Now that oops-tracing.rst has only information about
stack dumps found on OOPS, and bug-hunting.rst has only
information about how to identify the source code line
associated with a stack dump, let's merge them and
improve the information inside it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/bug-hunting.rst | 350 ++++++++++++++++++++++++-----
Documentation/admin-guide/index.rst | 1 -
Documentation/admin-guide/oops-tracing.rst | 241 --------------------
3 files changed, 289 insertions(+), 303 deletions(-)
delete mode 100644 Documentation/admin-guide/oops-tracing.rst
diff --git a/Documentation/admin-guide/bug-hunting.rst b/Documentation/admin-guide/bug-hunting.rst
index d245d4677ae2..08c4b1308189 100644
--- a/Documentation/admin-guide/bug-hunting.rst
+++ b/Documentation/admin-guide/bug-hunting.rst
@@ -1,68 +1,113 @@
Bug hunting
-+++++++++++
+===========
-Last updated: 28 October 2016
+Kernel bug reports often come with a stack dump like the one below::
-Fixing the bug
-==============
+ ------------[ cut here ]------------
+ WARNING: CPU: 1 PID: 28102 at kernel/module.c:1108 module_put+0x57/0x70
+ Modules linked in: dvb_usb_gp8psk(-) dvb_usb dvb_core nvidia_drm(PO) nvidia_modeset(PO) snd_hda_codec_hdmi snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_timer snd soundcore nvidia(PO) [last unloaded: rc_core]
+ CPU: 1 PID: 28102 Comm: rmmod Tainted: P WC O 4.8.4-build.1 #1
+ Hardware name: MSI MS-7309/MS-7309, BIOS V1.12 02/23/2009
+ 00000000 c12ba080 00000000 00000000 c103ed6a c1616014 00000001 00006dc6
+ c1615862 00000454 c109e8a7 c109e8a7 00000009 ffffffff 00000000 f13f6a10
+ f5f5a600 c103ee33 00000009 00000000 00000000 c109e8a7 f80ca4d0 c109f617
+ Call Trace:
+ [<c12ba080>] ? dump_stack+0x44/0x64
+ [<c103ed6a>] ? __warn+0xfa/0x120
+ [<c109e8a7>] ? module_put+0x57/0x70
+ [<c109e8a7>] ? module_put+0x57/0x70
+ [<c103ee33>] ? warn_slowpath_null+0x23/0x30
+ [<c109e8a7>] ? module_put+0x57/0x70
+ [<f80ca4d0>] ? gp8psk_fe_set_frontend+0x460/0x460 [dvb_usb_gp8psk]
+ [<c109f617>] ? symbol_put_addr+0x27/0x50
+ [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
+ [<f80bb3bf>] ? dvb_usb_exit+0x2f/0xd0 [dvb_usb]
+ [<c13d03bc>] ? usb_disable_endpoint+0x7c/0xb0
+ [<f80bb48a>] ? dvb_usb_device_exit+0x2a/0x50 [dvb_usb]
+ [<c13d2882>] ? usb_unbind_interface+0x62/0x250
+ [<c136b514>] ? __pm_runtime_idle+0x44/0x70
+ [<c13620d8>] ? __device_release_driver+0x78/0x120
+ [<c1362907>] ? driver_detach+0x87/0x90
+ [<c1361c48>] ? bus_remove_driver+0x38/0x90
+ [<c13d1c18>] ? usb_deregister+0x58/0xb0
+ [<c109fbb0>] ? SyS_delete_module+0x130/0x1f0
+ [<c1055654>] ? task_work_run+0x64/0x80
+ [<c1000fa5>] ? exit_to_usermode_loop+0x85/0x90
+ [<c10013f0>] ? do_fast_syscall_32+0x80/0x130
+ [<c1549f43>] ? sysenter_past_esp+0x40/0x6a
+ ---[ end trace 6ebc60ef3981792f ]---
-Nobody is going to tell you how to fix bugs. Seriously. You need to work it
-out. But below are some hints on how to use the tools.
+Such stack traces provide enough information to identify the line inside the
+Kernel's source code where the bug happened. Depending on the severity of
+the issue, it may also contain the word **Oops**, as on this one::
-objdump
--------
+ BUG: unable to handle kernel NULL pointer dereference at (null)
+ IP: [<c06969d4>] iret_exc+0x7d0/0xa59
+ *pdpt = 000000002258a001 *pde = 0000000000000000
+ Oops: 0002 [#1] PREEMPT SMP
+ ...
