It seems to me that kmalloc function in my code is returning overlapped memory. In my code a process from the user space calls a function in the kernel space (driver code). The kernel space function allocates memory as part of this kernel space function execution. The process in the user space calls the kernel space driver function twice consecutively. The kmalloc call get called twice (one for each invocation of the kernel space function). The kmalloc call allocates a memory of size 2592 bytes( a structure of this size). I noticed that in the 2nd invocation of the kmalloc, it returns a memory space which overlapped with the first time kmalloc call. I have logged the first byte address and last byte address of the structure allocated in the /var/log/message. Below is the address that I got on both the times:

Fist time kamalloc:
*createFcpTapeDriveCB, 1st address: 0xf5dbc800, 2nd address: 0xf5dbd220, size: 2592 *
*createFcpTapeDriveCB, last address: 0xf5dbd21c *

2nd time kmalloc:
*createFcpTapeDriveCB, 1st address: 0xf5dbc200, 2nd address: 0xf5dbcc20, size: 2592 *
*createFcpTapeDriveCB, last address: 0xf5dbcc1c *

Here the last address: is the address of the last variable in the structure (a 32 bit pointer). 1st address is the address of the first variable in the structure and 2nd address is the first address + sizeof(structure allocated).

Can anybody help me in this regard? Why kmalloc returns overlapped address? I am running kernel version:
[root@indigo2 fcpTape]# uname -a
Linux indigo2 2.4.19smp #3 SMP Mon Jun 16 23:25:54 IST 2003 i686 unknown


Thanks,
Dipak

Is kmalloc returning the address of a memory block or is kmalloc returning a pointer to the address of a memory block?


---------------
Steve Stites

This is my kmalloc call:

/* Call the kernel malloc to allocate memory for fcpTapeDriveCB. */
pFcpTapeDCB = (fcpTapeDriveCB *)kmalloc(GFP_KERNEL, sizeof(fcpTapeDriveCB));

It returns pointer to the allocated memory block.

Thanks,
Dipak

I am not certain but I would try:

pFcpTapeDCB = (fcpTapeDriveCB *)*kmalloc(GFP_KERNEL, sizeof(fcpTapeDriveCB));

instead of:

pFcpTapeDCB = (fcpTapeDriveCB *)kmalloc(GFP_KERNEL, sizeof(fcpTapeDriveCB));


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Steve Stites

The problem got fixed. Actually the problem was the way I was using the kmalloc:

pFcpTapeDCB = (fcpTapeDriveCB *)kmalloc(sizeof(fcpTapeDriveCB), GFP_KERNEL);

Here the priority should be the 2nd argument. I was using priority as the first argument.

Thanks a lot for your time.
Dipak