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x86/ftrace: Fix ebp in ftrace_regs_caller that screws up unwinder

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Fengguang Wu's zero day bot triggered a stack unwinder dump. This can
be easily triggered when CONFIG_FRAME_POINTERS is enabled and -mfentry
is in use on x86_32.

 ># cd /sys/kernel/debug/tracing
 ># echo 'p:schedule schedule' > kprobe_events
 ># echo stacktrace > events/kprobes/schedule/trigger

This is because the code that implemented fentry in the ftrace_regs_caller
tried to use the least amount of #ifdefs, and modified ebp when
CC_USE_FENTRY was defined to point to the parent ip as it does when
CC_USE_FENTRY is not defined. But when CONFIG_FRAME_POINTERS is set, it
corrupts the ebp register for this frame while doing the tracing.

NOTE, it does not corrupt ebp in any other way. It is just a bad frame
pointer when calling into the tracing infrastructure. The original ebp is
restored before returning from the fentry call. But if a stack trace is
performed inside the tracing, the unwinder will notice the bad ebp.

Instead of toying with ebp with CC_USING_FENTRY, just slap the parent ip
into the second parameter (%edx), and have an #else that does it the
original way.

The unwinder will unfortunately miss the function being traced, as the
stack frame is not set up yet for it, as it is for x86_64. But fixing that
is a bit more complex and did not work before anyway.

This has been tested with and without FRAME_POINTERS being set while using
-mfentry, as well as using an older compiler that uses mcount.

Analyzed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Fixes: 644e0e8dc7 ("x86/ftrace: Add -mfentry support to x86_32 with DYNAMIC_FTRACE set")
Reported-by: kernel test robot <fengguang.wu@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lists.01.org/pipermail/lkp/2017-April/006165.html
Link: http://lkml.kernel.org/r/20170420172236.7af7f6e5@gandalf.local.home
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Steven Rostedt (VMware) 2017-04-20 17:22:36 -04:00 committed by Thomas Gleixner
parent aa4f853461
commit dc912c3035
1 changed files with 4 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
arch/x86/kernel/ftrace_32.S
View File

@ -128,14 +128,14 @@ ENTRY(ftrace_regs_caller)
pushl %edx
pushl %ecx
pushl %ebx
#ifdef CC_USING_FENTRY
/* Load 4 off of the parent ip addr into ebp */
lea 14*4(%esp), %ebp
#endif
movl 12*4(%esp), %eax /* Load ip (1st parameter) */
subl $MCOUNT_INSN_SIZE, %eax /* Adjust ip */
#ifdef CC_USING_FENTRY
movl 15*4(%esp), %edx /* Load parent ip (2nd parameter) */
#else
movl 0x4(%ebp), %edx /* Load parent ip (2nd parameter) */
#endif
movl function_trace_op, %ecx /* Save ftrace_pos in 3rd parameter */
pushl %esp /* Save pt_regs as 4th parameter */