Why is pr_debug of the Linux kernel not giving any output?
CONFIG_DYNAMIC_DEBUG=y
https://www.kernel.org/doc/html/v4.11/admin-guide/dynamic-debug-howto.html
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: https://stackoverflow.com/a/37283021/895245
Add following to Makefile, assuming filename.c is the module source file.
CFLAGS_filename.o := -DDEBUG
not
CFLAGS_[filename].o := -DDEBUG
Refer https://www.kernel.org/doc/local/pr_debug.txt