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Previously extern function is added as BTF_KIND_VAR. This does not work
well with existing BTF infrastructure as function expected to use
BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO.
This patch added extern function to BTF_KIND_FUNC. The two bits 0:1
of btf_type.info are used to indicate what kind of function it is:
0: static
1: global
2: extern
Differential Revision: https://reviews.llvm.org/D71638
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Currently for extern variables with section attribute, those
BTF_KIND_VARs will not be placed in any DataSec. This is
inconvenient as any other generated BTF_KIND_VAR belongs to
one DataSec. This patch put these extern variables into
".extern" section so bpf loader can have a consistent
processing mechanism for all data sections and variables.
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extern variable usage in BPF is different from traditional
pure user space application. Recent discussion in linux bpf
mailing list has two use cases where debug info types are
required to use extern variables:
- extern types are required to have a suitable interface
in libbpf (bpf loader) to provide kernel config parameters
to bpf programs.
https://lore.kernel.org/bpf/CAEf4BzYCNo5GeVGMhp3fhysQ=_axAf=23PtwaZs-yAyafmXC9g@mail.gmail.com/T/#t
- extern types are required so kernel bpf verifier can
verify program which uses external functions more precisely.
This will make later link with actual external function no
need to reverify.
https://lore.kernel.org/bpf/87eez4odqp.fsf@toke.dk/T/#m8d5c3e87ffe7f2764e02d722cb0d8cbc136880ed
This patch added bpf support to consume such info into BTF,
which can then be used by bpf loader. Function processFuncPrototypes()
only adds extern function definitions into BTF. The functions
with actual definition have been added to BTF in some other places.
Differential Revision: https://reviews.llvm.org/D70697
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