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Message-ID: <7df70d3be4043296471f3cd3267898f953985d9a.camel@linux.dev>
Date: Mon, 03 Nov 2025 14:23:21 +0800
From: KaFai Wan <kafai.wan@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: 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>,
Eduard <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong Song
<yonghong.song@...ux.dev>, 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>, Henriette
Herzog <henriette.herzog@....de>, Luis Gerhorst <luis.gerhorst@....de>,
Harishankar Vishwanathan <harishankar.vishwanathan@...il.com>,
colin.i.king@...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 v3 1/2] bpf: Skip bounds adjustment for
conditional jumps on same scalar register
On Fri, 2025-10-31 at 09:37 -0700, Alexei Starovoitov wrote:
> On Fri, Oct 31, 2025 at 8:44 AM KaFai Wan <kafai.wan@...ux.dev> wrote:
> >
> > When conditional jumps are performed on the same scalar register
> > (e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly
> > attempts to adjust the register's min/max bounds. This leads to
> > invalid range bounds and triggers a BUG warning.
> >
> > The problematic BPF program:
> > 0: call bpf_get_prandom_u32
> > 1: w8 = 0x80000000
> > 2: r0 &= r8
> > 3: if r0 > r0 goto <exit>
> >
> > The instruction 3 triggers kernel warning:
> > 3: if r0 > r0 goto <exit>
> > true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0]
> > var_off=(0x0, 0x0)
> > true_reg2: const tnum out of sync with range bounds u64=[0x0, 0xffffffffffffffff]
> > s64=[0x8000000000000000, 0x7fffffffffffffff] var_off=(0x0, 0x0)
> >
> > Comparing a register with itself should not change its bounds and
> > for most comparison operations, comparing a register with itself has
> > a known result (e.g., r0 == r0 is always true, r0 < r0 is always false).
> >
> > Fix this by:
> > 1. Enhance is_scalar_branch_taken() to properly handle branch direction
> > computation for same register comparisons across all BPF jump operations
> > 2. Adds early return in reg_set_min_max() to avoid bounds adjustment
> > for unknown branch directions (e.g., BPF_JSET) on the same register
> >
> > The fix ensures that unnecessary bounds adjustments are skipped, preventing
> > the verifier bug while maintaining correct branch direction analysis.
> >
> > 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/
> > Signed-off-by: KaFai Wan <kafai.wan@...ux.dev>
> > ---
> > kernel/bpf/verifier.c | 33 +++++++++++++++++++++++++++++++++
> > 1 file changed, 33 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 542e23fb19c7..a571263f4ebe 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -15995,6 +15995,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct
> > bpf_reg_sta
> >
> > switch (opcode) {
> > case BPF_JEQ:
> > + if (reg1 == reg2)
> > + return 1;
> > /* constants, umin/umax and smin/smax checks would be
> > * redundant in this case because they all should match
> > */
> > @@ -16021,6 +16023,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct
> > bpf_reg_sta
> > }
> > break;
> > case BPF_JNE:
> > + if (reg1 == reg2)
> > + return 0;
> > /* constants, umin/umax and smin/smax checks would be
> > * redundant in this case because they all should match
> > */
> > @@ -16047,6 +16051,12 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct
> > bpf_reg_sta
> > }
> > break;
> > case BPF_JSET:
> > + if (reg1 == reg2) {
> > + if (tnum_is_const(t1))
> > + return t1.value != 0;
> > + else
> > + return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
> > + }
> > if (!is_reg_const(reg2, is_jmp32)) {
> > swap(reg1, reg2);
> > swap(t1, t2);
> > @@ -16059,48 +16069,64 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct
> > bpf_reg_sta
> > return 0;
> > break;
> > case BPF_JGT:
> > + if (reg1 == reg2)
> > + return 0;
> > if (umin1 > umax2)
> > return 1;
> > else if (umax1 <= umin2)
> > return 0;
> > break;
> > case BPF_JSGT:
> > + if (reg1 == reg2)
> > + return 0;
>
> This is uglier than the previous version.
> reg1 == reg2 is a syzbot territory.
> We shouldn't uglify the code everywhere because of it.
>
ok, will update in v4.
> pw-bot: cr
--
Thanks,
KaFai
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