Re: BTF generation and pruning (notes from office hours)

[Yonghong Song <yonghong.song@linux.dev>]

On 1/26/24 10:10 AM, Yonghong Song wrote:
>
> On 1/25/24 2:56 PM, David Faust wrote:
>> This morning in the BPF office hours we discussed BTF, starting from
>> some specific cases where gcc and clang differ, and ending up at the
>> broader question of what precisely should or should not be present
>> in generated BTF info and in what cases.
>>
>> Below is a summary/notes on the discussion so far. Apologies if I've
>> forgotten anything.
>>
>> Motivation: there are some cases where gcc emits more BTF information
>> than clang, in particular (not necessarily exhaustive):
>>    + clang does not emit BTF for unused static vars
>>    + clang does not emit BTF for variables which have been optimized
>>      away entirely
>>    + clang does not emit BTF for types which are only used by one
>>      of the above
>>    (See a couple of concrete examples at the bottom.)
>
> that is correct.
>
[...]
>
>> - This also comes with some drawbacks, in some cases BTF will not
>>    be emitted when it is desired. There is a BTF_TYPE_EMIT macro to
>>    work around that. It isn't a perfect solution.
>
> This is due to dwarf. The type most likely not in dwarf,
> I will take a look.

In yesterday's bpf office hour meeting, Alexei mentioned that struct
bpf_timer needs "BTF_TYPE_EMIT(struct bpf_timer)" to force dwarf
generating an entry although there are usages of "struct bpf_timer"
in the kernel.

I checked and found the only usage of "struct bpf_timer" is
sizeof(...) and __alignof__(...).

e.g.,
helpers.c:      BUILD_BUG_ON(sizeof(struct bpf_timer_kern) > sizeof(struct bpf_timer));
helpers.c:      BUILD_BUG_ON(__alignof__(struct bpf_timer_kern) != __alignof__(struct bpf_timer));

and unfortunately dwarf is not generated for such cases. The frontend
resolves the above sizeof(...) and __alignof__(...) as the constant before
generating debug info inside the compiler.

For example,

$ cat t2.c
struct bpf_timer {
         int a;
         int b;
};
                                                                                                                                      
int foo() {
   int v1, v2;
   v1 = sizeof(struct bpf_timer);
   v2 = __alignof__(struct bpf_timer);
   return v1 + v2;
}
$ clang -O2 -g -c t2.c
$ llvm-dwarfdump t2.o
t2.o:   file format elf64-x86-64
                                                                                                                                      
.debug_info contents:
0x00000000: Compile Unit: length = 0x00000046, format = DWARF32, version = 0x0005, unit_type = DW_UT_compile, abbr_offset = 0x0000, a
ddr_size = 0x08 (next unit at 0x0000004a)
                                                                                                                                      
0x0000000c: DW_TAG_compile_unit
               DW_AT_producer    ("clang version 19.0.0git (https://github.com/llvm/llvm-project.git 6d0080b5de26d8a8682ec6169851af3d0
4e30ccb)")
               DW_AT_language    (DW_LANG_C11)
               DW_AT_name        ("t2.c")
               DW_AT_str_offsets_base    (0x00000008)
               DW_AT_stmt_list   (0x00000000)
               DW_AT_comp_dir    ("/home/yhs/tmp10/btf")
               DW_AT_low_pc      (0x0000000000000000)
               DW_AT_high_pc     (0x0000000000000006)
               DW_AT_addr_base   (0x00000008)

0x00000023:   DW_TAG_subprogram
                 DW_AT_low_pc    (0x0000000000000000)
                 DW_AT_high_pc   (0x0000000000000006)
                 DW_AT_frame_base        (DW_OP_reg7 RSP)
                 DW_AT_call_all_calls    (true)
                 DW_AT_name      ("foo")
                 DW_AT_decl_file ("/home/yhs/tmp10/btf/t2.c")
                 DW_AT_decl_line (6)
                 DW_AT_type      (0x00000045 "int")
                 DW_AT_external  (true)

0x00000032:     DW_TAG_variable
                   DW_AT_const_value     (8)
                   DW_AT_name    ("v1")
                   DW_AT_decl_file       ("/home/yhs/tmp10/btf/t2.c")
                   DW_AT_decl_line       (7)
                   DW_AT_type    (0x00000045 "int")

0x0000003b:     DW_TAG_variable
                   DW_AT_const_value     (4)
                   DW_AT_name    ("v2")
                   DW_AT_decl_file       ("/home/yhs/tmp10/btf/t2.c")
                   DW_AT_decl_line       (7)
                   DW_AT_type    (0x00000045 "int")

0x00000044:     NULL

0x00000045:   DW_TAG_base_type
                 DW_AT_name      ("int")
                 DW_AT_encoding  (DW_ATE_signed)
                 DW_AT_byte_size (0x04)

0x00000049:   NULL

>
>>
>> So, the question is twofold:
>> 1. What ought to be represented in BTF for a BPF program?
>> 2. Is that/should that be followed for non-BPF program cases, such
>>     as generating BTF for vmlinux?
>>
[...]