Received-SPF: pass (google.com: domain of linux-kernel+bounces-1345-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Precedence: bulk
MIME-Version: 1.0
References: <CACkBjsbj4y4EhqpV-ZVt645UtERJRTxfEab21jXD1ahPyzH4_g@mail.gmail.com>
 <CAEf4BzZ0xidVCqB47XnkXcNhkPWF6_nTV7yt+_Lf0kcFEut2Mg@mail.gmail.com>
 <CACkBjsaEQxCaZ0ERRnBXduBqdw3MXB5r7naJx_anqxi0Wa-M_Q@mail.gmail.com>
 <480a5cfefc23446f7c82c5b87eef6306364132b9.camel@gmail.com>
 <917DAD9F-8697-45B8-8890-D33393F6CDF1@gmail.com> <9dee19c7d39795242c15b2f7aa56fb4a6c3ebffa.camel@gmail.com>
 <73d021e3f77161668aae833e478b210ed5cd2f4d.camel@gmail.com>
 <CAEf4BzYuV3odyj8A77ZW8H9jyx_YLhAkSiM+1hkvtH=OYcHL3w@mail.gmail.com>
 <526d4ac8f6788d3323d29fdbad0e0e5d09a534db.camel@gmail.com>
 <2b49b96de9f8a1cd6d78cc5aebe7c35776cd2c19.camel@gmail.com>
 <CAADnVQ+RVT1pO1hTzMawdkfc9B0xAxas2XmSk6+_EiqX9Xy9Ug@mail.gmail.com>
 <66b2a6c45045c207d8452ad3b5786a9dc0082d79.camel@gmail.com>
 <CAEf4BzaTTv7oP2vcfVYXjUnA958MqohkRDJ9J7qOCtGfpijROw@mail.gmail.com> <149715b52bdd9ec9453a8a817d8339bd1a86a4f7.camel@gmail.com>
In-Reply-To: <149715b52bdd9ec9453a8a817d8339bd1a86a4f7.camel@gmail.com>
From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date: Fri, 15 Dec 2023 09:01:20 -0800
Message-ID: <CAEf4Bzb0a0zRsT-UfQGR03Bp-7ztxd2OGDTgG3F4fnBpxFme9A@mail.gmail.com>
Subject: Re: [Bug Report] bpf: incorrectly pruning runtime execution path
To: Eduard Zingerman <eddyz87@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>, Hao Sun <sunhao.th@gmail.com>, 
	Alexei Starovoitov <ast@kernel.org>, Andrii Nakryiko <andrii@kernel.org>, 
	Daniel Borkmann <daniel@iogearbox.net>, bpf <bpf@vger.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, Dec 15, 2023 at 8:22=E2=80=AFAM Eduard Zingerman <eddyz87@gmail.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 r=
esults.
>
> {still working on this}
>
> > > By the way, I added some stats collection for find_equal_scalars() an=
d 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 ...", righ=
t?

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