Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Jul 11, 2014 at 03:24:28PM +0200, Jiri Kosina wrote:
> On Fri, 11 Jul 2014, Masami Hiramatsu wrote:
> 
> > >> I did some testing with kpatch and I found one minor issue.  The dynamically
> > >> allocated trampoline seems to confuse dump_stack() somewhat.
> > >>
> > >> I added a dump_stack() call in my ftrace_ops callback function
> > >> (kpatch_ftrace_handler) which had a filter on meminfo_proc_show().
> > > 
> > > Interesting. Are you using dwarf2 unwinder for stack dumping by any 
> > > chance? It seems to get things right here. Will look into it more 
> > > tomorrow.
> > 
> > Hmm, can dwarf2 unwinder work on the trampoline method? Since the 
> > trampoline just a copy of instructions which will not have CFI(which is 
> > stored in dwarf section), I guess it may not work... Frame pointer (push 
> > bp and save sp to bp on the entry) can work anyway.
> 
> That was exactly my idea and that's why I asked, thanks for confirming.
> 
> I am afraid we'll have to declare dynamic trampolines incompatible with 
> drawf2 stack dumping.

In this case, the problem wasn't related to DWARF, because dump_stack()
uses the frame pointer to unwind the stack.  I was able to fix the
problem with the following patch.

---

>From 951d2aec17885a62905df6b910dc705d99c63993 Mon Sep 17 00:00:00 2001
From: Josh Poimboeuf <jpoimboe@redhat.com>
Date: Fri, 11 Jul 2014 08:58:33 -0500
Subject: [PATCH] x86/dumpstack: fix stack traces for generated code

If a function in the stack trace is dynamically generated, for example an
ftrace dynamically generated trampoline, print_context_stack() gets confused
and ends up showing all the following addresses as unreliable:

  [  934.546013]  [<ffffffff81700312>] dump_stack+0x45/0x56
  [  934.546020]  [<ffffffff8125f5b0>] ? meminfo_proc_open+0x30/0x30
  [  934.546027]  [<ffffffffa080a494>] kpatch_ftrace_handler+0x14/0xf0 [kpatch]
  [  934.546058]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
  [  934.546062]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
  [  934.546067]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
  [  934.546071]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
  [  934.546075]  [<ffffffff8121423a>] ? seq_read+0x16a/0x3b0
  [  934.546081]  [<ffffffff8125768d>] ? proc_reg_read+0x3d/0x80
  [  934.546088]  [<ffffffff811f0668>] ? vfs_read+0x98/0x170
  [  934.546093]  [<ffffffff811f1345>] ? SyS_read+0x55/0xd0
  [  934.546099]  [<ffffffff81707969>] ? system_call_fastpath+0x16/0x1b

Once it encounters an address which is not in the kernel's text area, it gets
confused and stops updating the frame pointer.

The __kernel_text_address() check isn't needed when determining whether an
address is reliable.  It's only needed when deciding whether to print an
unreliable address.

Here's the same stack trace with this patch:

  [ 1314.612287]  [<ffffffff81700312>] dump_stack+0x45/0x56
  [ 1314.612290]  [<ffffffff8125f5b0>] ? meminfo_proc_open+0x30/0x30
  [ 1314.612293]  [<ffffffffa080a494>] kpatch_ftrace_handler+0x14/0xf0 [kpatch]
  [ 1314.612306]  [<ffffffffa00160c4>] 0xffffffffa00160c3
  [ 1314.612309]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
  [ 1314.612311]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
  [ 1314.612312]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
  [ 1314.612314]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
  [ 1314.612315]  [<ffffffff8121423a>] ? seq_read+0x16a/0x3b0
  [ 1314.612318]  [<ffffffff8125768d>] proc_reg_read+0x3d/0x80
  [ 1314.612320]  [<ffffffff811f0668>] vfs_read+0x98/0x170
  [ 1314.612322]  [<ffffffff811f1345>] SyS_read+0x55/0xd0
  [ 1314.612324]  [<ffffffff81707969>] system_call_fastpath+0x16/0x1b
---
 arch/x86/kernel/dumpstack.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index b74ebc7..db0a33c 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -102,14 +102,13 @@ print_context_stack(struct thread_info *tinfo,
 		unsigned long addr;
 
 		addr = *stack;
-		if (__kernel_text_address(addr)) {
-			if ((unsigned long) stack == bp + sizeof(long)) {
-				ops->address(data, addr, 1);
-				frame = frame->next_frame;
-				bp = (unsigned long) frame;
-			} else {
-				ops->address(data, addr, 0);
-			}
+		if ((unsigned long) stack == bp + sizeof(long)) {
+			ops->address(data, addr, 1);
+			frame = frame->next_frame;
+			bp = (unsigned long) frame;
+			print_ftrace_graph_addr(addr, data, ops, tinfo, graph);
+		} else if (__kernel_text_address(addr)) {
+			ops->address(data, addr, 0);
 			print_ftrace_graph_addr(addr, data, ops, tinfo, graph);
 		}
 		stack++;
-- 
1.9.3