linux-kernel - Re: [PATCH 2/9] objtool: Fix ORC unwinding in non-JIT BPF generated code

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Message-ID: <20190614013719.eqwpqfxukh6nhgec@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 13 Jun 2019 18:37:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>,
        Kairui Song <kasong@...hat.com>
Subject: Re: [PATCH 2/9] objtool: Fix ORC unwinding in non-JIT BPF generated
 code

On Thu, Jun 13, 2019 at 08:20:30PM -0500, Josh Poimboeuf wrote:
> On Thu, Jun 13, 2019 at 01:57:11PM -0700, Alexei Starovoitov wrote:
> > On Thu, Jun 13, 2019 at 08:20:59AM -0500, Josh Poimboeuf wrote:
> > > Objtool currently ignores ___bpf_prog_run() because it doesn't
> > > understand the jump table.  This results in the ORC unwinder not being
> > > able to unwind through non-JIT BPF code.
> > > 
> > > Luckily, the BPF jump table resembles a GCC switch jump table, which
> > > objtool already knows how to read.
> > > 
> > > Add generic support for reading any static local jump table array named
> > > "jump_table", and rename the BPF variable accordingly, so objtool can
> > > generate ORC data for ___bpf_prog_run().
> > > 
> > > Fixes: d15d356887e7 ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER")
> > > Reported-by: Song Liu <songliubraving@...com>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > > ---
> > >  kernel/bpf/core.c     |  5 ++---
> > >  tools/objtool/check.c | 16 ++++++++++++++--
> > >  2 files changed, 16 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > index 7c473f208a10..aa546ef7dbdc 100644
> > > --- a/kernel/bpf/core.c
> > > +++ b/kernel/bpf/core.c
> > > @@ -1299,7 +1299,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> > >  {
> > >  #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
> > >  #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
> > > -	static const void *jumptable[256] = {
> > > +	static const void *jump_table[256] = {
> > 
> > Nack to the change like above
> 
> "jump table" is two words, so does it not make sense to separate them
> with an underscore for readability?
> 
> I created a generic feature in objtool for this so that other code can
> also use it.  So a generic name (and typical Linux naming convention --
> separating words with an underscore) makes sense here.
> 
> > and to patches 8 and 9.
> 
> Well, it's your code, but ... can I ask why?  AT&T syntax is the
> standard for Linux, which is in fact the OS we are developing for.
> 
> It makes the code extra confusing for it to be annotated differently
> than all other Linux asm code.  And due to the inherent complexity of
> generating code at runtime, I'd think we'd want to make the code as
> readable as possible, for as many people as possible (i.e. other Linux
> developers).

imo your changes make it less readable.
please do not randomly change names and style based on your own preferences.
dst=src
mov(dst,src)
memcpy(dst,src)
if people want to have more bugs in assembler code. it's their call.
bpf_jit_comp.c is C code. dest is on the left.