-To debug a kernel, use objdump and look for the hex offset from the crash
-output to find the valid line of code/assembler. Without debug symbols, you
-will see the assembler code for the routine shown, but if your kernel has
-debug symbols the C code will also be available. (Debug symbols can be enabled
-in the kernel hacking menu of the menu configuration.) For example::
-
- $ objdump -r -S -l --disassemble net/dccp/ipv4.o
+Despite being an **Oops** or some other sort of stack trace, the offended
+line is usually required to identify and handle the bug. Along this chapter,
+we'll refer to "Oops" for all kinds of stack traces that need to be analized.
.. note::
- You need to be at the top level of the kernel tree for this to pick up
- your C files.
-
-If you don't have access to the code you can also debug on some crash dumps
-e.g. crash dump output as shown by Dave Miller::
-
- EIP is at +0x14/0x4c0
- ...
- Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
- 00 00 55 57 56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
- <8b> 83 3c 01 00 00 89 44 24 14 8b 45 28 85 c0 89 44 24 18 0f 85
-
- Put the bytes into a "foo.s" file like this:
-
- .text
- .globl foo
- foo:
- .byte .... /* bytes from Code: part of OOPS dump */
-
- Compile it with "gcc -c -o foo.o foo.s" then look at the output of
- "objdump --disassemble foo.o".
-
- Output:
-
- ip_queue_xmit:
- push %ebp
- push %edi
- push %esi
- push %ebx
- sub $0xbc, %esp
- mov 0xd0(%esp), %ebp ! %ebp = arg0 (skb)
- mov 0x8(%ebp), %ebx ! %ebx = skb->sk
- mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
+ ``ksymoops`` is useless on 2.6 or upper. Please use the Oops in its original
+ format (from ``dmesg``, etc). Ignore any references in this or other docs to
+ "decoding the Oops" or "running it through ksymoops".
+ If you post an Oops from 2.6+ that has been run through ``ksymoops``,
+ people will just tell you to repost it.
+
+Where is the Oops message is located?
+-------------------------------------
+
+Normally the Oops text is read from the kernel buffers by klogd and
+handed to ``syslogd`` which writes it to a syslog file, typically
+``/var/log/messages`` (depends on ``/etc/syslog.conf``). On systems with
+systemd, it may also be stored by the ``journald`` daemon, and accessed
+by running ``journalctl`` command.
+
+Sometimes ``klogd`` dies, in which case you can run ``dmesg > file`` to
+read the data from the kernel buffers and save it. Or you can
+``cat /proc/kmsg > file``, however you have to break in to stop the transfer,
+``kmsg`` is a "never ending file".
+
+If the machine has crashed so badly that you cannot enter commands or
+the disk is not available then you have three options:
+
+(1) Hand copy the text from the screen and type it in after the machine
+ has restarted. Messy but it is the only option if you have not
+ planned for a crash. Alternatively, you can take a picture of
+ the screen with a digital camera - not nice, but better than
+ nothing. If the messages scroll off the top of the console, you
+ may find that booting with a higher resolution (eg, ``vga=791``)
+ will allow you to read more of the text. (Caveat: This needs ``vesafb``,
+ so won't help for 'early' oopses)
+
+(2) Boot with a serial console (see
+ :ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
+ run a null modem to a second machine and capture the output there
+ using your favourite communication program. Minicom works well.
+
+(3) Use Kdump (see Documentation/kdump/kdump.txt),
+ extract the kernel ring buffer from old memory with using dmesg
+ gdbmacro in Documentation/kdump/gdbmacros.txt.
+
+Finding the bug's location
+--------------------------
+
+Reporting a bug works best if you point the location of the bug at the
+Kernel source file. There are two methods for doing that. Usually, using
+``gdb`` is easier, but the Kernel should be pre-compiled with debug info.
gdb
----
+^^^
-In addition, you can use GDB to figure out the exact file and line
+The GNU debug (``gdb``) is the best way to figure out the exact file and line
number of the OOPS from the ``vmlinux`` file.
-The usage of gdb requires a kernel compiled with ``CONFIG_DEBUG_INFO``.
+The usage of gdb works best on a kernel compiled with ``CONFIG_DEBUG_INFO``.
