Re: [PATCH 0/1] RFC: Revamp admin-guide/tainted-kernels.rst to make it more comprehensible

Thorsten Leemhuis
2019-01-03T09:33:04Z

Hi Jonathan! If you have a minute could you provide feedback on below
mail? I sent it right before Christmas to get it of my todo list, but
due to the timing it afaics fell through the cracks a bit, as I had
feared already (no worries). Ciao, Thorsten

Am 21.12.18 um 16:26 schrieb Thorsten Leemhuis:
> Hi! Am 17.12.18 um 19:24 schrieb Jonathan Corbet:
>> On Mon, 17 Dec 2018 16:20:42 +0100
>> Thorsten Leemhuis <linux@leemhuis.info> wrote:
>>
>>> +might be relevant later when investigating problems. Don't worry
>>> +yourself too much about this, most of the time it's not a problem to run
>> s/yourself//
>
> Thx for this and other suggestions or fixes, consider them implemented when
> not mentioned in this mail. Find the current state of the text at the end of
> this mail for reference.
>
>> [...]
>>> +At runtime, you can query the tainted state by reading
>>> +``/proc/sys/kernel/tainted``. If that returns ``0``, the kernel is not
>>> +tainted; any other number indicates the reasons why it is. You might
>>> +find that number in below table if there was only one reason that got
>>> +the kernel tainted. If there were multiple reasons you need to decode
>>> +the number, as it is a bitfield, where each bit indicates the absence or
>>> +presence of a particular type of taint. You can use the following python
>>> +command to decode::
>> Here's an idea if you feel like improving this: rather than putting an
>> inscrutable program inline, add a taint_status script to scripts/ that
>> prints out the status in fully human-readable form, with the explanation
>> for every set bit.
>
> I posted the script earlier today and noticed now that it prints only
> the fully human-readable form, not if a bit it set or unset. Would you
> prefer if it did that as well?
>
>>> +=== === ====== ========================================================
>>> +Bit Log Int Reason that got the kernel tainted
>>> +=== === ====== ========================================================
>>> + 1) G/P 0 proprietary module got loaded
>> I'd s/got/was/ throughout. Also, this is the kernel, we start counting at
>> zero! :)
>
> Hehe, yeah :-D At first I actually started at zero, but that looked
> odd as the old explanations (those already in the file) start to could at one.
> Having a off-by-one within one document is just confusing, that's why I
> decided against starting at zero here.
>
> Another reason that came to my mind when reading your comment: Yes, this
> is the kernel, but the document should be easy to understand even for
> inexperienced users (e.g. people that know how to open and use command
> line tools, but never learned programming). That's why I leaning towards
> starting with one everywhere. But yes, that can be confusing, that's
> why I added a note, albeit I'm not really happy with it yet:
>
> """
> Note: This document is aimed at users and thus starts to count at one here and
> in other places. Use ``seq 0 17`` instead to start counting at zero, as it's
> normal for developers.
> """
>
> See below for full context. Anyway: I can change the text to start at zero if
> you prefer it.
>
> Ciao, Thorsten
>
> ---
>
> Tainted kernels
> ---------------
>
> The kernel will mark itself as 'tainted' when something occurs that might be
> relevant later when investigating problems. Don't worry too much about this,
> most of the time it's not a problem to run a tainted kernel; the information is
> mainly of interest once someone wants to investigate some problem, as its real
> cause might be the event that got the kernel tainted. That's why bug reports
> from tainted kernels will often be ignored by developers, hence try to reproduce
> problems with an untainted kernel.
>
> Note the kernel will remain tainted even after you undo what caused the taint
> (i.e. unload a proprietary kernel module), to indicate the kernel remains not
> trustworthy. That's also why the kernel will print the tainted state when it
> notices an internal problem (a 'kernel bug'), a recoverable error
> ('kernel oops') or a non-recoverable error ('kernel panic') and writes debug
> information about this to the logs ``dmesg`` outputs. It's also possible to
> check the tainted state at runtime through a file in ``/proc/``.
>
>
> Tainted flag in bugs, oops or panics messages
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> You find the tainted state near the top in a line starting with 'CPU:'; if or
> why the kernel is shown after the Process ID ('PID:') and a shortened name of
> the command ('Comm:') that triggered the event:
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
> Oops: 0002 [#1] SMP PTI
> CPU: 0 PID: 4424 Comm: insmod Tainted: P W O 4.20.0-0.rc6.fc30 #1
> Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
> RIP: 0010:my_oops_init+0x13/0x1000 [kpanic]
> [...]
>
> You'll find a **'Not tainted: '** there if the kernel was not tainted at the
> time of the event; if it was, then it will print **'Tainted: '** and characters
> either letters or blanks. The meaning of those characters is explained in the
> table below. In above example it's '``Tainted: P W O ``' as as the
