Re: Getting empty callchain from perf_callchain_kernel()

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Wed, May 22, 2019 at 12:45:17PM -0500, Josh Poimboeuf wrote:
> On Wed, May 22, 2019 at 02:49:07PM +0000, Alexei Starovoitov wrote:
> > The one that is broken is prog_tests/stacktrace_map.c
> > There we attach bpf to standard tracepoint where
> > kernel suppose to collect pt_regs before calling into bpf.
> > And that's what bpf_get_stackid_tp() is doing.
> > It passes pt_regs (that was collected before any bpf)
> > into bpf_get_stackid() which calls get_perf_callchain().
> > Same thing with kprobes, uprobes.
> 
> Is it trying to unwind through ___bpf_prog_run()?
> 
> If so, that would at least explain why ORC isn't working.  Objtool
> currently ignores that function because it can't follow the jump table.

Here's a tentative fix (for ORC, at least).  I'll need to make sure this
doesn't break anything else.

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 242a643af82f..1d9a7cc4b836 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1562,7 +1562,6 @@ 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..2567027fce95 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1033,13 +1033,6 @@ static struct rela *find_switch_table(struct objtool_file *file,
 		if (text_rela->type == R_X86_64_PC32)
 			table_offset += 4;
 
-		/*
-		 * Make sure the .rodata address isn't associated with a
-		 * symbol.  gcc jump tables are anonymous data.
-		 */
-		if (find_symbol_containing(rodata_sec, table_offset))
-			continue;
-
 		rodata_rela = find_rela_by_dest(rodata_sec, table_offset);
 		if (rodata_rela) {
 			/*