Topic: GDB and UML (mostly works) by Philip Ries
Posted 3/31/2008 11:25:34 PM

I figured out how to get GDB to debug UML, mostly based on this page:
http://user-mode-linux.sourceforge.net/hacking.html

After you've started UML linux, look in your home folder for the ~/.uml directory.
One of its subdirectories will have the pid you want--it seems to be a directory
that changes rather every time rather than the one(s) that stick around after you
run UML. You can just try them out. Run 'gdb linux <pid>' (in another terminal)--
this should cause UML to hang while it's waiting for you to "continue" in GDB.
Set any breakpoints, then continue.

You can get the GDB prompt back by pressing Ctrl+C in GDB (UML will hang again
while it waits). So you don't have to get the right breakpoint the first time.

I noticed the code jumped around a bit when stepping (as Rob was describing in an
earlier thread), so I changed my UML Makefile to limit the optimizations. To do
that, I deleted "-O2" from HOSTCFLAGS and HOSTCXXFLAGS. I ran a 'make clean ARCH=um'
after that, though it may not be necessary.

When I did this the first time, my compilation got an error on mm/slab.c. Something
weird is going on there, and I #defined away the function calls that weren't linking
(it's like they were intended to be optimized out). Everything seems to be running
OK with this modification.

GDB hasn't missed a breakpoint so far, to my knowledge. The variables are viewable,
although sometimes one will give me "optimized out"--only to later appear correctly.

The only real annoyance is that it doesn't deal with macros/inlines well--they will
cause GDB to jump to other files inside one function.

If it helps: my UML is running at SKAS3. My .config has CONFIG_CC_OPTIMIZE_FOR_SIZE=n
(in addition to the plain debug options).

P.S. If you need to debug something near initialization, I read that it helps to add
init=/bin/sh to UML's command-line args so you can attach with GDB before the bootup
passes what you're looking for.