Why Is Pr_Debug of the Linux Kernel Not Giving Any Output?
I Have a Loadable Kernel Module and Its Init Is as Given Below Static Int __Init Id_Init(Void) { Struct Identity *Temp; /* Some Code Which Is Not Relevant to...
I have a loadable kernel module and its init is as given below
static int __init id_init(void)
{
struct identity *temp;
/* some code which is not relevant to the question */
temp = identity_find(3);
pr_debug("id 3 = %s\n", temp->name);
temp = identity_find(42);
if (temp == NULL)
pr_debug("id 42 not found\n");
/* some code which is not relevant to the question */
return 0;
}
Also I have enabled dynamic debugging enabled on the kernel version I am using - i.e CONFIG_DYNAMIC_DEBUG=y.
And in the Makefile of the module I have added a line CFLAGS_[id].o := -DDEBUG where id.c is the file name.
Now I checked in the /sys/kernel/debug/dynamic_debug/control after doing insmod of this module, in which I found the below lines
/home/pauldc/Programming/Kernel/id/id.c:69 [id]id_init =_ "id 42 not found\012"
/home/pauldc/Programming/Kernel/id/id.c:65 [id]id_init =_ "id 3 = %s\012"
Even after doing all this, to my disappointment I could not find the above two pr_debug statements in the output of dmesg. So what am I missing or doing wrong?
2 Answers
Add following to Makefile, assuming filename.c is the module source file.
CFLAGS_filename.o := -DDEBUG
not
CFLAGS_[filename].o := -DDEBUG
Refer
CONFIG_DYNAMIC_DEBUG=y
If you compile the kernel with this option, then you can do amazing things like:
echo 8 > /proc/sys/kernel/printk
echo 'file kernel/module.c +p' > /sys/kernel/debug/dynamic_debug/control
and this will selectively enable the pr_debug() you want.
We can then test this out with:
insmod mymodule.ko
which prints a lot of extra debug info as in:
[ 84.875592] init_module: umod=0000000073518b66, len=185416, uargs=000000009c6e375a
[ 84.876099] Core section allocation order:
[ 84.876257] .text
[ 84.876332] .note.gnu.build-id
[ 84.876418] .rodata.str1.1
[ 84.876492] .orc_unwind_ip
[ 84.876568] .orc_unwind
[ 84.876636] __mcount_loc
[ 84.876705] .data
[ 84.876760] .gnu.linkonce.this_module
[ 84.876856] .bss
[ 84.876919] Init section allocation order:
[ 84.877041] .symtab
[ 84.877121] .strtab
[ 84.877235] final section addresses:
[ 84.877352] 0xffffffffc0006000 .note.gnu.build-id
[ 84.877482] 0xffffffffc0005000 .text
[ 84.877580] 0xffffffffc0006024 .rodata.str1.1
[ 84.877695] 0xffffffffc0006040 .orc_unwind_ip
[ 84.877805] 0xffffffffc0006050 .orc_unwind
[ 84.877905] 0xffffffffc0006068 __mcount_loc
[ 84.878012] 0xffffffffc0007000 .data
[ 84.878107] 0xffffffffc0007000 .gnu.linkonce.this_module
[ 84.878238] 0xffffffffc0007340 .bss
[ 84.878331] 0xffffffffc000a000 .symtab
[ 84.878430] 0xffffffffc000a348 .strtab
[ 84.878657] Absolute symbol: 0x00000000
[ 84.878951] Absolute symbol: 0x00000000
[ 84.879713] hello init
And in particular, it contains the module load address:
[ 84.877482] 0xffffffffc0005000 .text
which is useful to convert addresses to lines.
For modules, we can do:
echo 8 > /proc/sys/kernel/printk
echo 'module myprintk +p' > /sys/kernel/debug/dynamic_debug/control
insmod /myprintk.ko
which allows us to easily test pr_debug by adding that to our own module.
Tested on kernel 4.16 with this setup.
printk(KERN_DEBUG != pr_debug when CONFIG_DYNAMIC_DEBUG=y
This is very inconsistent, but printk(KERN_DEBUG does show up when loglevel=8 even if we don't enable /sys/kernel/debug/dynamic_debug/control, this can be seen from:
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