linux-kernel - Re: [PATCH v3 07/29] x86: bpf

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Date:   Tue, 25 Apr 2017 16:41:48 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     David Miller <davem@...emloft.net>
Cc:     alexei.starovoitov@...il.com, mingo@...nel.org, mingo@...hat.com,
        tglx@...utronix.de, hpa@...or.com, x86@...nel.org,
        jpoimboe@...hat.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, daniel@...earbox.net, edumazet@...gle.com
Subject: Re: [PATCH v3 07/29] x86: bpf_jit, use ENTRY+ENDPROC

On 04/24/2017, 08:24 PM, David Miller wrote:
> From: Jiri Slaby <jslaby@...e.cz>
> Date: Mon, 24 Apr 2017 19:51:54 +0200
> 
>> For example what's the point of making the sk_load_word_positive_offset
>> label a global, callable function? Note that this is exactly the reason
>> why this particular two hunks look weird to you even though the
>> annotations only mechanically paraphrase what is in the current code.
> 
> So that it can be referenced by the eBPF JIT, because these are
> helpers for eBPF JIT generated code.  Every architecture implementing
> an eBPF JIT has this "mess".

I completely understand the needs for this, but I am complaining about
the way it is written. That is not the best -- unbalanced annotations, C
macros in lowercase (apart from that, C macros in .S need semicolons &
backslashes), FUNC macro, etc.

> You can't even put a tracepoint or kprobe on these things and expect
> to see "arguments" or "return PC" values in the usual spots.  This
> code has special calling conventions and register usage as Alexei
> explained.

Yes, I can see that.

> I would suggest that you read and understand how this assembler is
> designed, how it is called from the generated JIT code, and what it's
> semantics and register usage are, before trying to annotating it.

Of course I studied the code. I only missed macro CHOOSE_LOAD_FUNC which
I see now. So that answers why sk_load_word_positive_offset & similar
are marked as .globl.


But the original question I asked still remains: why do you mind calling
them BPF_FUNC_START & *_END, given:

1) the functions are marked by "FUNC" already:
$ git grep FUNC linus/master arch/x86/net/bpf_jit.S
linus/master:arch/x86/net/bpf_jit.S:#define FUNC(name) \
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_word)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_word_positive_offset)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_half)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_half_positive_offset)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_byte)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_byte_positive_offset)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_word_negative_offset)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_half_negative_offset)
linus/master:arch/x86/net/bpf_jit.S:FUNC(sk_load_byte_negative_offset)

2) they _are_ all callable from within the JIT code:
EMIT1_off32(0xE8, jmp_offset);

Yes, I fucked up the ENDs. They should be on different locations. But
the pieces are still functions from my POV and should be annotated
accordingly.

thanks,
-- 
js
suse labs