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From:   Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Subject: Re: [PATCH] x86 bpf: Fix extable offset calculation
To:     Alexei Starovoitov <alexei.starovoitov@gmail.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>,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>
References: <CAADnVQJux+8n-vpuK9FqTLuj4cXrp04pGkpvKaUdAPXLQ4c-PQ@mail.gmail.com>
 <20210622110026.1157847-1-ravi.bangoria@linux.ibm.com>
 <CAADnVQKLwEEZJ=_=g8RfgOrt9b1XN=dM9bt515pOrru=ADQR1Q@mail.gmail.com>
Message-ID: <960b5e26-e97d-2b1a-4628-df8525c0728b@linux.ibm.com>
Date:   Fri, 25 Jun 2021 11:52:24 +0530
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On 6/25/21 9:31 AM, Alexei Starovoitov wrote:
> 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.

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