linux-kernel - Re: [Bug Report] bpf: incorrectly pruning runtime execution path

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Message-ID: <CAEf4Bzb0a0zRsT-UfQGR03Bp-7ztxd2OGDTgG3F4fnBpxFme9A@mail.gmail.com>
Date: Fri, 15 Dec 2023 09:01:20 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Hao Sun <sunhao.th@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, bpf <bpf@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Bug Report] bpf: incorrectly pruning runtime execution path

On Fri, Dec 15, 2023 at 8:22 AM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Thu, 2023-12-14 at 21:20 -0800, Andrii Nakryiko wrote:
> [...]
> > > > can we detect that any register link is broken and force checkpoint here?
> > >
> > > Should be possible. I'll try this in the morning and check veristat results.
>
> {still working on this}
>
> > > By the way, I added some stats collection for find_equal_scalars() and see
> > > the following results when run on ./test_progs:
> > > - maximal number of registers with same id per call: 3
> > > - average number of registers with same id per call: 1.4
> >
> > What if we keep 8 extra bytes in jump/instruction history and encode
> > up to 8 linked registers/slots:
> >
> > 1. 1 bit to mark whether it's a src_reg set, or dst_reg set
> > 2. 1 bit to mark whether it's a stack slot or register
> > 3. 6 bits (0..63 values) to record register or slot number
> >
> > If we ever need more than 8 linked registers, we can just forcefully
> > some "links" by resetting some IDs?
>
> That should work as well.
> Probably don't need src/dst bit, as backtracker marks both as precise
> when processing conditional jump.

yeah, probably

>
> You mean "just forcefully [breaking] some "links" by resetting ...", right?

yeah, breaking, sorry, inattentive brain :)

>
> > BTW, is it only conditional jumps that need to record this linked
> > register sets? Did we previously discuss why we don't need this for
> > any other operation?
>
> Don't think that we discussed it.
> Here is my reasoning: the range transfer happens at find_equal_scalars()
> which is called only from check_cond_jmp_op().
> I think there are no other effects IDs have for scalar values.
> Thus, covering conditional jumps seems sufficient.
>
>

yep, makes sense, any other operation is breaking the link