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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Mar 29, 2023 at 06:36:34PM +0300, Eduard Zingerman escreveu:
> On Tue, 2023-03-28 at 18:17 -0300, Arnaldo Carvalho de Melo wrote:
> > Em Tue, Mar 28, 2023 at 06:30:05PM +0300, Eduard Zingerman escreveu:
> > > On Tue, 2023-03-28 at 12:26 -0300, Arnaldo Carvalho de Melo wrote:
> > > [...] 
> > > > Sure, but look:
> > > > 
> > > > > > -       struct qdisc_size_table *  stab;                 /*    32     8 */
> > > > > > +       struct qdisc_size_table    stab;                 /*    32     8 */
> > > > 
> > > > Its the DW_TAG_pointer_type that is getting lost somehow:
> > > > 
> > > >  <1><b0af32>: Abbrev Number: 81 (DW_TAG_structure_type)
> > > >     <b0af33>   DW_AT_name        : (indirect string, offset: 0xe825): Qdisc
> > > >     <b0af37>   DW_AT_byte_size   : 384
> > > >     <b0af39>   DW_AT_decl_file   : 223
> > > >     <b0af3a>   DW_AT_decl_line   : 72
> > > > 
> > > > <SNIP>
> > > > 
> > > >  <2><b0af77>: Abbrev Number: 65 (DW_TAG_member)
> > > >     <b0af78>   DW_AT_name        : (indirect string, offset: 0x4745ff): stab
> > > >     <b0af7c>   DW_AT_type        : <0xb0b76b>
> > > >     <b0af80>   DW_AT_decl_file   : 223
> > > >     <b0af81>   DW_AT_decl_line   : 99
> > > >     <b0af82>   DW_AT_data_member_location: 32
> > > > 
> > > > <SNIP>
> > > > 
> > > > <1><b0b76b>: Abbrev Number: 61 (DW_TAG_pointer_type)
> > > >     <b0b76c>   DW_AT_type        : <0xb0b77a>
> > > >  <2><b0b770>: Abbrev Number: 62 (User TAG value: 0x6000)
> > > >     <b0b771>   DW_AT_name        : (indirect string, offset: 0x378): btf_type_tag
> > > >     <b0b775>   DW_AT_const_value : (indirect string, offset: 0x6e93): rcu
> > > >  <2><b0b779>: Abbrev Number: 0
> > > >  <1><b0b77a>: Abbrev Number: 69 (DW_TAG_structure_type)
> > > >     <b0b77b>   DW_AT_name        : (indirect string, offset: 0x6e5ed): qdisc_size_table
> > > >     <b0b77f>   DW_AT_byte_size   : 64
> > > >     <b0b780>   DW_AT_decl_file   : 223
> > > >     <b0b781>   DW_AT_decl_line   : 56
> > > > 
> > > >  
> > > > So it's all there in the DWARF info:
> > > > 
> > > >    b0af77 has type 0xb0b76b which is a DW_TAG_pointer_type that has type
> > > >    0xb0b77a, that is DW_TAG_structure_type.
> > > > 
> > > > Now lets see how this was encoded into BTF:
> > > > 
> > > > [4725] STRUCT 'Qdisc' size=384 vlen=28
> > > > <SNIP>
> > > >         'stab' type_id=4790 bits_offset=256
> > > > <SNIP>
> > > > [4790] PTR '(anon)' type_id=4789
> > > > <SNIP>
> > > > [4789] TYPE_TAG 'rcu' type_id=4791
> > > > <SNIP>
> > > > [4791] STRUCT 'qdisc_size_table' size=64 vlen=5
> > > >         'rcu' type_id=320 bits_offset=0
> > > >         'list' type_id=87 bits_offset=128
> > > >         'szopts' type_id=4792 bits_offset=256
> > > >         'refcnt' type_id=16 bits_offset=448
> > > >         'data' type_id=4659 bits_offset=480
> > > > 
> > > > So it all seems ok, we should keep all the info and teach fprintf to
> > > > handle TYPE_TAG.
> > > > 
> > > > Which you tried, but somehow the '*' link is missing.
> > > 
> > > Yep, thanks a lot for the analysis, I will take a look.
> > > Hopefully will send v2 tomorrow.
> > 
> > So, with the patch below it gets equivalent, but some more tweaking is
> > needed to make the output completely match (spaces, "long usigned" ->
> > "unsigned long", which seems to be all equivalent):
> > 
> > --- /tmp/gcc    2023-03-28 18:13:42.022385428 -0300
