Re: [PATCH dwarves v2 1/5] fprintf: Correct names for types with btf_type_tag attribute

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2023-03-27 at 08:46 -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Mar 15, 2023 at 01:04:13AM +0200, Eduard Zingerman escreveu:
> > The following example contains a structure field annotated with
> > btf_type_tag attribute:
> > 
> >     #define __tag1 __attribute__((btf_type_tag("tag1")))
> > 
> >     struct st {
> >       int __tag1 *a;
> >     } g;
> > 
> > It is not printed correctly by `pahole -F dwarf` command:
> > 
> >     $ clang -g -c test.c -o test.o
> >     pahole -F dwarf test.o
> >     struct st {
> >     	tag1 *                     a;                    /*     0     8 */
> >     	...
> >     };
> > 
> > Note the type for variable `a`: `tag1` is printed instead of `int`.
> > This commit teaches `type__fprintf()` and `__tag_name()` logic to skip
> > `DW_TAG_LLVM_annotation` objects that are used to encode type tags.
> 
> I'm applying this now to make progress on this front, but longer term we
> should printf it too, as we want the output to match the original source
> code as much as possible from the type information.

Understood, thank you.

Also, I want to give a heads-up about ongoing discussion in:
https://reviews.llvm.org/D143967

The gist of the discussion is that for the code like:

  volatile __tag("foo") int;
  
Kernel expects BTF to be:

  __tag("foo") -> volatile -> int
  
And I encode it in DWARF as:

  volatile       -> int
    __tag("foo")
    
But GCC guys argue that DWARF should be like this:

  volatile       -> int
                      __tag("foo")

So, to get the BTF to a form acceptable for kernel some additional
pahole modifications might be necessary. (I will work on a prototype
for such modifications this week).

Maybe put this patch-set on-hold until that is resolved?

Thanks,
Eduard

> 
> - Arnaldo
>  
> > Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> > ---
> >  dwarves_fprintf.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/dwarves_fprintf.c b/dwarves_fprintf.c
> > index e8399e7..1e6147a 100644
> > --- a/dwarves_fprintf.c
> > +++ b/dwarves_fprintf.c
> > @@ -572,6 +572,7 @@ static const char *__tag__name(const struct tag *tag, const struct cu *cu,
> >  	case DW_TAG_restrict_type:
> >  	case DW_TAG_atomic_type:
> >  	case DW_TAG_unspecified_type:
> > +	case DW_TAG_LLVM_annotation:
> >  		type = cu__type(cu, tag->type);
> >  		if (type == NULL && tag->type != 0)
> >  			tag__id_not_found_snprintf(bf, len, tag->type);
> > @@ -786,6 +787,10 @@ next_type:
> >  			n = tag__has_type_loop(type, ptype, NULL, 0, fp);
> >  			if (n)
> >  				return printed + n;
> > +			if (ptype->tag == DW_TAG_LLVM_annotation) {
> > +				type = ptype;
> > +				goto next_type;
> > +			}
> >  			if (ptype->tag == DW_TAG_subroutine_type) {
> >  				printed += ftype__fprintf(tag__ftype(ptype),
> >  							  cu, name, 0, 1,
> > @@ -880,6 +885,14 @@ print_modifier: {
> >  		else
> >  			printed += enumeration__fprintf(type, &tconf, fp);
> >  		break;
> > +	case DW_TAG_LLVM_annotation: {
> > +		struct tag *ttype = cu__type(cu, type->type);
> > +		if (ttype) {
> > +			type = ttype;
> > +			goto next_type;
> > +		}
> > +		goto out_type_not_found;
> > +	}
> >  	}
> >  out:
> >  	if (type_expanded)
> > -- 
> > 2.39.1
> > 
>