Difference between revisions of "Kernel Debugging Tips"

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Here are some miscellaneous tips for debugging a kernel:
 
Here are some miscellaneous tips for debugging a kernel:
  
== Accessing the printk buffer after a silent hang on boot ==
+
== Using printks ==
Quinn Jensen writes:
+
To add your own debug message to the kernel, you can place a "printk()" in the kernel code.
  
Something I've found handy when the console is
+
See [[Debugging_by_printing#Usage| Debugging by printing -> Usage]] for more details.
silent is to dump the printk buffer from the boot
+
loader. To do it you have to know how your boot
+
loader maps memory compared to the kernel.  Here's
+
what I do with Redboot on i.MX31:
+
  
fgrep printk_buf System.map
+
=== Log levels ===
 +
Each kernel message can be pre-pended with a tag indicating the importance of the message.
  
this shows the linked address of the printk_buf, e.g.:
+
See [[Debugging_by_printing#Log_Levels| Debugging by printing -> Log_Levels]] for more details.
  
  c02338f0 b printk_buf.16194
+
=== Adding timing information ===
 +
Sometimes, it is useful to add timing information to the printk values, so you can see when a particular event occurred. The kernel includes an feature for doing
 +
this called printk times.
  
The address "c02338f0" is in kernel virtual, which,
+
See the help for CONFIG_PRINTK_TIMES in the file lib/Kconfig.debug for more information on this featureThis option is found on the "Kernel Hacking" menu when configuring the kernel.
in the case of i.MX31 ADS, redboot will have mapped
+
to 0x802338f0So, after resetting the target board,
+
but without letting it try to boot again, at the redboot
+
prompt,
+
  
dump -b 0x802338f0 -l 10000
+
The timestamps which are inserted into the printk output consist of seconds and microseconds, as absolute values from the start of machine operation (or from the start of kernel timekeeping).
  
And you see the printk buffer that never got flushed
+
There is also tool in the kernel source which will convert the timestamp values to relative values (so you can see the interval between events).  This tools is called show_delta and is located in the kernel 'scripts' directory.
to the UARTKnowing what's there can be '''very'''
+
 
useful in debugging your console.
+
See [[Printk Times]] for more information.
 +
 
 +
=== Viewing log messages ===
 +
The <tt>klogd</tt> program will extract messages from the kernel log buffer, and send them to the system log (which winds up in /var/log/messages on most systems).  This command runs in the background on most desktop or server systems, and continually transfers messages from the kernel log buffer to the system log.
 +
 
 +
You can view the contents of the log buffer directly, using the <tt>dmesg</tt> command.  Note that by default <tt>dmesg</tt> displays the messages from the buffer, but does not remove them.  So this command can be run multiple times to view the kernel printk messages.  See the dmesg man page for more things you can do with this tool.
 +
 
 +
=== Controlling console output ===
 +
In order to have the kernel boot be less "noisy", or in order to boot more quickly, it is sometimes useful to control the amount of messages displayed to the console during bootYou can do this by setting the kernel log level at boot time via a kernel command line option. See the "loglevel=" argument in <tt>Documentation/kernel-parameters.txt</tt>.
 +
 
 +
You can turn off all messages using the kernel command line option "quiet". See [[Disable Console]] for information on how much time this can save at boot up.
 +
 
 +
Note that even if the log level is changed, or "quiet" is used, although the printk messages are not print to console, they are still entered into the log buffer, and they can still be extracted and displayed later using the <tt>dmesg</tt> command.
 +
 
 +
=== Changing the size of the printk buffer ===
 +
See [[Debugging_by_printing#Internals_.2F_Changing_the_size_of_the_printk_buffer | Debugging by printing -> Internals / Changing the size of the printk buffer ]]
 +
 
 +
== Using kernel symbols ==
 +
You can look up the source code for a function address using your toolchain's addr2line program.
 +
See [[Find a kernel function line]] or [[Addr2line for kernel debugging]].
 +
 
 +
== Using a kernel debugger ==
 +
You can use the in-kernel debugger: [[KDB]]
 +
 
