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From:   KP Singh <kpsingh@chromium.org>
Date:   Wed, 4 Mar 2020 02:26:02 +0100
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        open list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Paul Turner <pjt@google.com>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>
Subject: Re: [PATCH bpf-next 2/7] bpf: JIT helpers for fmod_ret progs
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On 03-M?r 15:56, Alexei Starovoitov wrote:
> On Tue, Mar 03, 2020 at 11:28:12PM +0100, KP Singh wrote:
> > > > +static void align16_branch_target(u8 **pprog)
> > > > +{
> > > > +       u8 *target, *prog = *pprog;
> > > > +
> > > > +       target = PTR_ALIGN(prog, 16);
> > > > +       if (target != prog)
> > > > +               emit_nops(&prog, target - prog);
> > > > +       if (target != prog)
> > > > +               pr_err("calcultion error\n");
> > > 
> > > this wasn't in the original code, do you feel like it's more important
> > > to check this and print error?
> > > 
> > > also typo: calculation error, but then it's a bit brief and
> > > uninformative message. So I don't know, maybe just drop it?
> > 
> > Ah, good catch! this is deinitely not intended to be here.
> > It's a debug artifact and needs to dropped indeed.
> 
> That spurious pr_err() caught my attention as well.
> After further analysis there is a bug here.
> The function is missing last line:
>         *pprog = prog;

Great catch! Fixed.

> Without it the nop insertion is actually not happenning.
> Nops are being written, but next insns will overwrite them.
> When I noticed it by code review I applied the patches to my tree
> and run the tests and, as expected, all tests passed.
> The existing test_xdp_veth.sh emits the most amount of unaligned
> branches. Since then I've been thinking whether we could add a test
> to catch things like this and couldn't come up with a way to test it
> without burning a lot of code on it. So let's fix it and move on.
> Could you rename this helper? May be emit_align() and pass 16 into it?

Seems reasonable. Done.

> The code is not branch target specific. It's aligning the start
> of the next instruction.
> Also could you add a comment to:

Done. Sending v2 out.

- KP

>         align16_branch_target(&prog);
>         for (i = 0; i < fmod_ret->nr_progs; i++)
>                 emit_cond_near_jump(&branches[i], prog, branches[i],
>                                     X86_JNE);
>         kfree(branches);
> to say that the loop is updating prior location to jump to aligned
> branch target ?