Re: [PATCH 2/2] selftests/bpf: add verifier test for PTR_TO_MEM spill

[Yonghong Song]

On 1/12/21 7:43 AM, Daniel Borkmann wrote:
> On 1/12/21 4:35 PM, Gilad Reti wrote:
> > On Tue, Jan 12, 2021 at 4:56 PM KP Singh <kpsingh@xxxxxxxxxx> wrote:
> > > On Tue, Jan 12, 2021 at 10:16 AM Gilad Reti <gilad.reti@xxxxxxxxx> 
> > > wrote:
> > > > Add test to check that the verifier is able to recognize spilling of
> > > > PTR_TO_MEM registers.
> > >
> > > It would be nice to have some explanation of what the test does to
> > > recognize the spilling of the PTR_TO_MEM registers in the commit
> > > log as well.
> > >
> > > Would it be possible to augment an existing test_progs
> > > program like tools/testing/selftests/bpf/progs/test_ringbuf.c to test
> > > this functionality?
>
> How would you guarantee that LLVM generates the spill/fill, via inline asm?

You can make the following change to force the return value ("sample" 
here) of bpf_ringbuf_reserve() to spill on the stack.

diff --git a/tools/testing/selftests/bpf/progs/test_ringbuf.c 
b/tools/testing/selftests/bpf/progs/test_ringbuf.c
index 8ba9959b036b..011521170856 100644
--- a/tools/testing/selftests/bpf/progs/test_ringbuf.c
+++ b/tools/testing/selftests/bpf/progs/test_ringbuf.c
@@ -40,7 +40,7 @@ SEC("tp/syscalls/sys_enter_getpgid")
 int test_ringbuf(void *ctx)
 {
        int cur_pid = bpf_get_current_pid_tgid() >> 32;
-       struct sample *sample;
+       struct sample * volatile sample;
        int zero = 0;

        if (cur_pid != pid)

This change will cause verifier failure without Patch #1.

> > It may be possible, but from what I understood from Daniel's comment here
> >
> > https://lore.kernel.org/bpf/17629073-4fab-a922-ecc3-25b019960f44@xxxxxxxxxxxxx/ 
> >
> >
> > the test should be a part of the verifier tests (which is reasonable
> > to me since it is
> > a verifier bugfix)
>
> Yeah, the test_verifier case as you have is definitely the most straight
> forward way to add coverage in this case.