gdb break when program opens specific file

Question

Back story: While running a program under strace I notice that '/dev/urandom' is being open'ed. I would like to know where this call is coming from (it is not part of the program itself, it is part of the system).

So, using gdb, I am trying to break (using catch syscall open) program execution when the open call is issued, so I can see a backtrace. The problem is that open is being called alot, like several hundred times so I can't narrow down the specific call that is opening /dev/urandom. How should I go about narrowing down the specific call? Is there a way to filter by arguments, and if so how do I do it for a syscall?

Any advice would be helpful -- maybe I am going about this all wrong.

Answer 1

GDB is a pretty powerful tool, but has a bit of a learning curve.

Basically, you want to set up a conditional breakpoint.

First use the -i flag to strace or objdump -d to find the address of the open function or more realistically something in the chain of getting there, such as in the plt.

set a breakpoint at that address (if you have debug symbols, you can use those instead, omitting the *, but I'm assuming you don't - though you may well have them for library functions if nothing else.

break * 0x080482c8 

Next you need to make it conditional

(Ideally you could compare a string argument to a desired string. I wasn't getting this to work within the first few minutes of trying)

Let's hope we can assume the string is a constant somewhere in the program or one of the libraries it loads. You could look in /proc/pid/maps to get an idea of what is loaded and where, then use grep to verify the string is actually in a file, objdump -s to find it's address, and gdb to verify that you've actually found it in memory by combining the high part of the address from maps with the low part from the file. (EDIT: it's probably easier to use ldd on the executable than look in /proc/pid/maps)

Next you will need to know something about the abi of the platform you are working on, specifically how arguments are passed. I've been working on arm's lately, and that's very nice as the first few arguments just go in registers r0, r1, r2... etc. x86 is a bit less convenient - it seems they go on the stack, ie, *($esp+4), *($esp+8), *($esp+12).

So let's assume we are on an x86, and we want to check that the first argument in esp+4 equals the address we found for the constant we are trying to catch it passing. Only, esp+4 is a pointer to a char pointer. So we need to dereference it for comparison.

cond 1 *(char **)($esp+4)==0x8048514

Then you can type run and hope for the best

If you catch your breakpoint condition, and looking around with info registers and the x command to examine memory seems right, then you can use the return command to percolate back up the call stack until you find something you recognize.