Re: [PATCH bpf-next v4 06/20] bpf: Explore the landing pads no call site reaches

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2026-09-21 at 20:32 -0700, Yonghong Song wrote:
> 
> 
> On 9/21/26 4:58 PM, Eduard Zingerman wrote:
> > On Mon, 2026-09-21 at 14:01 -0700, Yonghong Song wrote:
> > > A cleanup record need not cover a call an exception can unwind out of: a
> > > frontend is free to emit a region around a helper or an ordinary kfunc,
> > > both nounwind here. Nothing marks a call site then, and that record's
> > > landing pad is reached by nothing at all -- leaving bpf_check_cfg() to
> > > refuse the program over code its own frontend had no way not to emit:
> > > 
> > >     0: call bpf_preempt_disable
> > >     1: call bpf_preempt_enable      record = { begin = 1, end = 2, pad = 4 }
> > >     2: r0 = 0
> > >     3: exit
> > >     4: r1 = pads_ran ll             landing pad
> > >     6: r2 = *(u64 *)(r1 + 0)
> > >     7: r2 |= RAN_NOUNWIND_REC
> > >     8: *(u64 *)(r1 + 0) = r2
> > >     9: call bpf_unwind_resume
> > >    10: exit
> > > 
> > > The range [1,2) holds one call, and it is a kfunc, so an exception cannot
> > > come out of it. cleanup_mark_call_sites() marks nothing, nothing pushes an
> > > edge to 4, and 4 through 10 are reachable from nothing: "unreachable insn
> > > 4". The pad is dead, which is correct -- no exception can ever arrive at it
> > > -- but the program is fine and has to load.
> > > 
> > > Walk every pad the table names that the edges did not reach, the way the
> > > walk is already re-seeded at an exception callback. From there the pad is
> > > code like any other: do_check() never enters it, because no call site
> > > dispatches to it, so the dead code sweep removes it along with everything
> > > else that was not reached.
> > > 
> > > Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> > > ---
> > Yonghong, have you observed such dead code being generated by rustc?
> > It is a bit surprising, tbh.
> 
> The above code is with inline asm. rustc won't be able to generate such 
> code.

I'd say we shouldn't include such mechanics only for testing.
What kinds of tests would require it?