Freeing allocated memory linux kernel device driver module - linux

Question

I am writing code for a linux kernel module that allocates space and stores some data in it, but the kmalloc allocation happens in the write function for the vfs api as i need the size of the buffer coming from the user application and i cannot access it outside the write function. where should i place the kfree() function? i cannot place it in under cleanup because it gives me an error whenever i try to uninstall the module.
ssize_t hcsr04_write(struct file *filp, const char *buffer, size_t length, loff_t * offset)
{
if (pulsecount < (5)){
pulseptr[pulsecount] = kmalloc(sizeof(buffer),GFP_ATOMIC);
sprintf (pulseptr[pulsecount],"%s",buffer);
pulsecount++;
}
else{
int j = 0;
while (j<4){
sprintf (pulseptr[j], "%s", (pulseptr[j+1]) ); // [5 , 20 , 30 , 70 , 50] ===> [20 , 30 , 70 , 50 , 50]
j++;
}
sprintf (pulseptr[4],"%s",buffer);
}
}
this is my write function.
static void __exit hcsr04_module_cleanup(void)
{
//if (pulseptr!= {NULL,NULL,NULL,NULL,NULL}){
kfree(pulseptr);
printk(KERN_INFO "Dynamic memory freed successfully.");
//}
//pulseptr = {NULL,NULL,NULL,NULL,NULL};
gpio_free( GPIO_OUT );
gpio_free( GPIO_IN );
hcsr04_lock = 0;
cdev_del(&hcsr04_cdev);
unregister_chrdev_region( hcsr04_dev, 1 );
kobject_put( hcsr04_kobject );
}
this is the cleanup function. if i execute rmmod command with the cleanup function like this i get the following error :
[ 93.294821] 8<--- cut here ---
[ 93.297928] Unable to handle kernel paging request at virtual address bcf03574
[ 93.305253] pgd = 3cdbb3d3
[ 93.307993] [bcf03574] *pgd=00000000
[ 93.311621] Internal error: Oops: 5 [#1] SMP ARM
[ 93.316301] Modules linked in: hcsr04(O-) nfc bnep bluetooth ecdh_generic ecc ipv6 hello(PO) g_serial libcomposite udc_core brcmfmac brcmutil sha256_generic libsha256 vc4 cfg80211 bcm2835_codec(C) rfkill bcm2835_isp(C) bcm2835_v4l2(C) v4l2_mem2mem cec bcm2835_mmal_vchiq(C) videobuf2_dma_contig snd_soc_core videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 snd_compress snd_pcm_dmaengine videobuf2_common snd_pcm videodev snd_timer mc snd raspberrypi_hwmon vc_sm_cma(C) uio_pdrv_genirq uio fixed
[ 93.360563] CPU: 0 PID: 478 Comm: rmmod Tainted: P C O 5.4.72-v7 #1
[ 93.368060] Hardware name: BCM2835
[ 93.371516] PC is at kfree+0x48/0x2bc
[ 93.375235] LR is at hcsr04_module_cleanup+0x18/0xcac [hcsr04]
[ 93.381148] pc : [<802fd004>] lr : [<7f17a36c>] psr: 20010013
[ 93.387501] sp : b80abf08 ip : b80abf38 fp : b80abf34
[ 93.392797] r10: 00000081 r9 : b80aa000 r8 : 801011c4
[ 93.398095] r7 : 7f17a36c r6 : 7e92dc38 r5 : 7f17c000 r4 : bcf03570
[ 93.404713] r3 : bab24000 r2 : 00000024 r1 : 00000000 r0 : 7f17c000
[ 93.411333] Flags: nzCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user
[ 93.418568] Control: 10c5383d Table: 38d5806a DAC: 00000055
[ 93.424394] Process rmmod (pid: 478, stack limit = 0x274b04e4)
[ 93.430307] Stack: (0xb80abf08 to 0xb80ac000)
[ 93.434728] bf00: 801c8158 80989110 7f17c0c0 00000000 7e92dc38 00000081
[ 93.443026] bf20: 801011c4 b80aa000 b80abf4c b80abf38 7f17a36c 802fcfc8 7f17c0c0 00000000
[ 93.451324] bf40: b80abfa4 b80abf50 801ca940 7f17a360 72736368 00003430 00000000 80da6b10
[ 93.459621] bf60: 80101068 7e92cbe8 00000000 801011c4 b80aa000 00000006 b80abfac b80abf88
[ 93.467918] bf80: 80228294 8022938c 00000000 80e05f88 00000000 7e92de2c 00000000 b80abfa8
[ 93.476216] bfa0: 80101000 801ca7e0 00000000 7e92de2c 01677694 00000800 6a0f1b00 7e92dbe4
[ 93.484513] bfc0: 00000000 7e92de2c 7e92dc38 00000081 7e92df1f 7e92dc34 01677190 00000001
[ 93.492811] bfe0: 004e1f70 7e92dbec 004c866f 76f37218 60010030 01677694 00000000 00000000
[ 93.501100] Backtrace:
[ 93.503593] [<802fcfbc>] (kfree) from [<7f17a36c>] (hcsr04_module_cleanup+0x18/0xcac [hcsr04])
[ 93.512331] r9:b80aa000 r8:801011c4 r7:00000081 r6:7e92dc38 r5:00000000 r4:7f17c0c0
[ 93.520195] [<7f17a354>] (hcsr04_module_cleanup [hcsr04]) from [<801ca940>] (sys_delete_module+0x16c/0x244)
[ 93.530073] r5:00000000 r4:7f17c0c0
[ 93.533704] [<801ca7d4>] (sys_delete_module) from [<80101000>] (ret_fast_syscall+0x0/0x28)
[ 93.542084] Exception stack(0xb80abfa8 to 0xb80abff0)
[ 93.547209] bfa0: 00000000 7e92de2c 01677694 00000800 6a0f1b00 7e92dbe4
[ 93.555507] bfc0: 00000000 7e92de2c 7e92dc38 00000081 7e92df1f 7e92dc34 01677190 00000001
[ 93.563801] bfe0: 004e1f70 7e92dbec 004c866f 76f37218
[ 93.568922] r5:7e92de2c r4:00000000
[ 93.572549] Code: e3a02024 e5933000 e1a04624 e0243492 (e5943004)
[ 93.578730] ---[ end trace cff8773499967501 ]---
the error goes away once i comment out kfree

Answer 1

I Realised what i did wrong. I shouldve done kfree(pulseptr[0]) where 0 can be the index number of the memory locations i have allocated.