decode_stacktrace: make stack dump output useful again

Right now when people try to report issues in the kernel they send stack
dumps to eachother, which looks something like this:

  [    6.906437]  [<ffffffff811f0e90>] ? backtrace_test_irq_callback+0x20/0x20
  [    6.907121]  [<ffffffff84388ce8>] dump_stack+0x52/0x7f
  [    6.907640]  [<ffffffff811f0ec8>] backtrace_regression_test+0x38/0x110
  [    6.908281]  [<ffffffff813596a0>] ? proc_create_data+0xa0/0xd0
  [    6.908870]  [<ffffffff870a8040>] ? proc_modules_init+0x22/0x22
  [    6.909480]  [<ffffffff810020c2>] do_one_initcall+0xc2/0x1e0
  [...]

However, most of the text you get is pure garbage.

The only useful thing above is the function name.  Due to the amount of
different kernel code versions and various configurations being used,
the kernel address and the offset into the function are not really
helpful in determining where the problem actually occured.

Too often the result of someone looking at a stack dump is asking the
person who sent it for a translation for one or more 'addr2line'
translations.  Which slows down the entire process of debugging the
issue (and really annoying).

The decode_stacktrace script is an attempt to make the output more
useful and easy to work with by translating all kernel addresses in the
stack dump into line numbers.  Which means that the stack dump would
look like this:

  [  635.148361]  dump_stack (lib/dump_stack.c:52)
  [  635.149127]  warn_slowpath_common (kernel/panic.c:418)
  [  635.150214]  warn_slowpath_null (kernel/panic.c:453)
  [  635.151031]  _oalloc_pages_slowpath+0x6a/0x7d0
  [  635.152171]  ? zone_watermark_ok (mm/page_alloc.c:1728)
  [  635.152988]  ? get_page_from_freelist (mm/page_alloc.c:1939)
  [  635.154766]  __alloc_pages_nodemask (mm/page_alloc.c:2766)

It's pretty obvious why this is better than the previous stack dump
before.

Usage is pretty simple:

        ./decode_stacktrace.sh [vmlinux] [base path]

Where vmlinux is the vmlinux to extract line numbers from and base path
is the path that points to the root of the build tree, for example:

        ./decode_stacktrace.sh vmlinux /home/sasha/linux/ < input.log > output.log

The stack trace should be piped through it (I, for example, just pipe
the output of the serial console of my KVM test box through it).

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/decode_stacktrace.sh

@@ -0,0 +1,126 @@
+#!/bin/bash
+# (c) 2014, Sasha Levin <sasha.levin@oracle.com>
+#set -x
+
+if [[ $# != 2 ]]; then
+	echo "Usage:"
+	echo "	$0 [vmlinux] [base path]"
+	exit 1
+fi
+
+vmlinux=$1
+basepath=$2
+declare -A cache
+
+parse_symbol() {
+	# The structure of symbol at this point is:
+	#   [name]+[offset]/[total length]
+	#
+	# For example:
+	#   do_basic_setup+0x9c/0xbf
+
+
+	# Strip the symbol name so that we could look it up
+	local name=${symbol%+*}
+
+	# Use 'nm vmlinux' to figure out the base address of said symbol.
+	# It's actually faster to call it every time than to load it
+	# all into bash.
+	if [[ "${cache[$name]+isset}" == "isset" ]]; then
+		local base_addr=${cache[$name]}
+	else
+		local base_addr=$(nm "$vmlinux" | grep -i ' t ' | awk "/ $name\$/ {print \$1}" | head -n1)
+		cache["$name"]="$base_addr"
+	fi
+	# Let's start doing the math to get the exact address into the
+	# symbol. First, strip out the symbol total length.
+	local expr=${symbol%/*}
+
+	# 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))
+	local address=$(printf "%x\n" "$expr")
+
+	# Pass it to addr2line to get filename and line number
+        # Could get more than one result
+	if [[ "${cache[$address]+isset}" == "isset" ]]; then
+		local code=${cache[$address]}
+	else
+		local code=$(addr2line -i -e "$vmlinux" "$address")
+		cache[$address]=$code
+	fi
+
+	# addr2line doesn't return a proper error code if it fails, so
+	# we detect it using the value it prints so that we could preserve
+	# the offset/size into the function and bail out
+	if [[ $code == "??:0" ]]; then
+		return
+	fi
+
+	# Strip out the base of the path
+	code=${code//$basepath/""}
+
+	# In the case of inlines, move everything to same line
+	code=${code//$'\n'/' '}
+
+	# Replace old address with pretty line numbers
+	symbol="$name ($code)"
+}
+
+decode_code() {
+	local scripts=`dirname "${BASH_SOURCE[0]}"`
+
+	echo "$1" | $scripts/decodecode
+}
+
+handle_line() {
+	local words
+
+	# Tokenize
+	read -a words <<<"$1"
+
+	# Remove hex numbers. Do it ourselves until it happens in the
+	# kernel
+
+	# We need to know the index of the last element before we
+	# remove elements because arrays are sparse
+	local last=$(( ${#words[@]} - 1 ))
+
+	for i in "${!words[@]}"; do
+		# Remove the address
+		if [[ ${words[$i]} =~ \[\<([^]]+)\>\] ]]; then
+			unset words[$i]
+		fi
+
+		# Format timestamps with tabs
+		if [[ ${words[$i]} == \[ && ${words[$i+1]} == *\] ]]; then
+			unset words[$i]
+			words[$i+1]=$(printf "[%13s\n" "${words[$i+1]}")
+		fi
+	done
+
+	# The symbol is the last element, process it
+	symbol=${words[$last]}
+	unset words[$last]
+	parse_symbol # modifies $symbol
+
+	# Add up the line number to the symbol
+	echo "${words[@]}" "$symbol"
+}
+
+while read line; do
+	# Let's see if we have an address in the line
+	if [[ $line =~ \[\<([^]]+)\>\]  ]]; then
+		# Translate address to line numbers
+		handle_line "$line"
+	# Is it a code line?
+	elif [[ $line == *Code:* ]]; then
+                decode_code "$line"
+        else
+		# Nothing special in this line, show it as is
+		echo "$line"
+	fi
+done