Re: [PATCH] x86 bpf: Fix extable offset calculation

[Ravi Bangoria]

On 6/25/21 9:31 AM, Alexei Starovoitov wrote:
> On Tue, Jun 22, 2021 at 4:01 AM Ravi Bangoria
> <ravi.bangoria@xxxxxxxxxxxxx> wrote:
> > commit 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks
> > for PROBE_LDX instructions.") is emitting couple of instructions
> > before actual load. Consider those additional instructions while
> > calculating extable offset.
> >
> > Fixes: 4c5de127598e1 ("bpf: Emit explicit NULL pointer checks for PROBE_LDX instructions.")
> > Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
> > ---
> >   arch/x86/net/bpf_jit_comp.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 2a2e290fa5d8..231a8178cc11 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1297,7 +1297,7 @@ st:                       if (is_imm8(insn->off))
> >                          emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
> >                          if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
> >                                  struct exception_table_entry *ex;
> > -                               u8 *_insn = image + proglen;
> > +                               u8 *_insn = image + proglen + (u8)(start_of_ldx - temp);
>
> Great debugging and the fix. Thanks a lot.
> I've dropped (u8) cast, kept (), and applied to bpf tree.
> I think it looks cleaner without that cast.

Thanks.

> Could you send a followup patch with a selftest, so I don't make
> the same mistake again ? ;)

Unfortunately extable gets involved only for bad kernel pointers and
ideally there should not be any bad pointer in kernel. So there is no
easy way to create a proper selftest for this.

Ravi