docs: admin-guide: for kernel bugs refer to other kernel documentation

If something goes wrong

-----------------------

 - If you have problems that seem to be due to kernel bugs, please check

   the file MAINTAINERS to see if there is a particular person associated

   with the part of the kernel that you are having trouble with. If there

   isn't anyone listed there, then the second best thing is to mail

   them to me (torvalds@linux-foundation.org), and possibly to any other

   relevant mailing-list or to the newsgroup.

 - In all bug-reports, *please* tell what kernel you are talking about,

   how to duplicate the problem, and what your setup is (use your common

   sense).  If the problem is new, tell me so, and if the problem is

   old, please try to tell me when you first noticed it.

 - If the bug results in a message like::

     unable to handle kernel paging request at address C0000010

     Oops: 0002

     EIP:   0010:XXXXXXXX

     eax: xxxxxxxx   ebx: xxxxxxxx   ecx: xxxxxxxx   edx: xxxxxxxx

     esi: xxxxxxxx   edi: xxxxxxxx   ebp: xxxxxxxx

     ds: xxxx  es: xxxx  fs: xxxx  gs: xxxx

     Pid: xx, process nr: xx

     xx xx xx xx xx xx xx xx xx xx

   or similar kernel debugging information on your screen or in your

   system log, please duplicate it *exactly*.  The dump may look

   incomprehensible to you, but it does contain information that may

   help debugging the problem.  The text above the dump is also

   important: it tells something about why the kernel dumped code (in

   the above example, it's due to a bad kernel pointer). More information

   on making sense of the dump is in Documentation/admin-guide/bug-hunting.rst

 - If you compiled the kernel with CONFIG_KALLSYMS you can send the dump

   as is, otherwise you will have to use the ``ksymoops`` program to make

   sense of the dump (but compiling with CONFIG_KALLSYMS is usually preferred).

   This utility can be downloaded from

   https://www.kernel.org/pub/linux/utils/kernel/ksymoops/ .

   Alternatively, you can do the dump lookup by hand:

 - In debugging dumps like the above, it helps enormously if you can

   look up what the EIP value means.  The hex value as such doesn't help

   me or anybody else very much: it will depend on your particular

   kernel setup.  What you should do is take the hex value from the EIP

   line (ignore the ``0010:``), and look it up in the kernel namelist to

   see which kernel function contains the offending address.

   To find out the kernel function name, you'll need to find the system

   binary associated with the kernel that exhibited the symptom.  This is

   the file 'linux/vmlinux'.  To extract the namelist and match it against

   the EIP from the kernel crash, do::

     nm vmlinux | sort | less

   This will give you a list of kernel addresses sorted in ascending

   order, from which it is simple to find the function that contains the

   offending address.  Note that the address given by the kernel

   debugging messages will not necessarily match exactly with the

   function addresses (in fact, that is very unlikely), so you can't

   just 'grep' the list: the list will, however, give you the starting

   point of each kernel function, so by looking for the function that

   has a starting address lower than the one you are searching for but

   is followed by a function with a higher address you will find the one

   you want.  In fact, it may be a good idea to include a bit of

   "context" in your problem report, giving a few lines around the

   interesting one.

   If you for some reason cannot do the above (you have a pre-compiled

   kernel image or similar), telling me as much about your setup as

   possible will help.  Please read

   'Documentation/admin-guide/reporting-issues.rst' for details.

 - Alternatively, you can use gdb on a running kernel. (read-only; i.e. you

   cannot change values or set break points.) To do this, first compile the

   kernel with -g; edit arch/x86/Makefile appropriately, then do a ``make

   clean``. You'll also need to enable CONFIG_PROC_FS (via ``make config``).

   After you've rebooted with the new kernel, do ``gdb vmlinux /proc/kcore``.

   You can now use all the usual gdb commands. The command to look up the

   point where your system crashed is ``l *0xXXXXXXXX``. (Replace the XXXes

   with the EIP value.)

   gdb'ing a non-running kernel currently fails because ``gdb`` (wrongly)

   disregards the starting offset for which the kernel is compiled.

If you have problems that seem to be due to kernel bugs, please follow the

instructions at 'Documentation/admin-guide/reporting-issues.rst'.

Hints on understanding kernel bug reports are in

'Documentation/admin-guide/bug-hunting.rst'. More on debugging the kernel

with gdb is in 'Documentation/dev-tools/gdb-kernel-debugging.rst' and

'Documentation/dev-tools/kgdb.rst'.