Re: [PATCH v6 net-next 4/6] bpf: enable bpf syscall on x64 and i386

From: Alexei Starovoitov
Date: Mon Aug 25 2014 - 21:44:04 EST

On Mon, Aug 25, 2014 at 6:07 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
> From: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> Date: Mon, 25 Aug 2014 18:00:56 -0700
>> -
>> +asmlinkage long sys_bpf(int cmd, unsigned long arg2, unsigned long arg3,
>> + unsigned long arg4, unsigned long arg5);
> Please do not add interfaces with opaque types as arguments.
> It is impossible for the compiler to type check the args at
> compile time when userspace tries to use this stuff.

I share this concern. I went with single BPF syscall, because
alternative is 6 syscalls for every command and more
syscalls in the future when we'd need to add another command.
I think type casting is much lesser evil.
We already have similar muxing syscalls.
It feels to me that single mux/demux syscall is easier to support,
document, add new commands. Type casting, yeah, not pretty.
Most users will be using wrappers similar to those I've defined in libbpf.h
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