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Date: Fri, 11 Jul 2014 09:29:28 -0500
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Jiri Kosina <jkosina@...e.cz>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
Steven Rostedt <rostedt@...dmis.org>,
linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Namhyung Kim <namhyung@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
Seth Jennings <sjenning@...hat.com>,
Jiri Slaby <jslaby@...e.cz>
Subject: Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines
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@...hat.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
--
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