This can be set by running::
$ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
@@ -84,6 +129,7 @@ offset from the OOPS::
And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled::
+ $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
$ make vmlinux
$ gdb vmlinux
(gdb) l *vt_ioctl+0xda8
@@ -125,17 +171,199 @@ in gdb and list the relevant code::
$ gdb fs/jbd/jbd.ko
(gdb) l *log_wait_commit+0xa3
-Another very useful option of the Kernel Hacking section in menuconfig is
-Debug memory allocations. This will help you see whether data has been
-initialised and not set before use etc. To see the values that get assigned
-with this look at ``mm/slab.c`` and search for ``POISON_INUSE``. When using
-this an Oops will often show the poisoned data instead of zero which is the
-default.
-
-Once you have worked out a fix please submit it upstream. After all open
-source is about sharing what you do and don't you want to be recognised for
-your genius?
+.. note::
+
+ You can also do the same for any function call at the stack trace,
+ like this one::
+
+ [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
+
+ The position where the above call happened can be seen with::
+
+ $ gdb drivers/media/usb/dvb-usb/dvb-usb.o
+ (gdb) l *dvb_usb_adapter_frontend_exit+0x3a
+
+objdump
+^^^^^^^
+
+To debug a kernel, use objdump and look for the hex offset from the crash
+output to find the valid line of code/assembler. Without debug symbols, you
+will see the assembler code for the routine shown, but if your kernel has
+debug symbols the C code will also be available. (Debug symbols can be enabled
+in the kernel hacking menu of the menu configuration.) For example::
+
+ $ objdump -r -S -l --disassemble net/dccp/ipv4.o
+
+.. note::
+
+ You need to be at the top level of the kernel tree for this to pick up
+ your C files.
+
+If you don't have access to the code you can also debug on some crash dumps
+e.g. crash dump output as shown by Dave Miller::
+
+ EIP is at +0x14/0x4c0
+ ...
+ Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
+ 00 00 55 57 56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
+ <8b> 83 3c 01 00 00 89 44 24 14 8b 45 28 85 c0 89 44 24 18 0f 85
+
+ Put the bytes into a "foo.s" file like this:
+
+ .text
+ .globl foo
+ foo:
+ .byte .... /* bytes from Code: part of OOPS dump */
+
+ Compile it with "gcc -c -o foo.o foo.s" then look at the output of
+ "objdump --disassemble foo.o".
+
+ Output:
+
+ ip_queue_xmit:
+ push %ebp
+ push %edi
+ push %esi
+ push %ebx
+ sub $0xbc, %esp
+ mov 0xd0(%esp), %ebp ! %ebp = arg0 (skb)
+ mov 0x8(%ebp), %ebx ! %ebx = skb->sk
+ mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
+
+Reporting the bug
+-----------------
+
+Once you find where the bug happened, by inspecting its location,
+you could either try to fix it yourself or report it upstream.
+
+In order to report it upstream, you should identify the mailing list
+used for the development of the affected code. This can be done by using
+the ``get_maintainer.pl`` script.
+
+For example, if you find a bug at the gspca's conex.c file, you can get
+their maintainers with::
+
+ $ ./scripts/get_maintainer.pl -f drivers/media/usb/gspca/sonixj.c
+ Hans Verkuil <hverkuil@xxxxxxxxx> (odd fixer:GSPCA USB WEBCAM DRIVER,commit_signer:1/1=100%)
+ Mauro Carvalho Chehab <mchehab@xxxxxxxxxx> (maintainer:MEDIA INPUT INFRASTRUCTURE (V4L/DVB),commit_signer:1/1=100%)
+ Tejun Heo <tj@xxxxxxxxxx> (commit_signer:1/1=100%)
+ Bhaktipriya Shridhar <bhaktipriya96@xxxxxxxxx> (commit_signer:1/1=100%,authored:1/1=100%,added_lines:4/4=100%,removed_lines:9/9=100%)
+ linux-media@xxxxxxxxxxxxxxx (open list:GSPCA USB WEBCAM DRIVER)
+ linux-kernel@xxxxxxxxxxxxxxx (open list)
+
+Please notice that it will point to:
+
+- The last developers that touched on the source code. On the above example,
+ Tejun and Bhaktipriya (in this specific case, none really envolved on the
+ development of this file);
+- The driver maintainer (Hans Verkuil);
+- The subsystem maintainer (Mauro Carvalho Chehab)
+- The driver and/or subsystem mailing list (linux-media@xxxxxxxxxxxxxxx);
+- the Linux Kernel mailing list (linux-kernel@xxxxxxxxxxxxxxx).
