Read shouldn't return the size of message read?

[hkazu]

Hi, thanks for the blog about kernel module programming.

I'm studying the code and adapting a bit for my needs but noticed that the line:

exploringBB/extras/kernel/ebbcharmutex/ebbcharmutex.c

Line 139 in efb5987

return (size_of_message=0); // clear the position to the start and return 0

should return the size of the message to be read by the user space application. The reason that the included test application doesn't fail is because it does not catch this returned value (0):

exploringBB/extras/kernel/ebbcharmutex/testebbcharmutex.c

Line 44 in efb5987

if (ret < 0){

, but in any other utility it will not read the message sent by the kernel. Thanks again!

[ZoranStojsavljevic]

I did everything as advised. Did git clone https://github.com/derekmolloy/exploringBB.git
And then, just before diving into the meat of the reference kernel driver, I wrote several versions of the script, the latest shown here:
[vuser@fedora29-ssd ebbchar]$ ls -al
total 784
drwxr-xr-x. 3 vuser vboxusers 4096 Apr 3 17:55 .
drwxr-xr-x. 9 vuser vboxusers 4096 Apr 3 14:52 ..
-rw-r--r--. 1 vuser vboxusers 91 Apr 3 14:52 99-ebbchar.rules
-rw-r--r--. 1 vuser vboxusers 8629 Apr 3 14:52 ebbchar.c
-rw-r--r--. 1 vuser vboxusers 319816 Apr 3 16:56 ebbchar.ko
-rw-r--r--. 1 vuser vboxusers 431 Apr 3 16:56 .ebbchar.ko.cmd
-rw-r--r--. 1 vuser vboxusers 646 Apr 3 16:56 ebbchar.mod.c
-rw-r--r--. 1 vuser vboxusers 104680 Apr 3 16:56 ebbchar.mod.o
-rw-r--r--. 1 vuser vboxusers 30709 Apr 3 16:56 .ebbchar.mod.o.cmd
-rw-r--r--. 1 vuser vboxusers 216896 Apr 3 16:56 ebbchar.o
-rw-r--r--. 1 vuser vboxusers 38017 Apr 3 16:56 .ebbchar.o.cmd
-rw-r--r--. 1 vuser vboxusers 5 Apr 3 14:52 .gitignore
-rwxr-xr-x. 1 vuser vboxusers 101 Apr 3 17:55 loop.sh
-rwxr-xr-x. 1 vuser vboxusers 102 Apr 3 16:59 loop.sh~
-rw-r--r--. 1 vuser vboxusers 194 Apr 3 14:52 Makefile
-rw-r--r--. 1 vuser vboxusers 87 Apr 3 16:56 modules.order
-rw-r--r--. 1 vuser vboxusers 0 Apr 3 16:56 Module.symvers
-rwxr-xr-x. 1 vuser vboxusers 18848 Apr 3 16:56 test
-rw-r--r--. 1 vuser vboxusers 1787 Apr 3 14:52 testebbchar.c
drwxr-xr-x. 2 vuser vboxusers 4096 Apr 3 16:56 .tmp_versions
[vuser@fedora29-ssd ebbchar]$ pwd
/home/vuser/projects2/C_coding/lkm/exploringBB/extras/kernel/ebbchar
[vuser@fedora29-ssd ebbchar]$ cat loop.sh
#!/bin/bash
for (( c=1; c<=5000; c++ ))
do
sudo insmod ebbchar.ko
sudo rmmod -f ebbchar
done
[vuser@fedora29-ssd ebbchar]$

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After about 100 cycles, the kernel ooooops. And the whole system began to be unresponsive. Browser does not work... I'll try to capture dmesg and post here.

Thank you,
Zoran

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