decode_stacktrace: decode caller address

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)

[akpm@linux-foundation.org: fix spello]
Link: https://lkml.kernel.org/r/177275821652.1557019.18367881408364381866.stgit@mhiramat.tok.corp.google.com
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>	[arm64]
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Sasha Levin (Microsoft) <sashal@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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
+	# 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, subtract 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