 +
Also, you can use QEMU and gdb (and a high-level IDE like eclipse).
 +
 
 +
See [http://issaris.blogspot.com/2007/12/download-linux-kernel-sourcecode-from.html Debugging the Linux kernel using Eclipse/CDT and Qemu] for a great article on using Eclipse (with the CDT plugin) to debug the Linux kernel.
 +
 
 +
 
 +
== Debugging early boot problems ==
 +
See [[Debugging_by_printing#Debugging_early_boot_problems]]
 +
 
 +
== Triggering a kernel event ==
 +
=== Overloading the sync system call ===
 +
Sometimes, it is nice to trigger an event to happen in the kernel from user space. Instead of creating infrastructure to handle a /proc event, an ioctl() or making a new syscall, it can be quick and easy to just overload an existing function.  One function not used very often is sync.  (I have found that the sync system call is not normally called by user space programs (or during standard linux booting).
 +
 
 +
It is quite easy to put a hook to your own kernel program in the sys_sync() routine (located in fs/sync.c) and cause it to execute by issuing 'sync' from the shell command line. This is handy as a temporary mechanism to test things that you have put in the kernel.  
  
 
[[Category:Development Tools]]
 
[[Category:Development Tools]]
 +
 +
== Interpreting an Oops message ==
 +
When the kernel encounters an internal fault, it will print an Oops message.
 +
Here are some tips on using the Oops message to find the source of the problem.
 +
* See [http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob_plain;f=Documentation/oops-tracing.txt;hb=HEAD Documentation/oops-tracing.txt]
 +
* See [http://vmlinux.org/jocke/linux/howto-find-oops-location.shtml HOWTO find oops location] by Denis Vlasenko
 +
 +
== Compilation tricks for the kernel ==
 +
Sometimes, you want to modify how the compiler builds an individual kernel file.  The following are tips for doing tasks related to this.
 +
=== Build an individual file ===
 +
You can build an individual output object file, with:
 +
make fs/buffer.o
 +
 +
This will build JUST fs/buffer.o (if it needs rebuilding) and not the entire kernel.  To force it to need re-building, use 'touch' on the associated source file:
 +
touch fs/buffer.c
 +
 +
=== Create the preprocessed file for an individual source file ===
 +
Using the same technique, you can create the preprocessed file for a C source file.  This is useful if you're having trouble tracking down macro expansion or where defines/prototypes are coming from exactly.
 +
make fs/buffer.i
 +
 +
=== Create the assembly file for an individual source file ===
 +
Using the same technique, you can create the assembly file for a C source file.  This is useful to get an idea what actual machine instructions are generated from the C source code.
 +
make fs/buffer.s
 +
 +
Another way to get the raw assembly, is to dump the object file using 'objdump'
 +
objdump -d fs/buffer.o > fs/buffer.disassem
 +
 +
This will produce a disassembly of the object file, which should show how the assembly was translated into machine instructions.
 +
 +
If the object has been compiled with debug symbols (using '-g'), then you might get more information using the '-S' option with objdump:
 +
objdump -S -d fs/buffer.o >fs/buffer.disassem
 +
 +
You can also request that the toolchain show mixed source and assembly, by passing extra flags:
 +
make EXTRA_CFLAGS="-g -Wa,-a,-ad -fverbose-asm" fs/buffer.o >fs/buffer.mixed
 +
 +
=== Alter the flags for a compilation ===
 +
Sometimes, you need to alter the compilation flags for an individual file.  There are two ways to do this. One is to add the extra flags on the make command line:
 +
 +
make EXTRA_CFLAGS="-g -finstrument-functions" fs/buffer.o
 +
 +
This will work if the flags can be appended to the regular set of C flags used for compiling the object.
 +
 +
However, if you need to do something more complicated, like removing or modifying flags, then you can build your own command line by hand.  To do this, it is easiest to have 'make' produce the default compilation command (which will be several lines long), then copy, paste and edit it, to run on the command line directly.  To see the exact compile commands used to compile a particular object, use the V=1 option with the kernel build system:
 +
 +
make V=1 fs/buffer.o
 +
 +
For me, this produced something like this:
 +
 +
mipsel-linux-gcc -Wp,-MD,fs/.buffer.o.d -nostdinc -isystem /home/usr/local/mipsel-linux-glibc/bin/../lib/gcc/mipsisa32el-linux/3.4.3/include -D__KERNEL__ -Iinclude -Iinclude2 -I/home/tbird/work/linux/include -I/home/tbird/work/linux/fs -Ifs -Wall -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -ffreestanding -O2 -fomit-frame-pointer -g -I/home/tbird/work/linux/ -I /home/tbird/work/linux/include/asm/gcc -G 0 -mno-abicalls -fno-pic -pipe -finline-limit=100000 -mabi=32 -march=mips32r2 -Wa,-32 -Wa,-march=mips32r2 -Wa,-mips32r2 -Wa,--trap -I/home/tbird/work/linux/include/asm-mips/ati -Iinclude/asm-mips/ati -I/home/tbird/work/linux/include/asm-mips/mach-generic -Iinclude/asm-mips/mach-generic -Wdeclaration-after-statement  -DKBUILD_BASENAME=buffer -DKBUILD_MODNAME=buffer -c -o fs/buffer.o /home/tbird/work/linux/fs/buffer.c

