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

[Richard Sandiford <richard.sandiford@arm.com>]

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.

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)