+
+Usually, the fastest way to have your bug fixed is to report it to mailing
+list used for the development of the code (linux-media ML) copying the driver maintainer (Hans).
+
+If you are totally stumped as to whom to send the report, and
+``get_maintainer.pl`` didn't provide you anything useful, send it to
+linux-kernel@xxxxxxxxxxxxxxx.
+
+Thanks for your help in making Linux as stable as humanly possible.
+
+Fixing the bug
+--------------
+
+If you know programming, you could help us by not only reporting the bug,
+but also providing us with a solution. After all open source is about
+sharing what you do and don't you want to be recognised for your genius?
+
+If you decide to take this way, once you have worked out a fix please submit
+it upstream.
Please do read
ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though
to help your code get accepted.
+
+
+---------------------------------------------------------------------------
+
+Notes on Oops tracing with ``klogd``
+------------------------------------
+
+In order to help Linus and the other kernel developers there has been
+substantial support incorporated into ``klogd`` for processing protection
+faults. In order to have full support for address resolution at least
+version 1.3-pl3 of the ``sysklogd`` package should be used.
+
+When a protection fault occurs the ``klogd`` daemon automatically
+translates important addresses in the kernel log messages to their
+symbolic equivalents. This translated kernel message is then
+forwarded through whatever reporting mechanism ``klogd`` is using. The
+protection fault message can be simply cut out of the message files
+and forwarded to the kernel developers.
+
+Two types of address resolution are performed by ``klogd``. The first is
+static translation and the second is dynamic translation. Static
+translation uses the System.map file in much the same manner that
+ksymoops does. In order to do static translation the ``klogd`` daemon
+must be able to find a system map file at daemon initialization time.
+See the klogd man page for information on how ``klogd`` searches for map
+files.
+
+Dynamic address translation is important when kernel loadable modules
+are being used. Since memory for kernel modules is allocated from the
+kernel's dynamic memory pools there are no fixed locations for either
+the start of the module or for functions and symbols in the module.
+
+The kernel supports system calls which allow a program to determine
+which modules are loaded and their location in memory. Using these
+system calls the klogd daemon builds a symbol table which can be used
+to debug a protection fault which occurs in a loadable kernel module.
+
+At the very minimum klogd will provide the name of the module which
+generated the protection fault. There may be additional symbolic
+information available if the developer of the loadable module chose to
+export symbol information from the module.
+
+Since the kernel module environment can be dynamic there must be a
+mechanism for notifying the ``klogd`` daemon when a change in module
+environment occurs. There are command line options available which
+allow klogd to signal the currently executing daemon that symbol
+information should be refreshed. See the ``klogd`` manual page for more
+information.
+
+A patch is included with the sysklogd distribution which modifies the
+``modules-2.0.0`` package to automatically signal klogd whenever a module
+is loaded or unloaded. Applying this patch provides essentially
+seamless support for debugging protection faults which occur with
+kernel loadable modules.
+
+The following is an example of a protection fault in a loadable module
+processed by ``klogd``::
+
+ Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
+ Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
+ Aug 29 09:51:01 blizard kernel: *pde = 00000000
+ Aug 29 09:51:01 blizard kernel: Oops: 0002
+ Aug 29 09:51:01 blizard kernel: CPU: 0
+ Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
+ Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
+ Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
+ Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
+ Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
+ Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
+ Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
+ Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
+ Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
+ Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
+ Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3
+
+---------------------------------------------------------------------------
+
+::
+
+ Dr. G.W. Wettstein Oncology Research Div. Computing Facility
+ Roger Maris Cancer Center INTERNET: greg@xxxxxxxxxxxxx
+ 820 4th St. N.
+ Fargo, ND 58122
+ Phone: 701-234-7556
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 86a6ab98d6b6..2681cbd24cdd 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -27,7 +27,6 @@ problems and bugs in particular.
security-bugs
bug-hunting
bug-bisect
- oops-tracing
tainted-kernels
ramoops
dynamic-debug-howto
diff --git a/Documentation/admin-guide/oops-tracing.rst b/Documentation/admin-guide/oops-tracing.rst
deleted file mode 100644
index 1f5e2b716631..000000000000
--- a/Documentation/admin-guide/oops-tracing.rst
+++ /dev/null
@@ -1,241 +0,0 @@
-OOPS tracing
-============
-
-.. note::
-
- ``ksymoops`` is useless on 2.6 or upper. Please use the Oops in its original
- format (from ``dmesg``, etc). Ignore any references in this or other docs to
- "decoding the Oops" or "running it through ksymoops".