Latest revision as of 21:33, 3 April 2012

Here are some miscellaneous tips for debugging a kernel:

Using printks

To add your own debug message to the kernel, you can place a "printk()" in the kernel code.

See Debugging by printing -> Usage for more details.

Log levels

Each kernel message can be pre-pended with a tag indicating the importance of the message.

See Debugging by printing -> Log_Levels for more details.

Adding timing information

Sometimes, it is useful to add timing information to the printk values, so you can see when a particular event occurred. The kernel includes an feature for doing this called printk times.

See the help for CONFIG_PRINTK_TIMES in the file lib/Kconfig.debug for more information on this feature. This option is found on the "Kernel Hacking" menu when configuring the kernel.

The timestamps which are inserted into the printk output consist of seconds and microseconds, as absolute values from the start of machine operation (or from the start of kernel timekeeping).

There is also tool in the kernel source which will convert the timestamp values to relative values (so you can see the interval between events). This tools is called show_delta and is located in the kernel 'scripts' directory.

See Printk Times for more information.

Viewing log messages

The klogd program will extract messages from the kernel log buffer, and send them to the system log (which winds up in /var/log/messages on most systems). This command runs in the background on most desktop or server systems, and continually transfers messages from the kernel log buffer to the system log.

You can view the contents of the log buffer directly, using the dmesg command. Note that by default dmesg displays the messages from the buffer, but does not remove them. So this command can be run multiple times to view the kernel printk messages. See the dmesg man page for more things you can do with this tool.

Controlling console output

In order to have the kernel boot be less "noisy", or in order to boot more quickly, it is sometimes useful to control the amount of messages displayed to the console during boot. You can do this by setting the kernel log level at boot time via a kernel command line option. See the "loglevel=" argument in Documentation/kernel-parameters.txt.

You can turn off all messages using the kernel command line option "quiet". See Disable Console for information on how much time this can save at boot up.

Note that even if the log level is changed, or "quiet" is used, although the printk messages are not print to console, they are still entered into the log buffer, and they can still be extracted and displayed later using the dmesg command.

Changing the size of the printk buffer

See Debugging by printing -> Internals / Changing the size of the printk buffer

Using kernel symbols

You can look up the source code for a function address using your toolchain's addr2line program. See Find a kernel function line or Addr2line for kernel debugging.

Using a kernel debugger

You can use the in-kernel debugger: KDB

Also, you can use QEMU and gdb (and a high-level IDE like eclipse).

See Debugging the Linux kernel using Eclipse/CDT and Qemu for a great article on using Eclipse (with the CDT plugin) to debug the Linux kernel.


