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SubjectRe: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines
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Hi Masami,

On Mon, 14 Jul 2014 10:35:21 +0900, Masami Hiramatsu wrote:
> (2014/07/11 23:29), Josh Poimboeuf wrote:
> [...]
>>
>>>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.
>
> Right, this uses a module_alloc to get a memory for trampline, but
> it just allocates a page in executable vmalloc area. We need a hack
> in __kernel_text_address if we really want to use that.
>
>> 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.
>
> Yeah, I guess that is for the case that the frame pointer is broken.
>
>>
>> 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
>
> Here, this still has a wrong entry. Maybe the trampline doesn't setup
> frame pointer (bp) correctly.

Hmm.. are you saying about the hex address above? I guess it's a valid
entry in the (dynamic) trampoline, but has no symbol..


>
>> [ 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

But these seem to be wrong - there're duplicate entries and they should
show some of these functions (at least) correctly IMHO. I guess it's
because the trampoline didn't save rbp to the stack right below the
return address as dumpstack requires.

Thanks,
Namhyung


>> [ 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++;
>>


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