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

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Message-ID: <90ec497a230584b0e627d12eaf172236b7a5165b.camel@linux.dev>
Date: Sat, 25 Oct 2025 00:37:51 +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 Fri, 2025-10-24 at 09:21 -0700, Eduard Zingerman wrote:
> On Sat, 2025-10-25 at 00:13 +0800, KaFai Wan wrote:
> 
> [...]
> 
> > 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.
> 
> Well, yes, but do you really need to do forbid `if r0 > r0 goto ...` in unpriv?

Currently `if r0 > r0 goto ...` is forbid in unpriv, but we can allow it. 

-- 
Thanks,
KaFai