Re: [PATCH] Make sure DW_CFA_advance_loc4 is in the same frag

[Jan Beulich <jbeulich@suse.com>]

On 10.08.2023 04:21, Jinyang He wrote:
> The DW_CFA_advance_loc4 may be in different frags. Then fr_fix
> may caused something wrong. Referenced by commit b9d8f5601bcf
> ("Re: Optimise away eh_frame advance_loc 0").

I'm afraid I don't understand that earlier fix: frag_more(1) there
ought to guarantee fr_fix (once the frag is closed) to be >= 1.
It would then seem to me that ...

> --- a/gas/ehopt.c
> +++ b/gas/ehopt.c
> @@ -386,7 +386,7 @@ check_eh_frame (expressionS *exp, unsigned int *pnbytes)
>  	{
>  	  /* This might be a DW_CFA_advance_loc4.  Record the frag and the
>  	     position within the frag, so that we can change it later.  */
> -	  frag_grow (1);
> +	  frag_grow (1 + 4);
>  	  d->state = state_saw_loc4;
>  	  d->loc4_frag = frag_now;
>  	  d->loc4_fix = frag_now_fix ();

... here instead we also need

	  frag_more (1);
	  d->state = state_saw_loc4;
	  d->loc4_frag = frag_now;
	  d->loc4_fix = frag_now_fix () - 1;

Otoh if frags were always grown (there and here), couldn't we do away
with loc4_frag and loc4_fix?

As to your 2.41 question: You realize the release was already cut?
Putting this (or whichever else) fix there would be likely be okay, but
would have a real effect only if 2.41.1 was ever made.

Jan