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

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

The 01/03/2023 17:27, Wilco Dijkstra wrote:
> 
> > 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.
> 
> This works fine since all registers are initialized to REG_UNSAVED every frame.
> 
> In v2 I've removed some clutter and encode the signing state in REG_UNSAVED/
> REG_UNDEFINED.

this looks good to me.


> @@ -1206,8 +1205,10 @@ execute_cfa_program (const unsigned char *insn_ptr,
>           /* 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;
> +         if (fs->regs.how[reg] == REG_UNSAVED)
> +           fs->regs.how[reg] = REG_UNDEFINED;
> +         else
> +           fs->regs.how[reg] = REG_UNSAVED;
>  #else
>           /* ??? Hardcoded for SPARC register window configuration.  */
>           if (__LIBGCC_DWARF_FRAME_REGISTERS__ >= 32)

i would keep the assert: how[reg] must be either UNSAVED or UNDEFINED
here, other how[reg] means the toggle cfi instruction is mixed with
incompatible instructions for the pseudo reg.

and i would add a comment about this e.g. saying that UNSAVED/UNDEFINED
how[reg] is used for tracking the return address signing status and
other how[reg] is not allowed here.

otherwise the patch looks good.