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

Eduard Zingerman
2023-03-27T12:10:31Z

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
> >
>