- If you post an Oops from 2.6+ that has been run through ``ksymoops``,
- people will just tell you to repost it.
-
-Quick Summary
--------------
-
-Find the Oops and send it to the maintainer of the kernel area that seems to be
-involved with the problem. Don't worry too much about getting the wrong person.
-If you are unsure send it to the person responsible for the code relevant to
-what you were doing. If it occurs repeatably try and describe how to recreate
-it. That's worth even more than the oops.
-
-If you are totally stumped as to whom to send the report, send it to
-linux-kernel@xxxxxxxxxxxxxxx. Thanks for your help in making Linux as
-stable as humanly possible.
-
-Where is the Oops?
-----------------------
-
-Normally the Oops text is read from the kernel buffers by klogd and
-handed to ``syslogd`` which writes it to a syslog file, typically
-``/var/log/messages`` (depends on ``/etc/syslog.conf``). Sometimes ``klogd``
-dies, in which case you can run ``dmesg > file`` to read the data from the
-kernel buffers and save it. Or you can ``cat /proc/kmsg > file``, however you
-have to break in to stop the transfer, ``kmsg`` is a "never ending file".
-If the machine has crashed so badly that you cannot enter commands or
-the disk is not available then you have three options :
-
-(1) Hand copy the text from the screen and type it in after the machine
- has restarted. Messy but it is the only option if you have not
- planned for a crash. Alternatively, you can take a picture of
- the screen with a digital camera - not nice, but better than
- nothing. If the messages scroll off the top of the console, you
- may find that booting with a higher resolution (eg, ``vga=791``)
- will allow you to read more of the text. (Caveat: This needs ``vesafb``,
- so won't help for 'early' oopses)
-
-(2) Boot with a serial console (see
- :ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
- run a null modem to a second machine and capture the output there
- using your favourite communication program. Minicom works well.
-
-(3) Use Kdump (see Documentation/kdump/kdump.txt),
- extract the kernel ring buffer from old memory with using dmesg
- gdbmacro in Documentation/kdump/gdbmacros.txt.
-
-
-Full Information
-----------------
-
-.. note::
-
- the message from Linus below applies to 2.4 kernel. I have preserved it
- for historical reasons, and because some of the information in it still
- applies. Especially, please ignore any references to ksymoops.
-
- ::
-
- From: Linus Torvalds <torvalds@xxxxxxxx>
-
- How to track down an Oops.. [originally a mail to linux-kernel]
-
- The main trick is having 5 years of experience with those pesky oops
- messages ;-)
-
-Actually, there are things you can do that make this easier. I have two
-separate approaches::
-
- gdb /usr/src/linux/vmlinux
- gdb> disassemble <offending_function>
-
-That's the easy way to find the problem, at least if the bug-report is
-well made (like this one was - run through ``ksymoops`` to get the
-information of which function and the offset in the function that it
-happened in).
-
-Oh, it helps if the report happens on a kernel that is compiled with the
-same compiler and similar setups.
-
-The other thing to do is disassemble the "Code:" part of the bug report:
-ksymoops will do this too with the correct tools, but if you don't have
-the tools you can just do a silly program::
-
- char str[] = "\xXX\xXX\xXX...";
- main(){}
-
-and compile it with ``gcc -g`` and then do ``disassemble str`` (where the ``XX``
-stuff are the values reported by the Oops - you can just cut-and-paste
-and do a replace of spaces to ``\x`` - that's what I do, as I'm too lazy
-to write a program to automate this all).
-
-Alternatively, you can use the shell script in ``scripts/decodecode``.
-Its usage is::
-
- decodecode < oops.txt
-
-The hex bytes that follow "Code:" may (in some architectures) have a series
-of bytes that precede the current instruction pointer as well as bytes at and
-following the current instruction pointer. In some cases, one instruction
-byte or word is surrounded by ``<>`` or ``()``, as in ``<86>`` or ``(f00d)``.
-These ``<>`` or ``()`` markings indicate the current instruction pointer.
-
-Example from i386, split into multiple lines for readability::
-
- Code: f9 0f 8d f9 00 00 00 8d 42 0c e8 dd 26 11 c7 a1 60 ea 2b f9 8b 50 08 a1
- 64 ea 2b f9 8d 34 82 8b 1e 85 db 74 6d 8b 15 60 ea 2b f9 <8b> 43 04 39 42 54
- 7e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0
-
-Finally, if you want to see where the code comes from, you can do::
-
- cd /usr/src/linux
- make fs/buffer.s # or whatever file the bug happened in
-
-and then you get a better idea of what happens than with the gdb
-disassembly.
