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

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Date: Fri, 15 Dec 2023 04:28:20 +0200
From: Eduard Zingerman <eddyz87@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...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 Thu, 2023-12-14 at 18:16 -0800, Alexei Starovoitov wrote:
[...]
> > E.g. for the test-case at hand:
> > 
> >   0: (85) call bpf_get_prandom_u32#7    ; R0=scalar()
> >   1: (bf) r7 = r0                       ; R0=scalar(id=1) R7_w=scalar(id=1)
> >   2: (bf) r8 = r0                       ; R0=scalar(id=1) R8_w=scalar(id=1)
> >   3: (85) call bpf_get_prandom_u32#7    ; R0=scalar()
> >   --- checkpoint #1 r7.id = 1, r8.id = 1 ---
> >   4: (25) if r0 > 0x1 goto pc+0         ; R0=scalar(smin=smin32=0,smax=umax=smax32=umax32=1,...)
> >   --- checkpoint #2 r7.id = 1, r8.id = 1 ---
> >   5: (3d) if r8 >= r0 goto pc+3         ; R0=1 R8=0 | record r8.id=1 in jump history
> >   6: (0f) r8 += r8                      ; R8=0
> 
> 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.

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