netdev - Re: [PATCH bpf-next 5/6] libbpf: support libbpf-provided extern variables

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Date:   Wed, 20 Nov 2019 21:39:13 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 5/6] libbpf: support libbpf-provided extern
 variables

On 11/19/19 7:44 PM, Andrii Nakryiko wrote:
> So, to summarize, we proceed with uint64_t for everything, with added
> bits of weak vs strong handling. Then in parallel we'll work on adding
> BTF for externs and __builtin_extern_resolved (and corresponding new
> kind of BTF relocation) and will keep making this whole API even
> better, while already having something useful and extensible.

I didn't know extern can be weak. That was a good find.
Indeed undefined config_* can be represented as uint64 zero value.
But I still have an issue with 'proceed uint64_t for everything',
since strings and tri-state don't fit into uint64.

How about strtol be applied by libbpf only for things that parse 
successfully (like decimal and hex) and everything else will be 
represented raw ?
Like CONFIG=y will be 121.
CONFIG=m will be 109
CONFIG="abc" will be 0x636261
In other words CONFIG_A is an address and
'extern weak uint64_t CONFIG_A' reads raw bytes from that location.
When that CONFIG_A is undefined in /boot/config.gz the u64 read
from that address will return zero.
u8 read from that address will return zero too.