Re: bpftool gen object doesn't handle GCC built BPF ELF files

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

>> On Wed, Jul 6, 2022 at 10:13 AM James Hilliard
>> <james.hilliard1@gmail.com> wrote:
>>>
>>> Note I'm testing with the following patches:
>>> https://lore.kernel.org/bpf/20220706111839.1247911-1-james.hilliard1@gmail.com/
>>> https://lore.kernel.org/bpf/20220706140623.2917858-1-james.hilliard1@gmail.com/
>>>
>>> It would appear there's some compatibility issues with bpftool gen and
>>> GCC, not sure what side though is wrong here:
>>> /home/buildroot/buildroot/output/per-package/systemd/host/sbin/bpftool
>>> gen object src/core/bpf/restrict_ifaces/restrict-ifaces.bpf.o
>>> src/core/bpf/restrict_ifaces/restrict-ifaces.bpf.unstripped.o
>>> libbpf: failed to find BTF info for global/extern symbol 'sd_restrictif_i'
>>> Error: failed to link
>>> 'src/core/bpf/restrict_ifaces/restrict-ifaces.bpf.unstripped.o':
>>> Unknown error -2 (-2)
>>>
>>> Relevant difference seems to be this:
>>> GCC:
>>> [55] FUNC 'sd_restrictif_i' type_id=47 linkage=static
>>> Clang:
>>> [27] FUNC 'sd_restrictif_i' type_id=26 linkage=global
>
> For functions GCC generates a BTF_KIND_FUNC entry, which has no linkage
> information, or so we thought: I just looked at bpftool/btf.c and I
> found the linkage info for function types is expected to be encoded in
> the vlen field of BTF_KIND_FUNC entries (why not adding a btf_func
> instead???) which is surprising to say the least.
>
> We are changing GCC to encode the linkage info in vlen for these types.
> Thanks for reporting this.

Patch sent to GCC upstream:
https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598090.html

>> GCC is wrong, clearly. This function is global ([0]) and libbpf
>> expects it to be marked as such in BTF.
>>
>> https://github.com/systemd/systemd/blob/main/src/core/bpf/restrict_ifaces/restrict-ifaces.bpf.c#L42-L50
>>
>>
>>> GCC:
>>>
>>> [1] INT 'signed char' size=1 bits_offset=0 nr_bits=8 encoding=UNKN
>>> [2] INT 'unsigned char' size=1 bits_offset=0 nr_bits=8 encoding=CHAR
>>> [3] TYPEDEF '__u8' type_id=2
>>> [4] CONST '(anon)' type_id=3
>>
>> [...]