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[F,1/2] s390/ftrace: save traced function caller

Message ID 20200303163713.19363-2-frank.heimes@canonical.com
State New
Headers show
Series Enable proper kprobes on ftrace support (LP: 1865858) | expand

Commit Message

Frank Heimes March 3, 2020, 4:37 p.m. UTC 
From: Vasily Gorbik <gor@linux.ibm.com>

BugLink: https://bugs.launchpad.net/bugs/1865858

A typical backtrace acquired from ftraced function currently looks like
the following (e.g. for "path_openat"):

arch_stack_walk+0x15c/0x2d8
stack_trace_save+0x50/0x68
stack_trace_call+0x15a/0x3b8
ftrace_graph_caller+0x0/0x1c
0x3e0007e3c98 <- ftraced function caller (should be do_filp_open+0x7c/0xe8)
do_open_execat+0x70/0x1b8
__do_execve_file.isra.0+0x7d8/0x860
__s390x_sys_execve+0x56/0x68
system_call+0xdc/0x2d8

Note random "0x3e0007e3c98" stack value as ftraced function caller. This
value causes either imprecise unwinder result or unwinding failure.
That "0x3e0007e3c98" comes from r14 of ftraced function stack frame, which
it haven't had a chance to initialize since the very first instruction
calls ftrace code ("ftrace_caller"). (ftraced function might never
save r14 as well). Nevertheless according to s390 ABI any function
is called with stack frame allocated for it and r14 contains return
address. "ftrace_caller" itself is called with "brasl %r0,ftrace_caller".
So, to fix this issue simply always save traced function caller onto
ftraced function stack frame.

Reported-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
(cherry picked from commit b4adfe55915d8363e244e42386d69567db1719b9)
Signed-off-by: Frank Heimes <frank.heimes@canonical.com>
---
 arch/s390/kernel/mcount.S | 1 +
 1 file changed, 1 insertion(+)
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Patch

diff --git a/arch/s390/kernel/mcount.S b/arch/s390/kernel/mcount.S
index 3431b2d5e334..f942341429b1 100644
--- a/arch/s390/kernel/mcount.S
+++ b/arch/s390/kernel/mcount.S
@@ -41,6 +41,7 @@  EXPORT_SYMBOL(_mcount)
 ENTRY(ftrace_caller)
 	.globl	ftrace_regs_caller
 	.set	ftrace_regs_caller,ftrace_caller
+	stg	%r14,(__SF_GPRS+8*8)(%r15)	# save traced function caller
 	lgr	%r1,%r15
 #if !(defined(CC_USING_HOTPATCH) || defined(CC_USING_NOP_MCOUNT))
 	aghi	%r0,MCOUNT_RETURN_FIXUP