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Log every call and exit in embedded system


I am studying a FOSS embedded OS and would like to understand its
callgraph (especially assembly calls during OS initialization).
Since this is an embedded OS it means I have no access to profiling or
coverage features from GCC.
I've also looked into GDB manual for tracing and logging, but didn't
found anything that can solve my problem in a direct way. I tried
backtrace but couldn't get any usefull information.
The idea is to understand its callgraph, so it's a one time operation.
I'm not looking for profiling or statistics (of course that being able
to do this would be great).

ATM, my idea is to create a script that:
(1) parses objdump output to get symbols and
(2) create a GDB command file that sets a breakpoint in every symbol.
I'll then read the commands file, run the target and manually take
note of the function name and source code filename (because there are
symbol name collisions) everytime a breakpoint occurs.

Problems with this approach:
(1) unable to tell when a function returns, so it's impossible to tell
what is the level at which functions are being called. This is
especially true in assembly files (i.e., was it a JMP or was it a
(2) unable to track macros and inline code :(
(3) can GBD handle thousands of breakpoints?
(4) it's a slow manual process because when a breakpoint fires I'll
have to take note of the symbol:filename and issue a continue

Is this the only way? Can anyone give me any suggestions or hints?

OFFTOPIC: how can I define string variables for GDB command files? I tried "set" but it doesn't accept strings. What I would like to do is this: export SOURCE_ROOT=/path/to/source/code/with/lots/of/dirs/ dir $SOURCE_ROOT dir $SOURCE_ROOT/core dir $SOURCE_ROOT/util dir $SOURCE_ROOT/drivers dir $SOURCE_ROOT/drivers/ethernet

Thanks in advance!

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