Re: [PATCH] casts should drop qualifiers

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Tue, Nov 17, 2020 at 3:22 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Would doing the same unqualify_type() in degenerate() be sufficient?

Actually, that's a stupid suggestion. Forget I ever mentioned it.

I should have reacted to Martin Ucker pointing out

> > lvalue conversion drops qualifers in C.  In GCC, this is not
> > implemented correctly as it is unobvervable in standard C
> > (but it using typeof).

with the notable point that it is unobservable outside of "typeof".

I'm not actually entirely sure that is true: if you don't drop
qualifiers, it's potentially observable in code generation, in that a
"volatile" that didn't get dropped might perhaps cause unnecessary
memory ops. But from a kernel variable type standpoint where we want
to just drop qualifiers on variables using "typeof()", maybe the
simplest solution would be just special-casing typeof itself, using
something (entirely untested and probably complete garbage) like this:

  --- a/symbol.c
  +++ b/symbol.c
  @@ -509,6 +509,7 @@ static struct symbol
*examine_pointer_type(struct symbol *sym)

   static struct symbol *examine_typeof(struct symbol *sym)
   {
  +     int lvalue = lvalue_expression(sym->initializer);
        struct symbol *base = evaluate_expression(sym->initializer);
        unsigned long mod = 0;

  @@ -520,6 +521,8 @@ static struct symbol *examine_typeof(struct symbol *sym)
        }
        if (base->type == SYM_BITFIELD)
                warning(base->pos, "typeof applied to bitfield type");
  +     if (!lvalue)
  +             mod &= MOD_QUALIFIER;
        sym->type = SYM_NODE;
        sym->ctype.modifiers = mod;
        sym->ctype.base_type = base;

Hmm?

           Linus