Re: [ARM] Wrong BLX destination

[Simon Marchi <simon dot marchi at polymtl dot ca>]

On 2016-03-09 03:07, Christophe Lyon wrote:
> Hi,
>
> I haven't looked at this for a while, but:
> - which relocations are encoded by gas? (objdump -r). I suspect you
> have the ARM version of call (as opposed to the thumb one)

Indeed:

  00000000 R_ARM_CALL        hello1

> - I think you have to use the .thumb_func assembly directive right
> before helloX definition, and probably give it a function type too
>
> The linker will take care of making the call point to the right
> address, provided it has the right relocations.

When adding .thumb_func before the helloX definitions, the branches are 
now

    8220:       eb00013c        bl      8718 <__hello1_from_arm>
    8224:       eb000135        bl      8700 <__hello2_from_arm>
    8228:       eb000137        bl      870c <__hello3_from_arm>


and this additional code was added somewhere:

  00008700 <__hello2_from_arm>:
      8700:       e59fc000        ldr     ip, [pc]        ; 8708 
<__hello2_from_arm+0x8>
      8704:       e12fff1c        bx      ip
      8708:       0000827f        .word   0x0000827f

  0000870c <__hello3_from_arm>:
      870c:       e59fc000        ldr     ip, [pc]        ; 8714 
<__hello3_from_arm+0x8>
      8710:       e12fff1c        bx      ip
      8714:       00008281        .word   0x00008281

  00008718 <__hello1_from_arm>:
      8718:       e59fc000        ldr     ip, [pc]        ; 8720 
<__hello1_from_arm+0x8>
      871c:       e12fff1c        bx      ip
      8720:       0000827d        .word   0x0000827d

So while it doesn't simply use BLX with the H bit set, it seems that it 
would now work properly with this indirection.

Thanks for the tip!

Simon