netdev - Re: [PATCH 7/9] x86/unwind/orc: Fall back to using frame pointers for generated code

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Date:   Fri, 14 Jun 2019 08:34:48 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>,
        Kairui Song <kasong@...hat.com>
Subject: Re: [PATCH 7/9] x86/unwind/orc: Fall back to using frame pointers
 for generated code

On Thu, Jun 13, 2019 at 11:00:09PM -0700, Alexei Starovoitov wrote:
> > +	if (src_reg == BPF_REG_FP) {
> > +		/*
> > +		 * If the value was copied from RBP (real frame pointer),
> > +		 * adjust it to the BPF program's frame pointer value.
> > +		 *
> > +		 * add dst, -40
> > +		 */
> > +		EMIT4(add_1mod(0x48, dst_reg), 0x83, add_1reg(0xC0, dst_reg),
> > +		      0xD8);
> > +	}
> > +
> 
> That won't work. Any register can point to a stack.

Right, but if the stack pointer comes from BPF_REG_FP then won't the
above correct it?  Then if the pointer gets passed around to other
registers it will have the correct value.  Or did I miss your point?

> The register can point to a stack of a different JITed function as well.

Do you mean tail calls?  Or something else?  For tail calls the stack is
shared and the stack layout is the same.

-- 
Josh