CODEWITHSUNDEEP
linuxmodulekernel
csstudent2233
csstudent2233

Reputation: 679

Reading dmesg from a kernel module

I'm trying to simply read the kernel ring buffer from a kernel module. Also known as /proc/kmsg and dmesg output.

From looking around it seems that the call for this is sys_syslog(); from what I've read dmesg uses syslog() which uses do_syslog() which resides in printk.c.

I grep around and see that do_syslog() is defined in linux/syslog.h, so I include this file.

The actual call to do_syslog() in the test module looks like this:

read_bytes = do_syslog(2, temp_buffer, 1024, 0);

Where temp_buffer is just a char temp_buffer[1024] meant for test purposes.

The program compiles, however when I try to load the module it fails:

insmod: error inserting 'testing.ko': -1 Unknown symbol in module

From dmesg I see: "Unknown symbol do_syslog (err 0)"

Why is this an unknown symbol? Do I need to link against something?

The makefile is as follows:

obj-m += testing.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

Upvotes: 2

Views: 2281

Answers (1)

InternetSeriousBusiness
InternetSeriousBusiness

Reputation: 2635

You need to recompile a kernel with that symbol exported.

EXPORT_SYMBOL (do_syslog);

Upvotes: 2

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