netdev - Re: BPF relocations

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Message-ID: <b39b79d6-4fe2-f887-dc7e-03b62d6eb3e4@fb.com>
Date:   Thu, 11 May 2017 16:10:35 -0700
From:   Alexei Starovoitov <ast@...com>
To:     David Miller <davem@...emloft.net>
CC:     <daniel@...earbox.net>, <netdev@...r.kernel.org>
Subject: Re: BPF relocations

On 5/11/17 12:31 PM, David Miller wrote:
>
> I haven't done more work on bintuils BPF support because we
> need to figure out exactly what to do with relocations.  So
> I've been trying to spend time thinking about this.
>
> As far as I can tell the 64-bit BPF relocation llvm uses
> is used in two situations:
>
> 1) 64-bit relocations against data
>
> 2) 64-bit relocations against ldimm64 instructions
>
> If this is true it's a very bad decision that has ramifications for us
> right now.
>
> One must always explicitly define relocations as being against data or
> instruction fields.  You cannot use the same relocation for both kinds
> of transformations, somehow trying to figure out what to do
> "contextually".  That doesn't work.

why it doesn't work?
as far as i can see x86 doesn't care where the relo applies.
afaik relocations are divided into absolute, pc relative and pic 
relative and it doesn't matter whether they're against .text,
.eh_frame or .debug_* sections.

We have just two so far: absolute 32-bit and absolute 64-bit relocation.
I don't see what we would use pc-relative relo for.

If I remember correctly, the x64 and other cpus use pc-relative for
'call foo' insns since this is how those instructions work.
We don't have such calls yet.