[ARM] Wrong BLX destination

[Simon Marchi]

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