Debug httpd process

Question

I'm attempting to inspect memory of an httpd binary compiled statically with openssl. When I list the httpd processes, I get:

$ ps aux|grep httpd
root     58539  0.0  0.7  75364  3740 ?        Ss   14:49   0:00 /opt/httpd/bin/httpd
daemon   58850  0.0  0.5 364328  2556 ?        Sl   14:58   0:00 /opt/httpd/bin/httpd
daemon   58914  0.0  0.5 364328  2548 ?        Sl   14:59   0:00 /opt/httpd/bin/httpd
daemon   58942  0.0  0.5 364328  2544 ?        Sl   14:59   0:00 /opt/httpd/bin/httpd

I then tried attaching to each of the processes individually and applying a breakpoint to the function I am trying to debug. However, the breakpoint never gets triggered, although I know the function is being called. I assume this problem relates to something along the lines of httpd forking on a new process, but I'm a bit stuck on how to proceed. Is there a generic way to ensure my breakpoint is triggered even if the processes forks?

Answer 1

Depending on the worker model your apache is configured to use its likely a new worker is being spawned to handle your request if your bp isnt hitting. Like Guntram said you can disable forking and put httpd in debug mode by using the -X flag and starting gdb with:

gdb httpd -X

If for some reason you need to debug the multiple worker configured httpd you should use the gdb options set follow-fork-mode ask or set follow-fork-mode child if you know you'd like to follow all forks in advance. (https://sourceware.org/gdb/onlinedocs/gdb/Forks.html). This would also be considered the 'generic approach' and would apply to programs that don't allow you to run single process in the foreground.