Debugging a SIGSEGV Backtrace

Joaquin Fernandez

Before begin my post I need introduce you the term of backtrace, a backtrace is a series of the last function calls in your program (view $man backtrace), with a backtrace you can access the call stack, so, in other words how did you get to that point in your program.

Today working on a code I got a SIGSEGV and the obviously subsecuently crash. After checking the log I found this backlog (which was made using backtrace()):

[17101 XX 12:17:05 (+6)][23417] {sigsafe} src/common.c@1233: SIGSEGV(11), puntero 0xc0 desde 0x7f288f7c79aa
[17101 XX 12:17:05 (+6)][23417] {sigsafe} src/common.c@1252: [bt]: (0) /usr/lib64/twsmedia/[0x7f288f7c79aa]
[17101 XX 12:17:05 (+6)][23417] {sigsafe} src/common.c@1252: [bt]: (1) /usr/lib64/twsmedia/[0x7f288f7c79aa]
[17101 XX 12:17:05 (+6)][23417] {sigsafe} src/common.c@1252: [bt]: (2) /usr/sbin/mwconstructor[0x40a2c0]
[17101 XX 12:17:05 (+6)][23417] {sigsafe} src/common.c@1252: [bt]: (3) /lib64/[0x7f289173edf5]
[17101 XX 12:17:05 (+6)][23417] {sigsafe} src/common.c@1252: [bt]: (4) /lib64/[0x7f288834c1ad]

Viewing this log you know where the problem was … but… Which line is it?

The simplest way to debug(if you dont know this trick) is run gdb and try to reproduce the bug, but not every time its a great decision.

But, wait! what you want to do then?

We will search directly inside the .o of the library for the problematic line… Lets begin:

Use nm to find the function’s start position on .o file

nm src/twsmedia_widget.o | less

You will find something like this line

000000000000cc9a T twsmedia_widget_alarm_pool_draw

0xcc9a is the start line of twsmedia_widget_alarm_pool_draw function

Then, add 0x1680 offset (twsmedia_widget_alarm_pool_draw+0x1680)) to that pos, in this case resulting in 0xe31a

For last, call addr2line, to search for the specific line in the .text section of the object

$ addr2line -j .text -e src/twsmedia_widget.o 0x000000000000e31a 

And Problem solved! Now we have the problematic line:  src/twsmedia_widget.c:3139

Its important to highlight that this method won’t work in some scenarios like having static functions (because you won’t have the function names in the backtrace).

That’s all for now, see you later!