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Message-ID: <CAADnVQKdMcOkkqNa3LbGWqsz9iHAODFSinokj6htbGi0N66h_Q@mail.gmail.com>
Date: Wed, 22 Oct 2025 13:12:26 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: Yonghong Song <yonghong.song@...ux.dev>, KaFai Wan <kafai.wan@...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 Wed, Oct 22, 2025 at 12:46 PM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Wed, 2025-10-22 at 11:14 -0700, Yonghong Song wrote:
> >
> > On 10/22/25 9:44 AM, KaFai Wan wrote:
> > > When conditional jumps are performed on the same register (e.g., r0 <= r0,
> > > r0 > r0, r0 < r0) where the register holds a scalar with range, the verifier
> > > incorrectly attempts to adjust the register's min/max bounds. This leads to
> > > invalid range bounds and triggers a BUG warning:
> > >
> > > verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
> > > WARNING: CPU: 0 PID: 93 at kernel/bpf/verifier.c:2731 reg_bounds_sanity_check+0x163/0x220
> > > Modules linked in:
> > > CPU: 0 UID: 0 PID: 93 Comm: repro-x-3 Tainted: G W 6.18.0-rc1-ge7586577b75f-dirty #218 PREEMPT(full)
> > > Tainted: [W]=WARN
> > > Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> > > RIP: 0010:reg_bounds_sanity_check+0x163/0x220
> > > Call Trace:
> > > <TASK>
> > > reg_set_min_max.part.0+0x1b1/0x360
> > > check_cond_jmp_op+0x1195/0x1a60
> > > do_check_common+0x33ac/0x33c0
> > > ...
> > >
> > > The issue occurs in reg_set_min_max() function where bounds adjustment logic
> > > is applied even when both registers being compared are the same. Comparing a
> > > register with itself should not change its bounds since the comparison result
> > > is always known (e.g., r0 == r0 is always true, r0 < r0 is always false).
> > >
> > > Fix this by adding an early return in reg_set_min_max() when false_reg1 and
> > > false_reg2 point to the same register, skipping the unnecessary bounds
> > > adjustment that leads to the verifier bug.
> > >
> > > Reported-by: Kaiyan Mei <M202472210@...t.edu.cn>
> > > Reported-by: Yinhao Hu <dddddd@...t.edu.cn>
> > > Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/
> > > Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
> > > Signed-off-by: KaFai Wan <kafai.wan@...ux.dev>
> > > ---
> > > kernel/bpf/verifier.c | 4 ++++
> > > 1 file changed, 4 insertions(+)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 6d175849e57a..420ad512d1af 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -16429,6 +16429,10 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
> > > if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
> > > return 0;
> > >
> > > + /* If conditional jumps on the same register, skip the adjustment */
> > > + if (false_reg1 == false_reg2)
> > > + return 0;
> >
> > Your change looks good. But this is a special case and it should not
> > happen for any compiler generated code. So could you investigate
> > why regs_refine_cond_op() does not work? Since false_reg1 and false_reg2
> > is the same, so register refinement should keep the same. Probably
> > some minor change in regs_refine_cond_op(...) should work?
> >
> > > +
> > > /* fallthrough (FALSE) branch */
> > > regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32);
> > > reg_bounds_sync(false_reg1);
>
> I think regs_refine_cond_op() is not written in a way to handle same
> registers passed as reg1 and reg2. E.g. in this particular case the
> condition is reformulated as "r0 < r0", and then the following branch
> is taken:
>
> static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
> u8 opcode, bool is_jmp32)
> {
> ...
> case BPF_JLT: // condition is rephrased as r0 < r0
> if (is_jmp32) {
> ...
> } else {
> reg1->umax_value = min(reg1->umax_value, reg2->umax_value - 1);
> reg2->umin_value = max(reg1->umin_value + 1, reg2->umin_value);
> }
> break;
> ...
> }
>
> Note that intent is to adjust umax of the LHS (reg1) register and umin
> of the RHS (reg2) register. But here it ends up adjusting the same register.
>
> (a) before refinement: u64=[0x0, 0x80000000] s64=[0x0, 0x80000000] u32=[0x0, 0x80000000] s32=[0x80000000, 0x0]
> (b) after refinement: u64=[0x1, 0x7fffffff] s64=[0x0, 0x80000000] u32=[0x0, 0x80000000] s32=[0x80000000, 0x0]
> (c) after sync : u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0]
>
> At (b) the u64 range translated to s32 is > 0, while s32 range is <= 0,
> hence the invariant violation.
>
> I think it's better to move the reg1 == reg2 check inside
> regs_refine_cond_op(), or to handle this case in is_branch_taken().
hmm. bu then regs_refine_cond_op() will skip it, yet reg_set_min_max()
will still be doing pointless work with reg_bounds_sync() and sanity check.
The current patch makes more sense to me.
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