Re: ARM Linux EABI: unwinding through a segfault handler

[Paul Brook <paul@codesourcery.com>]

> I'm trying to understand what you mean, but your comments are rather
> obscure. Which file do you think I should change?  Just tell me where to
> look and I'll do it.

glibc/ports/sysdeps/unix/sysv/linux/arm/eabi/sigrestorer.S Currently has:

        .fnstart
        .save {r0-r15}
        .pad #32
        nop
ENTRY(__default_sa_restorer_v2)
        mov     r7, $SYS_ify(sigreturn)
        swi     0x0
        .fnend

This needs changing to:

        .fnstart
        .personality __gnu_personality_sigframe
        nop
ENTRY(__default_sa_restorer_v2)
        mov     r7, $SYS_ify(sigreturn)
        swi     0x0
        .fnend

The implementation of __gnu_personality_sigframe looks something like:

#include <unwind.h>

static void adjust_sp(context, int n)
{
  _Unwind_Word sp;

  _Unwind_VRS_Get (context, _UVRSC_CORE, R_SP, _UVRSD_UINT32, &sp);
  sp += n;
  _Unwind_VRS_Set (context, _UVRSC_CORE, R_SP, _UVRSD_UINT32, &sp);
}

static uint32_t get_cpsr(context)
{
  return sp[...];
}

_Unwind_Reason_Code
__gnu_personality_sigframe (_Unwind_State state,
                        _Unwind_Control_Block *ucbp,
                        _Unwind_Context *context)
{
  int reg;
  uint32_t cpsr;
  uint32_t pc;
  uint32_t *sp;

  /* We never have any handlers or cleanups.
   Just Unwind the signal frame and keep going.  */

  _Unwind_VRS_Get (context, _UVRSC_CORE, R_SP, _UVRSD_UINT32, &sp);
  sp -= ...;
  if (GLRO (dl_hwcap) & HWCAP_ARM_VFP)
    {
      foreach(vfp_reg)
        _Unwind_VRS_Set(context, _UVRSC_VFP, ..., _UVRSD_DOUBLE, &sp[...]);
      /* Don't forget about VFP3 (i.e. D16-D31 if they exist).  */
    }
  foreach(core_reg)
    _Unwind_VRS_Set (context, _UVRSC_CORE, ..., _UVRSD_UINT32, &sp[...]);
  pc = sp[...];
  cpsr = sp[...];
  /* Set mode bit from saved CPSR.  */
  pc &= ~1;
  if (cpsr & CPSR_T)
    pc |= 1;
  /* Advance PC past the faulting instruction.  */
  if ((cpsr & CPSR_T) && (*(uint16_t *)(pc & ~1) < 0xe800))
    pc += 2;
  else
    pc += 4;
  _Unwind_VRS_Set (context, _UVRSC_CORE, R_PC, _UVRSD_UINT32, &pc);

  /* We don't/can't restore CPSR and FPSR.  However the EABI requires these
     have fixed values at public entry points.  Hope that subsequent
     catch/cleanup handlers are ok with that value.  */
  return _URC_CONTINUE_UNWIND;
}

For bonus points have __gnu_personality_sigframe automagically handle bit old 
and new layouts, removing the _v1/_v2 hacks.

Paul