On Mon, May 20, 2019 at 04:19:54PM +0930, Alan Modra wrote:
> On Thu, May 16, 2019 at 05:52:42PM -0500, Segher Boessenkool wrote:
> > Hi Umesh,
> >
> > On Thu, May 16, 2019 at 06:12:48PM +0530, Umesh Kalappa wrote:
> > > We are very new to Power abi and we are thinking to handle this case
> > > in loader like go through the relocations like R_PPC64_REL24 and
> > > found symbol has the localentry ,then compute the delta (GEP - LEP )
> > > and patch the caller address like (sym.value - delta).
> >
> > I wonder if you have found a bug in the compiler after all. Most things
> > are supposed to work without the linker/loader having to do special
> > things; e.g. using the global entry point should always work, using the
> > local entry point is just an optimisation.
>
> That isn't true for direct calls. If using the global entry point,
> the linker must provide stub code to load up r12 with the global entry
> address and modify the nop after the bl. The linker must also adjust
> calls using the local entry point; the call instruction (and
> relocation) specify the function symbol not the function plus local
> entry offset.
>
> So I don't think there is any compiler bug here, just a broken kernel
> module loader. Incidentally, if thunks are broken then it's very
> likely local function calls are broken too.
The ABI says
"When a linker causes control to transfer to a global entry point, it
must insert a glue code sequence that loads r12 with the global
entry-point address. Code at the global entry point can assume that
register r12 points to the GEP."
But in the testcase the jump *already* was to the global entry point:
.globl _ZN12Intermediate1vEv
...
.set .LTHUNK0,_ZN12Intermediate1vEv
...
.globl _ZThn8_N12Intermediate1vEv
.type _ZThn8_N12Intermediate1vEv, @function
_ZThn8_N12Intermediate1vEv:
.cfi_startproc
.LCF2:
0: addis 2,12,.TOC.-.LCF2@ha
addi 2,2,.TOC.-.LCF2@l
.localentry _ZThn8_N12Intermediate1vEv,.-_ZThn8_N12Intermediate1vEv
addi 3,3,-8
b .LTHUNK0
.cfi_endproc
.LFE27:
.size _ZThn8_N12Intermediate1vEv,.-_ZThn8_N12Intermediate1vEv
Segher
