< June 2006 >
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Thu, 01 Jun 2006:

Valgrind is one of the most common tools people use to debug memory. Recently while I was debugging APC, the primary problem I have is of php Zend code writing into shared memory without acquiring the locks required. I had been debugging that with gdb for a while, but gdb is just dead slow for watching writes to 16 Mb of memory and generating backtraces.

The result of all that pain was a quick patch on valgrind 3.1.1. The patch would log all writes to a memory block with backtraces. But valgrind does not have a terminal to type into midway, unlike gdb. So the question was how to indicate a watchpoint. Valgrind magic functions were the answer. The magic functions can pass a parameter to valgrind while in execution. This is a source hack and is a hell of a lot easier to do than actually breaking in gdb and marking a breakpoint everytime you run it. So here's how the code looks like :-

#include "valgrind/memcheck.h"

int main()
	int * k = malloc(sizeof(int));
	int x = VALGRIND_SET_WATCHPOINT(k, sizeof(int));

That is marked out in the normal code with the following assembly fragment.

    movl    $1296236555, -56(%ebp)
    movl    8(%ebp), %eax
    movl    %eax, -52(%ebp)
    movl    $4, -48(%ebp)
    movl    $0, -44(%ebp)
    movl    $0, -40(%ebp)
    leal    -56(%ebp), %eax
    movl    $0, %edx
    roll $29, %eax ; roll $3, %eax
    rorl $27, %eax ; rorl $5, %eax
    roll $13, %eax ; roll $19, %eax
    movl    %edx, %eax
    movl    %eax, -12(%ebp)

This doesn't do anything at all on a normal x86 cpu but inside the valgrind executor, it is picked up and delivered to mc_handle_client_request where I handle the case and add the address and size, to the watch points list.

So whenever a helperc_STOREV* function is called, the address passed in is checked against the watchpoints list, which is stored in the corresponding primary map of access bits. All of these bright ideas were completely stolen from Richard Walsh patch for valgrind 2.x. But of course, if it weren't for the giants on whose shoulders I stand ...

bash$ valgrind a.out

==6493== Watchpoint 0 event: write
==6493==    at 0x804845E: modify (in /home/gopalv/hacks/valgrind-tests/a.out)
==6493==    by 0x80484EA: main (in /home/gopalv/hacks/valgrind-tests/a.out)
==6493== This watchpoint has been triggered 1 time
==6493== This watchpoint was set at:
==6493==    at 0x80484DB: main (in /home/gopalv/hacks/valgrind-tests/a.out)

Now, I can actually run a huge ass set of tests on php5 after marking the APC shared memory as watched and see all the writes, filter out all the APC writes and continue to copy out the other written segments into local memory for Zend's pleasure.

Writing software gives you that high of creating something out of nearly nothing. Since I am neither a poet nor a painter, there's no other easy way to get that high (unless ... *ahem*).

Mathemeticians stand on each other's shoulders while computer scientists stand on each other's toes.
                -- Richard Hamming

posted at: 23:44 | path: /hacks | permalink | Tags: , ,