-
-Now, the trick is just then to combine all the data you have: the C
-sources (and general knowledge of what it **should** do), the assembly
-listing and the code disassembly (and additionally the register dump you
-also get from the "oops" message - that can be useful to see **what** the
-corrupted pointers were, and when you have the assembler listing you can
-also match the other registers to whatever C expressions they were used
-for).
-
-Essentially, you just look at what doesn't match (in this case it was the
-"Code" disassembly that didn't match with what the compiler generated).
-Then you need to find out **why** they don't match. Often it's simple - you
-see that the code uses a NULL pointer and then you look at the code and
-wonder how the NULL pointer got there, and if it's a valid thing to do
-you just check against it..
-
-Now, if somebody gets the idea that this is time-consuming and requires
-some small amount of concentration, you're right. Which is why I will
-mostly just ignore any panic reports that don't have the symbol table
-info etc looked up: it simply gets too hard to look it up (I have some
-programs to search for specific patterns in the kernel code segment, and
-sometimes I have been able to look up those kinds of panics too, but
-that really requires pretty good knowledge of the kernel just to be able
-to pick out the right sequences etc..)
-
-**Sometimes** it happens that I just see the disassembled code sequence
-from the panic, and I know immediately where it's coming from. That's when
-I get worried that I've been doing this for too long ;-)
-
- Linus
-
-
----------------------------------------------------------------------------
-
-Notes on Oops tracing with ``klogd``
-------------------------------------
-
-In order to help Linus and the other kernel developers there has been
-substantial support incorporated into ``klogd`` for processing protection
-faults. In order to have full support for address resolution at least
-version 1.3-pl3 of the ``sysklogd`` package should be used.
-
-When a protection fault occurs the ``klogd`` daemon automatically
-translates important addresses in the kernel log messages to their
-symbolic equivalents. This translated kernel message is then
-forwarded through whatever reporting mechanism ``klogd`` is using. The
-protection fault message can be simply cut out of the message files
-and forwarded to the kernel developers.
-
-Two types of address resolution are performed by ``klogd``. The first is
-static translation and the second is dynamic translation. Static
-translation uses the System.map file in much the same manner that
-ksymoops does. In order to do static translation the ``klogd`` daemon
-must be able to find a system map file at daemon initialization time.
-See the klogd man page for information on how ``klogd`` searches for map
-files.
-
-Dynamic address translation is important when kernel loadable modules
-are being used. Since memory for kernel modules is allocated from the
-kernel's dynamic memory pools there are no fixed locations for either
-the start of the module or for functions and symbols in the module.
-
-The kernel supports system calls which allow a program to determine
-which modules are loaded and their location in memory. Using these
-system calls the klogd daemon builds a symbol table which can be used
-to debug a protection fault which occurs in a loadable kernel module.
-
-At the very minimum klogd will provide the name of the module which
-generated the protection fault. There may be additional symbolic
-information available if the developer of the loadable module chose to
-export symbol information from the module.
-
-Since the kernel module environment can be dynamic there must be a
-mechanism for notifying the ``klogd`` daemon when a change in module
-environment occurs. There are command line options available which
-allow klogd to signal the currently executing daemon that symbol
-information should be refreshed. See the ``klogd`` manual page for more
-information.
-
-A patch is included with the sysklogd distribution which modifies the
-``modules-2.0.0`` package to automatically signal klogd whenever a module
-is loaded or unloaded. Applying this patch provides essentially
-seamless support for debugging protection faults which occur with
-kernel loadable modules.
-
-The following is an example of a protection fault in a loadable module
-processed by ``klogd``::
-
- Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
- Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
- Aug 29 09:51:01 blizard kernel: *pde = 00000000
- Aug 29 09:51:01 blizard kernel: Oops: 0002
- Aug 29 09:51:01 blizard kernel: CPU: 0
- Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
- Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
- Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
- Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
- Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
- Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
- Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
- Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
- Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
- Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
- Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3
-
----------------------------------------------------------------------------
-
-::
-
- Dr. G.W. Wettstein Oncology Research Div. Computing Facility
- Roger Maris Cancer Center INTERNET: greg@xxxxxxxxxxxxx
- 820 4th St. N.
- Fargo, ND 58122
- Phone: 701-234-7556
-
-
----------------------------------------------------------------------------
-
--
2.7.4
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