Re: [PATCH bpf-next] bpf: Fix register equivalence tracking.

[John Fastabend <john.fastabend@gmail.com>]

Andrii Nakryiko wrote:
> On Wed, Oct 14, 2020 at 10:59 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > The 64-bit JEQ/JNE handling in reg_set_min_max() was clearing reg->id in either
> > true or false branch. In the case 'if (reg->id)' check was done on the other
> > branch the counter part register would have reg->id == 0 when called into
> > find_equal_scalars(). In such case the helper would incorrectly identify other
> > registers with id == 0 as equivalent and propagate the state incorrectly.

One thought. It seems we should never have reg->id=0 in find_equal_scalars()
would it be worthwhile to add an additional check here? Something like,

  if (known_reg->id == 0)
	return

Or even a WARN_ON_ONCE() there? Not sold either way, but maybe worth thinking
about.

> > Fix it by preserving ID across reg_set_min_max().
> > In other words any kind of comparison operator on the scalar register
> > should preserve its ID to recognize:
> > r1 = r2
> > if (r1 == 20) {
> >   #1 here both r1 and r2 == 20
> > } else if (r2 < 20) {
> >   #2 here both r1 and r2 < 20
> > }
> >
> > The patch is addressing #1 case. The #2 was working correctly already.
> >
> > Fixes: 75748837b7e5 ("bpf: Propagate scalar ranges through register assignments.")
> > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > ---
> 
> Number of underscores is a bit subtle a difference, but this fixes the bug, so:
> 
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> 

Nice catch,

Acked-by: John Fastabend <john.fastabend@gmail.com>

> 
> >  kernel/bpf/verifier.c                         | 38 ++++++++++++-------
> >  .../testing/selftests/bpf/verifier/regalloc.c | 26 +++++++++++++
> >  2 files changed, 51 insertions(+), 13 deletions(-)
> >
> 
> [...]