-----Original Message-----
From: Smital Desai [mailto:Smital.Desai at lntinfotech.com]
Sent: Tuesday, January 31, 2012 5:47 PM
To: Jeff Haran; Kernel Newbies
Subject: RE: Query regarding Kmemleak

________________________________________
From: Jeff Haran [jharan at bytemobile.com]
Sent: Wednesday, February 01, 2012 6:00 AM
To: Smital Desai; Kernel Newbies
Subject: RE: Query regarding Kmemleak

quoted

-----Original Message-----
From: kernelnewbies-bounces at kernelnewbies.org

[mailto:kernelnewbies-

quoted

bounces at kernelnewbies.org] On Behalf Of Smital Desai
Sent: Tuesday, January 31, 2012 4:02 PM
To: Kernel Newbies
Subject: Query regarding Kmemleak

Hello ,

         I am  just trying to understand Kmemleak support.

         In a test program ,  allocating some memory using and

vmalloc

() and

quoted

freeing it in exit () routine.
         But when i run a kmemleak , It reports this as a leak.

         But isn't it a generic reuirement to allocate something for

the lifetime of

quoted

the module and free when we remove the module. ?   Can somebody
explain ?

         I have provided the kmemleak output and my test program

below.

quoted

Thanks,
Smital Desai

==========================================================

quoted

=======================

cat  /sys/kernel/debug/memleak

unreferenced object 0xe1b18000 (size 512):
  comm "insmod", pid 1066, jiffies 4294953148 (age 54.630s)
  hex dump (first 32 bytes):
    36 1c 00 00 02 16 00 00 3c 1c 00 00 02 16 00 00

6.......<.......

quoted

    43 1c 00 00 02 16 00 00 4f 1c 00 00 02 16 00 00

C.......O.......

quoted

  backtrace:
    [<c0234ecc>] __vmalloc_node_range+0x1a8/0x1d0
    [<c0234f2c>] __vmalloc_node+0x38/0x44
    [<c023507c>] vmalloc+0x28/0x30
    [<bf00200c>] 0xbf00200c
    [<c0100458>] do_one_initcall+0x94/0x164
    [<c01d639c>] sys_init_module+0x70/0x190
    [<c0105d60>] ret_fast_syscall+0x0/0x30
    [<ffffffff>] 0xffffffff
unreferenced object 0xe1b1a000 (size 512):
  comm "insmod", pid 1066, jiffies 4294953148 (age 54.630s)
  hex dump (first 32 bytes):
    00 00 00 00 00 2c 00 00 00 00 00 00 2a 03 00 00

.....,......*...

quoted

    00 00 00 00 00 2c 00 00 bc 00 00 00 02 2e 00 00

.....,..........

quoted

  backtrace:
    [<c0234ecc>] __vmalloc_node_range+0x1a8/0x1d0
    [<c0234f2c>] __vmalloc_node+0x38/0x44
    [<c023507c>] vmalloc+0x28/0x30
    [<bf00201c>] 0xbf00201c
    [<c0100458>] do_one_initcall+0x94/0x164
    [<c01d639c>] sys_init_module+0x70/0x190
    [<c0105d60>] ret_fast_syscall+0x0/0x30
    [<ffffffff>] 0xffffffff

==========================================================

quoted

===========================



                                                            "test.c"

==========================================================

quoted

========================
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kmemleak.h>

char *ptr1, *ptr2;

/*
 * Some very simple testing. This function needs to be extended for
 * proper testing.
 */
static int __init kmemleak_test_init(void)
{
        ptr1 = vmalloc(512);
        ptr2 = vmalloc(512);

        return 0;
}
module_init(kmemleak_test_init);

static void __exit kmemleak_test_exit(void)
{
       vfree(ptr1);
       vfree(ptr2);
}
module_exit(kmemleak_test_exit);

MODULE_LICENSE("GPL");

==========================================================

quoted

========================



Thanks and Regard
 Smital Desai

The contents of this e-mail and any attachment(s) may contain

confidential

quoted

or privileged information for the intended recipient(s). Unintended
recipients are prohibited from taking action on the basis of

information in this

quoted

e-mail and  using or disseminating the information,  and must notify

the

quoted

sender and delete it from their system. L&T Infotech will not accept
responsibility or liability for the accuracy or completeness of, or

the presence

quoted

of any virus or disabling code in this e-mail"

_______________________________________________
Kernelnewbies mailing list
Kernelnewbies at kernelnewbies.org
http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

quoted

quoted

Just to be clear:

quoted

quoted

Did you rmmod your module before doing the cat on
/sys/kernel/debug/memleak?

quoted

quoted

If not, I think the above output of memleak is what would be

expected.

quoted

quoted

Jeff Haran

Hello Jeff,

     Yeh , you are correct and i didn't remove the module .
     But in that case what if module stays there for the system's life

time

keeping the allocated memory , it will get reported by kmemleak as

potential

__leak__ . ( Which essentially is false postitive )

     And the case i am mentioning can be pretty much the requirement

of

atleast few drivers which keep memory allocated for system's lifetime

, how

to avoid this false postitive in such cases .?

Thanks,
Smital Desai

I don't see how the system could distinguish this memory allocation from
a memory leak, but not being an expert on the topic I look forward to
being corrected by others more knowledgeable. 8^)

A "memory leak" after all is just allocated memory that the coder
forgets to free when his code is otherwise done with and with a kernel
module something on the outside can't really know the module is done
with it until the module is rmmod'ed. There's no lifetime specified when
you call vmalloc(). I've seen people who were tasked with finding memory
leaks do tricks like keep track of the address of every allocated memory
block and then periodically scan through memory looking to see if those
addresses exists anywhere, the theory being that if one doesn't then the
subject code has forgotten about the memory allocation and is thus
highly likely to be a leak. But I don't see how one could ever eliminate
all false positives.

Jeff Haran