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

[Alexei Starovoitov]

On Wed, Jan 7, 2026 at 7:52 PM Edward Adam Davis <eadavis@xxxxxx> wrote:
>
> On Wed, 7 Jan 2026 19:02:37 -0800, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> 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.

If you think it's still possible to construct a program that
passes empty const string into this helper then please craft
a selftest that demonstrates that.