Forwarded: Re: [syzbot] [bpf?] WARNING in bpf_bprintf_prepare (3)

[syzbot <syzbot+b0cff308140f79a9c4cb@syzkaller.appspotmail.com>]

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Subject: Re: [syzbot] [bpf?] WARNING in bpf_bprintf_prepare (3)
Author: chandna.sahil@gmail.com

On Mon, Oct 20, 2025 at 02:08:24PM -0700, syzbot wrote:
>Hello,
>
>syzbot found the following issue on:
>
>HEAD commit:    a1e83d4c0361 selftests/bpf: Fix redefinition of 'off' as d..
>git tree:       bpf
>console output: https://syzkaller.appspot.com/x/log.txt?x=12d21de2580000
>kernel config:  https://syzkaller.appspot.com/x/.config?x=9ad7b090a18654a7
>dashboard link: https://syzkaller.appspot.com/bug?extid=b0cff308140f79a9c4cb
>compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
>syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=160cf542580000
>C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128d5c58580000
>
>Downloadable assets:
>disk image: https://storage.googleapis.com/syzbot-assets/2f6a7a0cd1b7/disk-a1e83d4c.raw.xz
>vmlinux: https://storage.googleapis.com/syzbot-assets/873984cfc71e/vmlinux-a1e83d4c.xz
>kernel image: https://storage.googleapis.com/syzbot-assets/16711d84070c/bzImage-a1e83d4c.xz
>
>The issue was bisected to:
>
>commit 7c33e97a6ef5d84e98b892c3e00c6d1678d20395
>Author: Sahil Chandna <chandna.sahil@gmail.com>
>Date:   Tue Oct 14 18:56:35 2025 +0000
>
>    bpf: Do not disable preemption in bpf_test_run().
>
>bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=172fe492580000
>final oops:     https://syzkaller.appspot.com/x/report.txt?x=14afe492580000
>console output: https://syzkaller.appspot.com/x/log.txt?x=10afe492580000
>
>IMPORTANT: if you fix the issue, please add the following tag to the commit:
>Reported-by: syzbot+b0cff308140f79a9c4cb@syzkaller.appspotmail.com
>Fixes: 7c33e97a6ef5 ("bpf: Do not disable preemption in bpf_test_run().")
#syz test

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 1b61bb25ba0e..6a128179a26f 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1021,7 +1021,9 @@ u32 bpf_flow_dissect(struct bpf_prog *prog, struct bpf_flow_dissector *ctx,
  		     (int)FLOW_DISSECTOR_F_STOP_AT_ENCAP);
  	flow_keys->flags = flags;
  
+	preempt_disable();
  	result = bpf_prog_run_pin_on_cpu(prog, ctx);
+	preempt_enable();
  
  	flow_keys->nhoff = clamp_t(u16, flow_keys->nhoff, nhoff, hlen);
  	flow_keys->thoff = clamp_t(u16, flow_keys->thoff,
--