Re: pahole and gcc-14 issues

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

On Mon, Apr 28, 2025 at 04:36:07PM -0300, Arnaldo Carvalho de Melo wrote:
> On Mon, Apr 28, 2025 at 04:21:14PM +0100, Alan Maguire wrote:
> > In the bad case, the bpf_prog active member:
>  
> > <2><3d594>: Abbrev Number: 4 (DW_TAG_member)
> >     <3d595>   DW_AT_name        : (indirect string, offset: 0x3b976): active
> >     <3d599>   DW_AT_decl_file   : 125
> >     <3d59a>   DW_AT_decl_line   : 1654
> >     <3d59c>   DW_AT_decl_column : 17
> >     <3d59d>   DW_AT_type        : <0x4bd05>
>  
> > ...is a pointer:
> > 
> >  <1><4bd05>: Abbrev Number: 58 (DW_TAG_pointer_type)
> >     <4bd06>   DW_AT_byte_size   : 8
> >     <4bd07>   DW_AT_address_class: 2
> >     <4bd08>   DW_AT_type        : <0x301cd>
>  
> > ...which points at an int
>  
> >  <1><301cd>: Abbrev Number: 214 (DW_TAG_base_type)
> >     <301cf>   DW_AT_byte_size   : 4
> >     <301d0>   DW_AT_encoding    : 5     (signed)
> >     <301d1>   DW_AT_name        : int
> >     <301d5>   DW_AT_name        : int
>  
> > ...but note the the DW_AT_address_class attribute in the latter case and
> > the two DW_AT_name values. We don't use that address attribute in pahole
> > as far as I can see, but it might be enough to cause problems.

I just looked at a vmlinux built with gcc 15 and we have those:

 <1><7812a39>: Abbrev Number: 7 (DW_TAG_structure_type)
    <7812a3a>   DW_AT_name        : (indirect string, offset: 0xc0e4): alloc_tag_counters
    <7812a3e>   DW_AT_byte_size   : 16
    <7812a3f>   DW_AT_decl_file   : 76
    <7812a40>   DW_AT_decl_line   : 18
    <7812a41>   DW_AT_decl_column : 8
    <7812a42>   DW_AT_sibling     : <0x7812a61>
 <2><7812a46>: Abbrev Number: 1 (DW_TAG_member)
    <7812a47>   DW_AT_name        : (indirect string, offset: 0x3c288f): bytes
    <7812a4b>   DW_AT_decl_file   : 76
    <7812a4c>   DW_AT_decl_line   : 19
    <7812a4d>   DW_AT_decl_column : 6
    <7812a4e>   DW_AT_type        : <0x780f217>
    <7812a52>   DW_AT_data_member_location: 0
 <2><7812a53>: Abbrev Number: 1 (DW_TAG_member)
    <7812a54>   DW_AT_name        : (indirect string, offset: 0x255770): calls
    <7812a58>   DW_AT_decl_file   : 76
    <7812a59>   DW_AT_decl_line   : 20
    <7812a5a>   DW_AT_decl_column : 6
    <7812a5b>   DW_AT_type        : <0x780f217>
    <7812a5f>   DW_AT_data_member_location: 8

 <1><7812a88>: Abbrev Number: 80 (DW_TAG_pointer_type)
    <7812a89>   DW_AT_byte_size   : 8
    <7812a89>   DW_AT_address_class: 2
    <7812a89>   DW_AT_type        : <0x7812a39>

 <1><7812a61>: Abbrev Number: 25 (DW_TAG_structure_type)
    <7812a62>   DW_AT_name        : (indirect string, offset: 0x585bd1): alloc_tag
    <7812a66>   DW_AT_byte_size   : 40
    <7812a67>   DW_AT_alignment   : 8
    <7812a68>   DW_AT_decl_file   : 76
    <7812a69>   DW_AT_decl_line   : 28
    <7812a6a>   DW_AT_decl_column : 8
    <7812a6a>   DW_AT_sibling     : <0x7812a88>
 <2><7812a6e>: Abbrev Number: 54 (DW_TAG_member)
    <7812a6f>   DW_AT_name        : ct
    <7812a72>   DW_AT_decl_file   : 76
    <7812a73>   DW_AT_decl_line   : 29
    <7812a74>   DW_AT_decl_column : 19
    <7812a75>   DW_AT_type        : <0x78129ea>
    <7812a79>   DW_AT_alignment   : 8
    <7812a79>   DW_AT_data_member_location: 0
 <2><7812a7a>: Abbrev Number: 1 (DW_TAG_member)
    <7812a7b>   DW_AT_name        : (indirect string, offset: 0x565566): counters
    <7812a7f>   DW_AT_decl_file   : 76
    <7812a80>   DW_AT_decl_line   : 30
    <7812a81>   DW_AT_decl_column : 38
    <7812a82>   DW_AT_type        : <0x7812a88>
    <7812a86>   DW_AT_data_member_location: 32

struct alloc_tag {
        struct codetag                  ct;
        struct alloc_tag_counters __percpu      *counters;
} __aligned(8);

Its the __percpu, something else to catch in the DWARF loader and then
to use when pretty printing.

⬢ [acme@toolbx linux]$ pahole -F dwarf -C alloc_tag ../build/v6.15.0-rc2+/vmlinux
struct alloc_tag {
	struct codetag             ct __attribute__((__aligned__(8))); /*     0    32 */
	struct alloc_tag_counters * counters;            /*    32     8 */

	/* size: 40, cachelines: 1, members: 2 */
	/* forced alignments: 1 */
	/* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));
⬢ [acme@toolbx linux]$

- Arnaldo
 
> Looks like broken DWARF, there should be just one DW_AT_name per
> DW_TAG_base_type, what is the language for the CU where the bad cases
> appear? Is some sort of LTO being used?