netdev - Re: [PATCH] bpf: Format string can't be empty

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Message-ID: <tencent_1F9A6FB02D856F9C9550E80AEC3ECD30790A@qq.com>
Date: Thu,  8 Jan 2026 11:52:00 +0800
From: Edward Adam Davis <eadavis@...com>
To: alexei.starovoitov@...il.com
Cc: andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	daniel@...earbox.net,
	eadavis@...com,
	eddyz87@...il.com,
	haoluo@...gle.com,
	john.fastabend@...il.com,
	jolsa@...nel.org,
	kpsingh@...nel.org,
	linux-kernel@...r.kernel.org,
	martin.lau@...ux.dev,
	netdev@...r.kernel.org,
	sdf@...ichev.me,
	song@...nel.org,
	syzbot+2c29addf92581b410079@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com,
	yonghong.song@...ux.dev
Subject: Re: [PATCH] bpf: Format string can't be empty

On Wed, 7 Jan 2026 19:02:37 -0800, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index db72b96f9c8c..88da2d0e634c 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -827,7 +827,7 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
> >         char fmt_ptype, cur_ip[16], ip_spec[] = "%pXX";
> >
> >         fmt_end = strnchr(fmt, fmt_size, 0);
> > -       if (!fmt_end)
> > +       if (!fmt_end || fmt_end == fmt)
> >                 return -EINVAL;
> 
> I don't think you root caused it correctly.
> The better fix and analysis:
I am keeping my analysis and patch.
The root cause of the problem is that the format string does not contain
a null terminator ('\0').
Filtering out map type 0x22 to solve the problem is too hasty, as it
would prevent all instructions from calling functions with constant
string arguments.