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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jun 22, 2021 at 4:01 AM Ravi Bangoria
<ravi.bangoria@linux.ibm.com> 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@linux.ibm.com>
> ---
>  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.
Could you send a followup patch with a selftest, so I don't make
the same mistake again ? ;)