Re: [PATCHi v1] PowerPC: Create call stubs for compiled modules

[Abhay Kandpal <abhay@linux.ibm.com>]

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Hi Ulrich,

Thanks for the suggestion. I tried|-mlongcall| and it doesn't work on PowerPC,
though not for the reason I expected.

gcc does generate the right calling sequence with it - saves r2,
loads the target into r12, uses mtctr/bctrl, restores r2:

   24:    std     r2,24(r1)
   30:    addis   r12,r2,0
             30: R_PPC64_PLT16_HA    _setjmp
   34:    ld      r12,0(r12)
             34: R_PPC64_PLT16_LO_DS    _setjmp
   80:    mtctr   r12
             80: R_PPC64_PLTSEQ    longjmp
   84:    bctrl
             84: R_PPC64_PLTCALL    longjmp
   88:    ld      r2,24(r1)

But it obtains the target address from a PLT slot addressed off r2,
so the calls need R_PPC64_PLT16_HA / R_PPC64_PLT16_LO_DS, which BFD's generic linker rejects:

warning: Compiled module "/tmp/gdbobj-6tZVEd/out1.o" section ".text": dangerous relocation: generic linker can't handle R_PPC64_PLT16_HA
warning: Compiled module "/tmp/gdbobj-6tZVEd/out1.o" section ".text": dangerous relocation: generic linker can't handle R_PPC64_PLT16_LO_DS

|-mlongcall| also converts the intra-module call to a PLT call,
so it fails earlier than before - in|_gdb_expr| rather than in the callee.
Same result with|-mcmodel=large -mlongcall| and with|-fno-plt -mlongcall| (|-mno-plt| is not recognised on PowerPC).

So on PowerPC|-mlongcall| gives the correct convention but still requires a PLT,
which is the one thing GDB can't supply. Resolving PLT16 would mean building a table
within +-32KB of the module's TOC and computing slot offsets - more machinery than the stub, not less.

With the patch, GDB builds the target address as immediates instead, needing no table:

call site:
   bl      <stub>
   ld      r2,24(r1)          ; the nop, rewritten
stub:
   std     r2,24(r1)
   lis     r12,target@highest
   ori     r12,r12,target@higher
   rldicr  r12,r12,32,31
   oris    r12,r12,target@h
   ori     r12,r12,target@l
   mtctr   r12
   bctr

At entry to|_setjmp|, r12 holds the callee's entry address and r2 the correct TOC;
before the patch r2 pointed past the end of libc, which is the SIGSEGV.

The patch applies cleanly to master and gives 526 passes, 0 failures in gdb.compile on powerpc64le.

Thanks
Abhay


On 18/08/26 17:49, Ulrich Weigand wrote:
> Abhay Kandpal<abhay@linux.ibm.com> wrote:
>
>> The compile command loads a module into inferior memory and relocates
>> it itself, without a linker.  For R_PPC64_REL24 it patches the branch
>> to point directly at the target.  On ELFv2 that is not a valid call to
>> another module: the callee derives its TOC pointer from r12, which
> only
>> a PLT-style call stub sets up, and the caller's TOC pointer is never
>> restored because the nop following the bl is left alone.
> On other platforms, the way this is supposed to work is to use a
> set of compiler command-line options that result in code that does
> not require PLTs for external calls.  Typically, this means to use
> -mcmodel=large.
>
> However, it seems that on PowerPC, while that option exists, it
> generates code that still needs PLTs.  There is another option
> -mlongcall that should avoid this, however.
>
> I'm wondering if we were to just add -mlongcall to the platform-
> specific compiler options for PowerPC, we could fix this issue
> without having to reimplement a full PLT solution in GDB ...
>
> Bye,
> Ulrich

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