PR 32721, internal error in tc-i386.c:parse_register

[Alan Modra]

On Thu, Feb 20, 2025 at 08:58:22AM +0100, Jan Beulich wrote:
> On 20.02.2025 02:33, Alan Modra wrote:
> > pr30117 showed one of the assertions added by 4d1bb7955a8b was too
> > strict.  oss-fuzz also found the second assertion to be too strict,
> > with this testcase distilled from 7k of garbage source:
> > 
> >  A=%eax%%!
> >  Y=A
> >  Z=A
> >  or $6,Z
> > 
> > 	PR 32721
> > 	* config/tc-i386.c (parse_register): Move "know" into
> > 	condition.  Simplify.
> 
> Hmm, I will want to look into this in more detail. We better wouldn't ...
> 
> > --- a/gas/config/tc-i386.c
> > +++ b/gas/config/tc-i386.c
> > @@ -16838,10 +16838,9 @@ parse_register (const char *reg_string, char **end_op)
> >  	{
> >  	  const expressionS *e = symbol_get_value_expression (symbolP);
> >  
> > -	  if (e->X_op == O_register)
> > +	  if (e->X_op == O_register
> > +	      && (valueT) e->X_add_number < i386_regtab_size)
> >  	    {
> > -	      know (e->X_add_number >= 0
> > -		    && (valueT) e->X_add_number < i386_regtab_size);
> 
> ... manufacture O_register expressions not representing any valid register.
> IOW I think the assertion is legitimate and a fix is going to be needed
> elsewhere.

Yes, there is something going on in expression evaluation that should
be investigated.  For example, it is very odd that the line "Y=A" is
necessary to trigger the bug.

This testcase also triggers the assertion:

 A=%eax-999
 Y=A
 Z=A
 or $6,Z

-- 
Alan Modra