Debugging early boot problems

See Debugging_by_printing#Debugging_early_boot_problems

Triggering a kernel event

Overloading the sync system call

Sometimes, it is nice to trigger an event to happen in the kernel from user space. Instead of creating infrastructure to handle a /proc event, an ioctl() or making a new syscall, it can be quick and easy to just overload an existing function. One function not used very often is sync. (I have found that the sync system call is not normally called by user space programs (or during standard linux booting).

It is quite easy to put a hook to your own kernel program in the sys_sync() routine (located in fs/sync.c) and cause it to execute by issuing 'sync' from the shell command line. This is handy as a temporary mechanism to test things that you have put in the kernel.

Interpreting an Oops message

When the kernel encounters an internal fault, it will print an Oops message. Here are some tips on using the Oops message to find the source of the problem.

Compilation tricks for the kernel

Sometimes, you want to modify how the compiler builds an individual kernel file. The following are tips for doing tasks related to this.

Build an individual file

You can build an individual output object file, with:

make fs/buffer.o

This will build JUST fs/buffer.o (if it needs rebuilding) and not the entire kernel. To force it to need re-building, use 'touch' on the associated source file:

touch fs/buffer.c

Create the preprocessed file for an individual source file

Using the same technique, you can create the preprocessed file for a C source file. This is useful if you're having trouble tracking down macro expansion or where defines/prototypes are coming from exactly.

make fs/buffer.i

Create the assembly file for an individual source file

Using the same technique, you can create the assembly file for a C source file. This is useful to get an idea what actual machine instructions are generated from the C source code.

make fs/buffer.s

Another way to get the raw assembly, is to dump the object file using 'objdump'

objdump -d fs/buffer.o > fs/buffer.disassem

This will produce a disassembly of the object file, which should show how the assembly was translated into machine instructions.

If the object has been compiled with debug symbols (using '-g'), then you might get more information using the '-S' option with objdump:

objdump -S -d fs/buffer.o >fs/buffer.disassem

You can also request that the toolchain show mixed source and assembly, by passing extra flags:

make EXTRA_CFLAGS="-g -Wa,-a,-ad -fverbose-asm" fs/buffer.o >fs/buffer.mixed

Alter the flags for a compilation

Sometimes, you need to alter the compilation flags for an individual file. There are two ways to do this. One is to add the extra flags on the make command line:

make EXTRA_CFLAGS="-g -finstrument-functions" fs/buffer.o

This will work if the flags can be appended to the regular set of C flags used for compiling the object.

However, if you need to do something more complicated, like removing or modifying flags, then you can build your own command line by hand. To do this, it is easiest to have 'make' produce the default compilation command (which will be several lines long), then copy, paste and edit it, to run on the command line directly. To see the exact compile commands used to compile a particular object, use the V=1 option with the kernel build system:

make V=1 fs/buffer.o

For me, this produced something like this:

mipsel-linux-gcc -Wp,-MD,fs/.buffer.o.d -nostdinc -isystem /home/usr/local/mipsel-linux-glibc/bin/../lib/gcc/mipsisa32el-linux/3.4.3/include -D__KERNEL__ -Iinclude -Iinclude2 -I/home/tbird/work/linux/include -I/home/tbird/work/linux/fs -Ifs -Wall -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -ffreestanding -O2 -fomit-frame-pointer -g -I/home/tbird/work/linux/ -I /home/tbird/work/linux/include/asm/gcc -G 0 -mno-abicalls -fno-pic -pipe -finline-limit=100000 -mabi=32 -march=mips32r2 -Wa,-32 -Wa,-march=mips32r2 -Wa,-mips32r2 -Wa,--trap -I/home/tbird/work/linux/include/asm-mips/ati -Iinclude/asm-mips/ati -I/home/tbird/work/linux/include/asm-mips/mach-generic -Iinclude/asm-mips/mach-generic -Wdeclaration-after-statement -DKBUILD_BASENAME=buffer -DKBUILD_MODNAME=buffer -c -o fs/buffer.o /home/tbird/work/linux/fs/buffer.c