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

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

Jakub Jelinek <jakub@redhat.com> writes:
> On Thu, Jan 12, 2023 at 01:28:59PM +0100, Jakub Jelinek wrote:
>> > Although we don't AFAIK support using DW_CFA_undefined with RA signing,
>> > the failure mode would be non-obvious: it would effectively toggle the
>> > bit on.
>> 
>> We don't install unwind-dw2.h nor give user code access to the how array
>> (and it just lives on the stack of __frame_state_for/uw_init_context_1
>> functions and address of it is passed to functions called from it),
>> so I hope all this is private to the libgcc unwinder.  After all, otherwise
>> e.g. the change how "how" is represented couldn't be done.
>> That said, if new enum entries are added in the generic code, then
>> I think uw_update_context_1 will warn about unhandled case in a switch,
>> unless we e.g. change
>>       case REG_UNSAVED:
>>       case REG_UNDEFINED:
>>         break;
>> to
>>       default:
>> 	break;
>> (and provided that the new enums would want such handling).

If we have a new enum, I think we should handle it explicitly.  The fact
that the information isn't propagated between frames is a key part of
the semantics.

>> Another option is to just define the arch dependent value for how field
>> in the arch code, right now it is unsigned char type, so using say
>> (unsigned char) ~0 or (unsigned char) ~0 and (unsigned char) ~1 as arch
>> specific values might be ok too.
>
> Yet another option would be to define 1-2 extra REG_ values in the generic
> unwind-dw2.h header, but name them
>   REG_ARCH_SPECIFIC_1,
>   REG_ARCH_SPECIFIC_2,
> or so, and then the machine specific code can
> #define REG_AARCH64_TOGGLE_ON REG_ARCH_SPECIFIC_1
> Of course, all this depends on whether the arch specific codes can be
> handled in uw_update_context_1 by doing break; there and nothing else.

Yeah, personally I'd prefer for target-independent code to provide
the toggle representation, even if it isn't widely used.

Thanks,
Richard