> > +++ /tmp/clang  2023-03-28 18:13:45.854486885 -0300
> > @@ -73,15 +73,15 @@
> >         unsigned int               flags;                /*    16     4 */
> >         u32                        limit;                /*    20     4 */
> >         const struct Qdisc_ops  *  ops;                  /*    24     8 */
> > -       struct qdisc_size_table *  stab;                 /*    32     8 */
> > +       struct qdisc_size_table  * stab;                 /*    32     8 */
> >         struct hlist_node          hash;                 /*    40    16 */
> >         u32                        handle;               /*    56     4 */
> >         u32                        parent;               /*    60     4 */
> >         /* --- cacheline 1 boundary (64 bytes) --- */
> >         struct netdev_queue *      dev_queue;            /*    64     8 */
> > -       struct net_rate_estimator * rate_est;            /*    72     8 */
> > -       struct gnet_stats_basic_sync * cpu_bstats;       /*    80     8 */
> > -       struct gnet_stats_queue *  cpu_qstats;           /*    88     8 */
> > +       struct net_rate_estimator  * rate_est;           /*    72     8 */
> > +       struct gnet_stats_basic_sync  * cpu_bstats;      /*    80     8 */
> > +       struct gnet_stats_queue  * cpu_qstats;           /*    88     8 */
> >         int                        pad;                  /*    96     4 */
> >         refcount_t                 refcnt;               /*   100     4 */
> > 
> > @@ -96,8 +96,8 @@
> > 
> >         /* XXX 4 bytes hole, try to pack */
> > 
> > -       long unsigned int          state;                /*   216     8 */
> > -       long unsigned int          state2;               /*   224     8 */
> > +       unsigned long              state;                /*   216     8 */
> > +       unsigned long              state2;               /*   224     8 */
> >         struct Qdisc *             next_sched;           /*   232     8 */
> >         struct sk_buff_head        skb_bad_txq;          /*   240    24 */
> > 
> > @@ -112,7 +112,7 @@
> >         /* XXX 40 bytes hole, try to pack */
> > 
> >         /* --- cacheline 6 boundary (384 bytes) --- */
> > -       long int                   privdata[];           /*   384     0 */
> > +       long                       privdata[];           /*   384     0 */
> > 
> >         /* size: 384, cachelines: 6, members: 28 */
> >         /* sum members: 260, holes: 4, sum holes: 124 */
> > @@ -145,19 +145,19 @@
> >         /* XXX 4 bytes hole, try to pack */
> > 
> >         struct netdev_queue *      (*select_queue)(struct Qdisc *, struct tcmsg *); /*     8     8 */
> > -       int                        (*graft)(struct Qdisc *, long unsigned int, struct Qdisc *, struct Qdisc * *, struct netlink_ext_ack *); /*    16     8 */
> > +       int                        (*graft)(struct Qdisc *, unsigned long, struct Qdisc *, struct Qdisc * *, struct netlink_ext_ack *); /*    16     8 */
> > -       struct Qdisc *             (*leaf)(struct Qdisc *, long unsigned int); /*    24     8 */
> > +       struct Qdisc *             (*leaf)(struct Qdisc *, unsigned long); /*    24     8 */
> > -       void                       (*qlen_notify)(struct Qdisc *, long unsigned int); /*    32     8 */
> > +       void                       (*qlen_notify)(struct Qdisc *, unsigned long); /*    32     8 */
> > -       long unsigned int          (*find)(struct Qdisc *, u32); /*    40     8 */
