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

[Indu Bhagat <indu.bhagat@oracle.com>]

On 7/6/22 10:20 AM, Andrii Nakryiko wrote:
> 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
>>
> 
> 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
> 

How about updating the BTF format documentation in btf.rst to reflect 
the specification for BTF_KIND_FUNC ?

Thanks

diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
index f49aeef62d0c..b3a9d5ac882c 100644
--- a/Documentation/bpf/btf.rst
+++ b/Documentation/bpf/btf.rst
@@ -369,7 +369,7 @@ No additional type data follow ``btf_type``.
    * ``name_off``: offset to a valid C identifier
    * ``info.kind_flag``: 0
    * ``info.kind``: BTF_KIND_FUNC
-  * ``info.vlen``: 0
+  * ``info.vlen``: linkage information (static=0, global=1)
    * ``type``: a BTF_KIND_FUNC_PROTO type

  No additional type data follow ``btf_type``.

> 
>> 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
> 
> [...]
>