Re: [PATCH] libgcc: Fix uninitialized RA signing on AArch64 [PR107678]

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 12/05/2022 19:04, Richard Sandiford wrote:
> Wilco Dijkstra <Wilco.Dijkstra@arm.com> writes:
> > A recent change only initializes the regs.how[] during Dwarf unwinding
> > which resulted in an uninitialized offset used in return address signing
> > and random failures during unwinding.  The fix is to use REG_SAVED_OFFSET
> > as the state where the return address signing bit is valid, and if the
> > state is REG_UNSAVED, initialize it to 0.
> >
> > Passes bootstrap & regress, OK for commit?
> >
> > libgcc/
> >         PR target/107678
> >         * unwind-dw2.c (execute_cfa_program): Initialize offset of
> >         DWARF_REGNUM_AARCH64_RA_STATE if in REG_UNSAVED state.
> >         * config/aarch64/aarch64-unwind.h (aarch64_frob_update_contex):
> >         Check state is REG_SAVED_OFFSET before using offset for RA state.
> >
> > ---
> >
> > diff --git a/libgcc/config/aarch64/aarch64-unwind.h b/libgcc/config/aarch64/aarch64-unwind.h
> > index 26db9cbd9e5c526e0c410a4fc6be2bedb7d261cf..597133b3d708a50a366c8bfeff57475f5522b3f6 100644
> > --- a/libgcc/config/aarch64/aarch64-unwind.h
> > +++ b/libgcc/config/aarch64/aarch64-unwind.h
> > @@ -71,21 +71,15 @@ aarch64_demangle_return_addr (struct _Unwind_Context *context,
> >  }
> >  
> >  /* Do AArch64 private initialization on CONTEXT based on frame info FS.  Mark
> > -   CONTEXT as return address signed if bit 0 of DWARF_REGNUM_AARCH64_RA_STATE is
> > -   set.  */
> > +   CONTEXT as having a signed return address if DWARF_REGNUM_AARCH64_RA_STATE
> > +   is initialized (REG_SAVED_OFFSET state) and the offset has bit 0 set.  */
> >  
> >  static inline void
> >  aarch64_frob_update_context (struct _Unwind_Context *context,
> >  			     _Unwind_FrameState *fs)
> >  {
> > -  const int reg = DWARF_REGNUM_AARCH64_RA_STATE;
> > -  int ra_signed;
> > -  if (fs->regs.how[reg] == REG_UNSAVED)
> > -    ra_signed = fs->regs.reg[reg].loc.offset & 0x1;
> > -  else
> > -    ra_signed = _Unwind_GetGR (context, reg) & 0x1;
> > -  if (ra_signed)
> > -    /* The flag is used for re-authenticating EH handler's address.  */
> > +  if (fs->regs.how[DWARF_REGNUM_AARCH64_RA_STATE] == REG_SAVED_OFFSET
> > +      && (fs->regs.reg[DWARF_REGNUM_AARCH64_RA_STATE].loc.offset & 1) != 0)
> >      context->flags |= RA_SIGNED_BIT;
> >    else
> >      context->flags &= ~RA_SIGNED_BIT;
> 
> Hmm, but the point of the original patch was to support code generators
> that emit DW_CFA_val_expression instead of DW_CFA_AARCH64_negate_ra_state.
> Doesn't this patch undo that?
> 
> Also, if I understood correctly, the reason we use REG_UNSAVED is to
> ensure that state from one frame isn't carried across to a parent frame,
> in cases where the parent frame lacks any signing.  That is, each frame
> should start out with a zero bit even if a child frame is unwound while
> it has a set bit.

yes.

i don't think how[*RA_STATE] can ever be set to REG_SAVED_OFFSET,
this pseudo reg is not spilled to the stack, it is reset to 0 in
each frame and then toggled within a frame.

unwind-dw2.c has

	case DW_CFA_GNU_window_save:
#if defined (__aarch64__) && !defined (__ILP32__)
	  /* This CFA is multiplexed with Sparc.  On AArch64 it's used to toggle
	     return address signing status.  */
	  reg = DWARF_REGNUM_AARCH64_RA_STATE;
	  gcc_assert (fs->regs.how[reg] == REG_UNSAVED);
	  fs->regs.reg[reg].loc.offset ^= 1;

for this to work, loc.offset must be reset in uw_frame_state_for.
we may need a new hook for that.


> 
> Thanks,
> Richard
> 
> > diff --git a/libgcc/unwind-dw2.c b/libgcc/unwind-dw2.c
> > index eaceace20298b9b13344aff9d1fe9ee5f9c7bd73..87f2ae065b67982ce48f74e45523d9c754a7661c 100644
> > --- a/libgcc/unwind-dw2.c
> > +++ b/libgcc/unwind-dw2.c
> > @@ -1203,11 +1203,16 @@ execute_cfa_program (const unsigned char *insn_ptr,
> >  
> >  	case DW_CFA_GNU_window_save:
> >  #if defined (__aarch64__) && !defined (__ILP32__)
> > -	  /* This CFA is multiplexed with Sparc.  On AArch64 it's used to toggle
> > -	     return address signing status.  */
> > -	  reg = DWARF_REGNUM_AARCH64_RA_STATE;
> > -	  gcc_assert (fs->regs.how[reg] == REG_UNSAVED);
> > -	  fs->regs.reg[reg].loc.offset ^= 1;
> > +	 /* This CFA is multiplexed with Sparc.  On AArch64 it's used to toggle
> > +	    the return address signing status.  It is initialized at the first
> > +	    use and the state is stored in bit 0 of the offset.  */
> > +	 reg = DWARF_REGNUM_AARCH64_RA_STATE;
> > +	 if (fs->regs.how[reg] == REG_UNSAVED)
> > +	   {
> > +	     fs->regs.how[reg] = REG_SAVED_OFFSET;
> > +	     fs->regs.reg[reg].loc.offset = 0;
> > +	   }
> > +	 fs->regs.reg[reg].loc.offset ^= 1;
> >  #else
> >  	  /* ??? Hardcoded for SPARC register window configuration.  */
> >  	  if (__LIBGCC_DWARF_FRAME_REGISTERS__ >= 32)