linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Skip bounds adjustment for conditional jumps on same register

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Message-ID: <b190c9b2837b28cf579aa38126de50e29e0add32.camel@linux.dev>
Date: Sat, 25 Oct 2025 00:13:19 +0800
From: KaFai Wan <kafai.wan@...ux.dev>
To: Eduard Zingerman <eddyz87@...il.com>, Alexei Starovoitov
	 <alexei.starovoitov@...il.com>
Cc: Yonghong Song <yonghong.song@...ux.dev>, Alexei Starovoitov
 <ast@...nel.org>,  Daniel Borkmann <daniel@...earbox.net>, John Fastabend
 <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, Martin
 KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, KP Singh
 <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,  Shuah Khan
 <shuah@...nel.org>, Paul Chaignon <paul.chaignon@...il.com>, Matan Shachnai
 <m.shachnai@...il.com>, Luis Gerhorst <luis.gerhorst@....de>, 
 colin.i.king@...il.com, Harishankar Vishwanathan
 <harishankar.vishwanathan@...il.com>, bpf <bpf@...r.kernel.org>, LKML
 <linux-kernel@...r.kernel.org>, "open list:KERNEL SELFTEST FRAMEWORK"
 <linux-kselftest@...r.kernel.org>, Kaiyan Mei <M202472210@...t.edu.cn>, 
 Yinhao Hu <dddddd@...t.edu.cn>
Subject: Re: [PATCH bpf-next 1/2] bpf: Skip bounds adjustment for
 conditional jumps on same register

On Thu, 2025-10-23 at 10:38 -0700, Eduard Zingerman wrote:
> On Thu, 2025-10-23 at 19:26 +0800, KaFai Wan wrote:
> 
> [...]
> 
> > > @@ -16173,6 +16173,25 @@ static int is_pkt_ptr_branch_taken(struct
> > > bpf_reg_state *dst_reg,
> > >  static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state
> > > *reg2,
> > >                            u8 opcode, bool is_jmp32)
> > >  {
> > > +       if (reg1 == reg2) {
> > > +               switch (opcode) {
> > > +               case BPF_JGE:
> > > +               case BPF_JLE:
> > > +               case BPF_JSGE:
> > > +               case BPF_JSLE:
> > > +               case BPF_JEQ:
> > > +               case BPF_JSET:
> > 
> > Others are fine, but BPF_JSET on the same register could be 0 (if value is 0).
> > And it's unknown to take the branch if 0 within the range.
> 
> Right, missed that one.
> 
> > 
> > > +                       return 1;
> > > +               case BPF_JGT:
> > > +               case BPF_JLT:
> > > +               case BPF_JSGT:
> > > +               case BPF_JSLT:
> > > +               case BPF_JNE:
> > > +                       return 0;
> > > +               default:
> > > +                       return -1;
> > > +               }
> > > +       }
> > > 
> > > But that's too much code for an artificial case.
> > > Idk, either way is fine with me.
> > 
> > There is is_scalar_branch_taken() in is_branch_taken(), I missed it. I'll a)
> > check the opcode one by one in is_scalar_branch_taken(), and b) keep this patch
> > for unknown BPF_JSET branch.
> 
> Sounds good to me. Note that the logic is correct for both scalar and
> non-scalar cases, so I don't think we have to constrain it to
> is_scalar_branch_taken() (don't think there is a need to check if
> pointer comparisons are allowed, as no new information is inferred
> from comparisons with self).

For non-scalar cases we only allow pointer comparison on pkt_ptr, this check is before
is_branch_taken()

	src_reg = &regs[insn->src_reg];
	if (!(reg_is_pkt_pointer_any(dst_reg) && reg_is_pkt_pointer_any(src_reg)) &&
	    is_pointer_value(env, insn->src_reg)) {
		verbose(env, "R%d pointer comparison prohibited\n",
			insn->src_reg);
		return -EACCES;
	} 

and in the end of check_cond_jmp_op() (after is_branch_taken()), we checked again

	} else if (!try_match_pkt_pointers(insn, dst_reg, &regs[insn->src_reg],
					   this_branch, other_branch) &&
		   is_pointer_value(env, insn->dst_reg)) {
		verbose(env, "R%d pointer comparison prohibited\n",
			insn->dst_reg);
		return -EACCES;
	}

this time we check if it is valid comparison on pkt_ptr in try_match_pkt_pointers(). 

Currently we just allow 4 opcode (BPF_JGT, BPF_JLT, BPF_JGE, BPF_JLE) on pkt_ptr, and with
conditions. But we bypass these prohibits in privileged mode (is_pointer_value() always 
return false in privileged mode).

So the logic skip these prohibits for pkt_ptr in unprivileged mode.

-- 
Thanks,
KaFai