[PATCH v3] decode_stacktrace: Decode caller address

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Decode the caller address instead of the return address by default.
This also introduced -R option to provide return address decoding
mode.

This changes the decode_stacktrace.sh to decode the line info 1byte
before the return address which will be the call(branch) instruction
address. If the return address is a symbol address (zero offset from
it), it falls back to decoding the return address.

This improves results especially when optimizations have changed the
order of the lines around the return address, or when the return
address does not have the actual line information.

With this change;
 Call Trace:
  <TASK>
  dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
  lockdep_rcu_suspicious (kernel/locking/lockdep.c:6876)
  event_filter_pid_sched_process_fork (kernel/trace/trace_events.c:1057)
  kernel_clone (include/trace/events/sched.h:396 include/trace/events/sched.h:396 kernel/fork.c:2664)
  __x64_sys_clone (kernel/fork.c:2795 kernel/fork.c:2779 kernel/fork.c:2779)
  do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
  ? entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  ? trace_irq_disable (include/trace/events/preemptirq.h:36)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)


Without this (or give -R option);
 Call Trace:
  <TASK>
  dump_stack_lvl (lib/dump_stack.c:122)
  lockdep_rcu_suspicious (kernel/locking/lockdep.c:6877)
  event_filter_pid_sched_process_fork (kernel/trace/trace_events.c:?)
  kernel_clone (include/trace/events/sched.h:? include/trace/events/sched.h:396 kernel/fork.c:2664)
  __x64_sys_clone (kernel/fork.c:2779)
  do_syscall_64 (arch/x86/entry/syscall_64.c:?)
  ? entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
  ? trace_irq_disable (include/trace/events/preemptirq.h:36)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 Changes in v3:
   - Decode call address by default.
   - Add -R option for decoding return address.
 Changes in v2:
   - Do not search, but just decode return_address - 1.
---
 scripts/decode_stacktrace.sh |   26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/scripts/decode_stacktrace.sh b/scripts/decode_stacktrace.sh
index 8d01b741de62..5a25844003cc 100755
--- a/scripts/decode_stacktrace.sh
+++ b/scripts/decode_stacktrace.sh
@@ -5,9 +5,11 @@
 
 usage() {
 	echo "Usage:"
-	echo "	$0 -r <release>"
-	echo "	$0 [<vmlinux> [<base_path>|auto [<modules_path>]]]"
+	echo "	$0 [-R] -r <release>"
+	echo "	$0 [-R] [<vmlinux> [<base_path>|auto [<modules_path>]]]"
 	echo "	$0 -h"
+	echo "Options:"
+	echo "  -R: decode return address instead of caller address."
 }
 
 # Try to find a Rust demangler
@@ -33,11 +35,17 @@ fi
 READELF=${UTIL_PREFIX}readelf${UTIL_SUFFIX}
 ADDR2LINE=${UTIL_PREFIX}addr2line${UTIL_SUFFIX}
 NM=${UTIL_PREFIX}nm${UTIL_SUFFIX}
+decode_retaddr=false
 
 if [[ $1 == "-h" ]] ; then
 	usage
 	exit 0
-elif [[ $1 == "-r" ]] ; then
+elif [[ $1 == "-R" ]] ; then
+	decode_retaddr=true
+	shift 1
+fi
+
+if [[ $1 == "-r" ]] ; then
 	vmlinux=""
 	basepath="auto"
 	modpath=""
@@ -176,13 +184,23 @@ parse_symbol() {
 	# Let's start doing the math to get the exact address into the
 	# symbol. First, strip out the symbol total length.
 	local expr=${symbol%/*}
+	# Also parse the offset from symbol.
+	local offset=${expr#*+}
+	offset=$((offset))
 
 	# Now, replace the symbol name with the base address we found
 	# before.
 	expr=${expr/$name/0x$base_addr}
 
 	# Evaluate it to find the actual address
-	expr=$((expr))
+	# The stack trace shows the return address, which is the next
+	# instruction after the actual call, so as long as it's in the same
+	# symbol, substract one from that to point the call instruction.
+	if [[ $decode_retaddr == false && $offset != 0 ]]; then
+		expr=$((expr-1))
+	else
+		expr=$((expr))
+	fi
 	local address=$(printf "%x\n" "$expr")
 
 	# Pass it to addr2line to get filename and line number