netdev - Re: [RESEND PATCH v4 0/8] bpf powerpc: Add BPF_PROBE

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Message-ID: <61bc0e8e-8ab9-f837-1b44-1e193567fff7@csgroup.eu>
Date:   Thu, 14 Oct 2021 10:33:50 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     David Laight <David.Laight@...LAB.COM>,
        'Hari Bathini' <hbathini@...ux.ibm.com>,
        "naveen.n.rao@...ux.ibm.com" <naveen.n.rao@...ux.ibm.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Cc:     "paulus@...ba.org" <paulus@...ba.org>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "kafai@...com" <kafai@...com>,
        "songliubraving@...com" <songliubraving@...com>,
        "yhs@...com" <yhs@...com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kpsingh@...nel.org" <kpsingh@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RESEND PATCH v4 0/8] bpf powerpc: Add BPF_PROBE_MEM support in
 powerpc JIT compiler



Le 14/10/2021 à 10:15, David Laight a écrit :
> From: Hari Bathini
>> Sent: 12 October 2021 13:31
>>
>> Patch #1 & #2 are simple cleanup patches. Patch #3 refactors JIT
>> compiler code with the aim to simplify adding BPF_PROBE_MEM support.
>> Patch #4 introduces PPC_RAW_BRANCH() macro instead of open coding
>> branch instruction. Patch #5 & #7 add BPF_PROBE_MEM support for PPC64
>> & PPC32 JIT compilers respectively. Patch #6 & #8 handle bad userspace
>> pointers for PPC64 & PPC32 cases respectively.
> 
> I thought that BPF was only allowed to do fairly restricted
> memory accesses - so WTF does it need a BPF_PROBE_MEM instruction?
> 


Looks like it's been added by commit 2a02759ef5f8 ("bpf: Add support for 
BTF pointers to interpreter")

They say in the log:

     Pointer to BTF object is a pointer to kernel object or NULL.
     The memory access in the interpreter has to be done via 
probe_kernel_read
     to avoid page faults.