> kernel got tainted earlier because a proprietary Module (``P``) was loaded, a
> warning occurred (``W``), and an externally-built module was loaded (``O``).
> To decode other letters use the table below.
>
>
> Decoding tainted state at runtime
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> At runtime, you can query the tainted state by reading
> ``cat /proc/sys/kernel/tainted``. If that returns ``0``, the kernel is not
> tainted; any other number indicates the reasons why it is. The easiest way to
> decode that number is the script ``tools/debugging/kernel-chktaint``, which your
> distribution might ship as part of a package called ``linux-tools`` or
> ``kernel-tools``; if it doesn't you can download the script from
> `git.kernel.org <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/tools/debugging/kernel-chktaint>`_.
> and execute it with ``sh kernel-chktaint``
>
> If you do not want to run that script you can try to decode the number yourself.
> That's easy if there was only one reason that got your kernel tainted, as in
> this case you can find the number with the table below. If there were multiple
> reasons you need to decode the number, as it is a bitfield, where each bit
> indicates the absence or presence of a particular type of taint. It's best to
> leave that to the aforementioned script, but if you need something quick you can
> use this shell command to check which bits are set:
>
> $ for i in $(seq 18); do echo $i $(($(cat /proc/sys/kernel/tainted)>>($i-1)&1));done
>
> Note: This document is aimed at users and thus starts to count at one here and
> in other places. Use ``seq 0 17`` instead to start counting at zero, as it's
> normal for developers.
>
> Table for decoding tainted state
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> ==== === ====== ========================================================
> Pos. Log Number Reason that got the kernel tainted
> ==== === ====== ========================================================
> 1) G/P 0 proprietary module was loaded
> 2) _/F 2 module was force loaded
> 3) _/S 4 SMP kernel oops on an officially SMP incapable processor
> 4) _/R 8 module was force unloaded
> 5) _/M 16 processor reported a Machine Check Exception (MCE)
> 6) _/B 32 bad page referenced or some unexpected page flags
> 7) _/U 64 taint requested by userspace application
> 8) _/D 128 kernel died recently, i.e. there was an OOPS or BUG
> 9) _/A 256 ACPI table overridden by user
> 10) _/W 512 kernel issued warning
> 11) _/C 1024 staging driver was loaded
> 12) _/I 2048 workaround for bug in platform firmware applied
> 13) _/O 4096 externally-built ("out-of-tree") module was loaded
> 14) _/E 8192 unsigned module was loaded
> 15) _/L 16384 soft lockup occurred
> 16) _/K 32768 Kernel live patched
> 17) _/K 65536 Auxiliary taint, defined for and used by distros
> 18) _/K 131072 Kernel was built with the struct randomization plugin
> ==== === ====== ========================================================
>
> Note: To make reading easier ``_`` is representing a blank in this
> table.
>
> More detailed explanation for tainting
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> 1) ``G`` if all modules loaded have a GPL or compatible license, ``P`` if
> any proprietary module has been loaded. Modules without a
> MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
> insmod as GPL compatible are assumed to be proprietary.
>
> 2) ``F`` if any module was force loaded by ``insmod -f``, ``' '`` if all
> modules were loaded normally.
>
> 3) ``S`` if the oops occurred on an SMP kernel running on hardware that
> hasn't been certified as safe to run multiprocessor.
> Currently this occurs only on various Athlons that are not
> SMP capable.
>
> 4) ``R`` if a module was force unloaded by ``rmmod -f``, ``' '`` if all
> modules were unloaded normally.
>
> 5) ``M`` if any processor has reported a Machine Check Exception,
> ``' '`` if no Machine Check Exceptions have occurred.
>
> 6) ``B`` if a page-release function has found a bad page reference or
> some unexpected page flags.
>
> 7) ``U`` if a user or user application specifically requested that the
> Tainted flag be set, ``' '`` otherwise.
>
> 8) ``D`` if the kernel has died recently, i.e. there was an OOPS or BUG.
>
> 9) ``A`` if the ACPI table has been overridden.
>
> 10) ``W`` if a warning has previously been issued by the kernel.
> (Though some warnings may set more specific taint flags.)
>
> 11) ``C`` if a staging driver has been loaded.
>
> 12) ``I`` if the kernel is working around a severe bug in the platform
> firmware (BIOS or similar).
>
> 13) ``O`` if an externally-built ("out-of-tree") module has been loaded.
>
> 14) ``E`` if an unsigned module has been loaded in a kernel supporting
> module signature.
>
> 15) ``L`` if a soft lockup has previously occurred on the system.
>
> 16) ``K`` if the kernel has been live patched.
>
> 17) ``X`` Auxiliary taint, defined for and used by Linux distributors.
>
> 18) ``T`` Kernel was build with randstruct plugin, which can intentionally
> produce extremely unusual kernel structure layouts (even performance
> pathological ones), which is important to know when debugging. Set at
> build time.
>