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From: Jordan Niethe <jniethe5@gmail.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Hari Bathini <hbathini@linux.ibm.com>,
	naveen.n.rao@linux.ibm.com, Michael Ellerman <mpe@ellerman.id.au>,
	ast@kernel.org, daniel@iogearbox.net,
	Ravi Bangoria <ravi.bangoria@linux.ibm.com>,
	songliubraving@fb.com, netdev@vger.kernel.org,
	john.fastabend@gmail.com, andrii@kernel.org, kpsingh@kernel.org,
	Paul Mackerras <paulus@samba.org>,
	yhs@fb.com, bpf@vger.kernel.org,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	kafai@fb.com
Subject: Re: [PATCH v4 5/8] bpf ppc64: Add BPF_PROBE_MEM support for JIT
Date: Thu, 30 Sep 2021 14:18:38 +1000	[thread overview]
Message-ID: <CACzsE9oNCaXoizCt-KzKS48A1L7v4_em-nLVfVLeeuWky1mrTA@mail.gmail.com> (raw)
In-Reply-To: <9628c18d-001e-9777-e800-486a83844ac1@csgroup.eu>

On Wed, Sep 29, 2021 at 9:50 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 29/09/2021 à 13:18, Hari Bathini a écrit :
> > From: Ravi Bangoria <ravi.bangoria@linux.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 ?

The main thing is on x86, the extable has another field , handler:
struct exception_table_entry { int insn, fixup, handler; };
handler can be used to perform other things before continuing on to fixup.
So for bpf the handler is used to clear the dest register (which is
encoded in the low byte of fixup).
More detail in 3dec541b2e63 ("bpf: Add support for BTF pointers to x86 JIT").

arm64 is an example of an arch that doesn't have a handler field in the extable.
They did something along the lines of this rather than adding a
handler field to the extable.
See 800834285361 ("bpf, arm64: Add BPF exception tables")

>
>
> > 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)
> >

  reply	other threads:[~2021-09-30  4:18 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-29 11:18 [PATCH v4 0/8] bpf powerpc: Add BPF_PROBE_MEM support in powerpc JIT compiler Hari Bathini
2021-09-29 11:18 ` [PATCH v4 1/8] bpf powerpc: Remove unused SEEN_STACK Hari Bathini
2021-09-29 11:18 ` [PATCH v4 2/8] bpf powerpc: Remove extra_pass from bpf_jit_build_body() Hari Bathini
2021-09-29 11:18 ` [PATCH v4 3/8] bpf powerpc: refactor JIT compiler code Hari Bathini
2021-09-29 11:42   ` Christophe Leroy
2021-09-29 11:18 ` [PATCH v4 4/8] powerpc/ppc-opcode: introduce PPC_RAW_BRANCH() macro Hari Bathini
2021-09-29 11:18 ` [PATCH v4 5/8] bpf ppc64: Add BPF_PROBE_MEM support for JIT Hari Bathini
2021-09-29 11:43   ` Christophe Leroy
2021-09-29 11:47   ` Christophe Leroy
2021-09-30  4:18     ` Jordan Niethe [this message]
2021-09-29 11:18 ` [PATCH v4 6/8] bpf ppc64: Access only if addr is kernel address Hari Bathini
2021-09-29 11:44   ` LEROY Christophe
2021-09-29 11:18 ` [PATCH v4 7/8] bpf ppc32: Add BPF_PROBE_MEM support for JIT Hari Bathini
2021-09-29 11:44   ` Christophe Leroy
2021-09-29 11:18 ` [PATCH v4 8/8] bpf ppc32: Access only if addr is kernel address Hari Bathini
2021-09-29 11:45   ` Christophe Leroy
2021-09-30 20:57 ` [PATCH v4 0/8] bpf powerpc: Add BPF_PROBE_MEM support in powerpc JIT compiler Daniel Borkmann
2021-10-01 21:22   ` Naveen N. Rao
2021-10-03 22:49   ` Michael Ellerman
2021-10-04  8:05     ` Daniel Borkmann
2021-10-04  8:43       ` Michael Ellerman

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