> > +       unsigned long              (*find)(struct Qdisc *, u32); /*    40     8 */
> > -       int                        (*change)(struct Qdisc *, u32, u32, struct nlattr * *, long unsigned int *, struct netlink_ext_ack *); /*    48     8 */
> > +       int                        (*change)(struct Qdisc *, u32, u32, struct nlattr * *, unsigned long *, struct netlink_ext_ack *); /*    48     8 */
> > -       int                        (*delete)(struct Qdisc *, long unsigned int, struct netlink_ext_ack *); /*    56     8 */
> > +       int                        (*delete)(struct Qdisc *, unsigned long, struct netlink_ext_ack *); /*    56     8 */
> > 
> > diff --git a/dwarves_fprintf.c b/dwarves_fprintf.c
> > index 1e6147a82056c188..1ecc24321bf8f975 100644
> > --- a/dwarves_fprintf.c
> > +++ b/dwarves_fprintf.c
> > @@ -788,8 +788,15 @@ next_type:
> >  			if (n)
> >  				return printed + n;
> >  			if (ptype->tag == DW_TAG_LLVM_annotation) {
> > -				type = ptype;
> > -				goto next_type;
> > +				// FIXME: Just skip this for now, we need to print this annotation
> > +				// to match the original source code.
> > +
> > +				if (ptype->type == 0) {
> > +					printed += fprintf(fp, "%-*s %s", tconf.type_spacing, "void *", name);
> > +					break;
> > +				}
> > +
> > +				ptype = cu__type(cu, ptype->type);
> >  			}
> >  			if (ptype->tag == DW_TAG_subroutine_type) {
> >  				printed += ftype__fprintf(tag__ftype(ptype),
> 
> This explains why '*' was missing, but unfortunately it breaks apart
> when there are multiple type tags, e.g.:
> 
> 
>     $ cat tag-test.c
>     #define __t __attribute__((btf_type_tag("t1")))
>     
>     struct foo {
>       int  (__t __t *a)(void);
>     } g;
>     $ clang -g -c tag-test.c -o tag-test.o && pahole -J tag-test.o && pahole --sort -F dwarf tag-test.o
>     struct foo {
>     	int ()(void)   *           a;                    /*     0     8 */
>     
>     	/* size: 8, cachelines: 1, members: 1 */
>     	/* last cacheline: 8 bytes */
>     };
>     $ clang -g -c tag-test.c -o tag-test.o && pahole -J tag-test.o && pahole --sort -F btf tag-test.o
>     struct foo {
>     	int ()(void)   *           a;                    /*     0     8 */
>     
>     	/* size: 8, cachelines: 1, members: 1 */
>     	/* last cacheline: 8 bytes */
>     };
>     
> What I don't understand is why pointer's type is LLVM_annotation.

Well, that is how it is encoded in BTF and then you supported it in:

Author: Eduard Zingerman <eddyz87@gmail.com>
Date:   Wed Mar 15 01:04:14 2023 +0200

    btf_loader: A hack for BTF import of btf_type_tag attributes`


I find it natural, and think that annots thing is a variation on how to
store modifiers for types, see, this DW_TAG_LLVM_annotation is in the
same class as 'restrict', 'const', 'volatile', "atomic", etc

I understand that for encoding _DWARF_ people preferred to make it as a
child DIE to avoid breaking existing DWARF consumers, but in
pahole's dwarf_loader.c we can just make it work like BTF and insert the
btf_type_tag in the chain, just like 'const', etc, no?

pahole wants to print those annotation just like it prints 'const',
'volatile', etc.

> Probably, the cleanest solution would be to make DWARF and BTF loaders
> work in a similar way and attach LLVM_annotation as `annots` field of
> the `struct btf_type_tag_ptr_type`. Thus, avoiding 'LLVM_annotation's
> in the middle of type chains. I'll work on this.

-- 

- Arnaldo