RE: [PATCH v2 2/5] objtool: Fix ORC unwinding in non-JIT BPF generated code

[David Laight]

From: Alexei Starovoitov
> Sent: 14 June 2019 21:59
> 
> On Fri, Jun 14, 2019 at 12:56:41PM -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@xxxxxx>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> >  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] = {
> >  		[0 ... 255] = &&default_label,
> >  		/* Now overwrite non-defaults ... */
> >  		BPF_INSN_MAP(BPF_INSN_2_LBL, BPF_INSN_3_LBL),
> > @@ -1315,7 +1315,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> >  #define CONT_JMP ({ insn++; goto select_insn; })
> >
> >  select_insn:
> > -	goto *jumptable[insn->code];
> > +	goto *jump_table[insn->code];
> >
> >  	/* ALU */
> >  #define ALU(OPCODE, OP)			\
> > @@ -1558,7 +1558,6 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> >  		BUG_ON(1);
> >  		return 0;
> >  }
> > -STACK_FRAME_NON_STANDARD(___bpf_prog_run); /* jump table */
> >
> >  #define PROG_NAME(stack_size) __bpf_prog_run##stack_size
> >  #define DEFINE_BPF_PROG_RUN(stack_size) \
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index 172f99195726..8341c2fff14f 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -18,6 +18,8 @@
> >
> >  #define FAKE_JUMP_OFFSET -1
> >
> > +#define JUMP_TABLE_SYM_PREFIX "jump_table."
> 
> since external tool will be looking at it should it be named
> "bpf_jump_table." to avoid potential name conflicts?
> Or even more unique name?
> Like "bpf_interpreter_jump_table." ?

If external code might need to process such symbols then
jump_table_bpf_interpreter would (probably) make the symbols
easier to locate.

Oh, and blue is a good colour :-)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)