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

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	David Ahern <dsahern@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH] x86 bpf: Fix extable offset calculation
Date: Thu, 24 Jun 2021 21:01:05 -0700	[thread overview]
Message-ID: <CAADnVQKLwEEZJ=_=g8RfgOrt9b1XN=dM9bt515pOrru=ADQR1Q@mail.gmail.com> (raw)
In-Reply-To: <20210622110026.1157847-1-ravi.bangoria@linux.ibm.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 ? ;)