.uk> 于2024年2月7日周三 03:20写道：
>
> On Wed, 7 Feb 2024, YunQiang Su wrote:
>
> > The asm code like
> >       b       (0)
> > will generate binary like:
> >
> > 00000000 <.text>:
> >       0: 1000ffff b 0x0
> >            0: R_MIPS_PC16 *ABS*
> >       4: 00000000 nop
> > If this object is linked into an PIC/PIC dynamic executable,
> > this branch instruction will jump to a wrong address, with low
> > 16bit unset.
>
>  NAK, `mips_pic' may be set to SVR4_PIC and still produce de facto non-PIC
> code, as per the original SVR4 MIPS ABI (with lazy binding stubs rather
> than PLT).  We continue supporting this mode.
>
>  Originally NO_PIC was intended for bare metal code only, statically
> linked only, which was later extended to Linux, etc. with the addition of
> PLT and copy reloc support.
>
>  PR31343 has to be sorted in the linker.
>

So, what's the expected behavior?
Let's use the example:
        .set noreorder
        .set nomicromips
        .set nomips16
        b    (4)
        INSN0
        INSN1
        INSN2
        INSN3

At least we have 2 choice:
1. jump to INSN0, if we treat (4) same with (. + 4).
2. jump to INSN1, if we treat (4) as the offset in ISA document:
    "An 18-bit signed offset (the 16-bit offset field shifted left 2
bits) is added
     to the address of the instruction following the branch (not the
branch itself),
     in the branch delay slot, to form a PC-relative effective target address."

Currently, this syntax is not supported by ld even for static at all:

xx:
      b (8)
      ssnop
      ssnop
      ssnop
      ssnop

mipsel-linux-gnu-gcc -mno-shared -mno-abicalls -static -nostdlib 1.s
warning: cannot find entry symbol __start; defaulting to 00400110
/tmp/ccdHuq6V.o: in function `xx':
(.text+0x0): relocation truncated to fit: R_MIPS_PC16 against `*UND*'

I don't think that we should generate such confusion binary at all.
So, I prefer to disable this syntax for all cases, even for static objects:
only branch to label is allowed.