netdev - Re: [PATCH v4 5/8] bpf ppc64: Add BPF_PROBE

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Date:   Wed, 29 Sep 2021 13:47:35 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Hari Bathini <hbathini@...ux.ibm.com>, naveen.n.rao@...ux.ibm.com,
        mpe@...erman.id.au, ast@...nel.org, daniel@...earbox.net
Cc:     paulus@...ba.org, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, netdev@...r.kernel.org, bpf@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH v4 5/8] bpf ppc64: Add BPF_PROBE_MEM support for JIT



Le 29/09/2021 à 13:18, Hari Bathini a écrit :
> From: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
> 
> BPF load instruction with BPF_PROBE_MEM mode can cause a fault
> inside kernel. Append exception table for such instructions
> within BPF program.
> 
> Unlike other archs which uses extable 'fixup' field to pass dest_reg
> and nip, BPF exception table on PowerPC follows the generic PowerPC


For my curiosity, can you explain why we don't want and can't do the 
same on powerpc as on other archs ?


> exception table design, where it populates both fixup and extable
> sections within BPF program. fixup section contains two instructions,
> first instruction clears dest_reg and 2nd jumps to next instruction
> in the BPF code. extable 'insn' field contains relative offset of
> the instruction and 'fixup' field contains relative offset of the
> fixup entry. Example layout of BPF program with extable present:
> 
>               +------------------+
>               |                  |
>               |                  |
>     0x4020 -->| ld   r27,4(r3)   |
>               |                  |
>               |                  |
>     0x40ac -->| lwz  r3,0(r4)    |
>               |                  |
>               |                  |
>               |------------------|
>     0x4280 -->| li  r27,0        |  \ fixup entry
>               | b   0x4024       |  /
>     0x4288 -->| li  r3,0         |
>               | b   0x40b0       |
>               |------------------|
>     0x4290 -->| insn=0xfffffd90  |  \ extable entry
>               | fixup=0xffffffec |  /
>     0x4298 -->| insn=0xfffffe14  |
>               | fixup=0xffffffec |
>               +------------------+
> 
>     (Addresses shown here are chosen random, not real)
>