Tainted flag in bugs, oops or panics messages¶

You find the tainted state near the top in a line starting with ‘CPU:’; if orwhy the kernel was tainted is shown after the Process ID (‘PID:’) and a shortenedname of the command (‘Comm:’) that triggered the event:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000Oops: 0002 [#1] SMP PTICPU: 0 PID: 4424 Comm: insmod Tainted: P        W  O      4.20.0-0.rc6.fc30 #1Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011RIP: 0010:my_oops_init+0x13/0x1000 [kpanic][...]

You’ll find a ‘Not tainted: ‘ there if the kernel was not tainted at thetime of the event; if it was, then it will print ‘Tainted: ‘ and characterseither letters or blanks. In the example above it looks like this:

Tainted: P        W  O

The meaning of those characters is explained in the table below. In this casethe 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 readingcat/proc/sys/kernel/tainted. If that returns0, the kernel is nottainted; any other number indicates the reasons why it is. The easiest way todecode that number is the scripttools/debugging/kernel-chktaint, which yourdistribution might ship as part of a package calledlinux-tools orkernel-tools; if it doesn’t, you can download the script fromgit.kernel.organd execute it withshkernel-chktaint, which would print something likethis on the machine that had the statements in the logs that were quoted earlier:

Kernel is Tainted for following reasons: * Proprietary module was loaded (#0) * Kernel issued warning (#9) * Externally-built ('out-of-tree') module was loaded  (#12)See Documentation/admin-guide/tainted-kernels.rst in the Linux kernel or https://www.kernel.org/doc/html/latest/admin-guide/tainted-kernels.html for a more details explanation of the various taint flags.Raw taint value as int/string: 4609/'P        W  O     '

You can try to decode the number yourself. That’s easy if there was only onereason that got your kernel tainted, as in this case you can find the numberwith the table below. If there were multiple reasons you need to decode thenumber, as it is a bitfield, where each bit indicates the absence or presence ofa particular type of taint. It’s best to leave that to the aforementionedscript, but if you need something quick you can use this shell command to checkwhich bits are set:

$ for i in $(seq 18); do echo $(($i-1)) $(($(cat /proc/sys/kernel/tainted)>>($i-1)&1));done

Table for decoding tainted state¶

Note: The character_ is representing a blank in this table to make readingeasier.

More detailed explanation for tainting¶

0. G if all modules loaded have a GPL or compatible license,P ifany proprietary module has been loaded. Modules without aMODULE_LICENSE or with a MODULE_LICENSE that is not recognised byinsmod as GPL compatible are assumed to be proprietary.

1. F if any module was force loaded byinsmod-f,'' if allmodules were loaded normally.

2. S if the kernel is running on a processor or system that is out ofspecification: hardware has been put into an unsupported configuration,therefore proper execution cannot be guaranteed.Kernel will be tainted if, for example:

   • on x86: PAE is forced through forcepae on intel CPUs (such as Pentium M)which do not report PAE but may have a functional implementation, an SMPkernel is running on non officially capable SMP Athlon CPUs, MSRs arebeing poked at from userspace.

   • on arm: kernel running on certain CPUs (such as Keystone 2) withouthaving certain kernel features enabled.

   • on arm64: there are mismatched hardware features between CPUs, thebootloader has booted CPUs in different modes.

   • certain drivers are being used on non supported architectures (such asscsi/snic on something else than x86_64, scsi/ips on nonx86/x86_64/itanium, have broken firmware settings for theirqchip/irq-gic on arm64 ...).

   • x86/x86_64: Microcode late loading is dangerous and will result intainting the kernel. It requires that all CPUs rendezvous to make surethe update happens when the system is as quiescent as possible. However,a higher priority MCE/SMI/NMI can move control flow away from thatrendezvous and interrupt the update, which can be detrimental to themachine.

3. R if a module was force unloaded byrmmod-f,'' if allmodules were unloaded normally.

4. M if any processor has reported a Machine Check Exception,'' if no Machine Check Exceptions have occurred.

5. B If a page-release function has found a bad page reference or someunexpected page flags. This indicates a hardware problem or a kernel bug;there should be other information in the log indicating why this taintingoccurred.

6. U if a user or user application specifically requested that theTainted flag be set,'' otherwise.

7. D if the kernel has died recently, i.e. there was an OOPS or BUG.

8. A if an ACPI table has been overridden.

9. W if a warning has previously been issued by the kernel.(Though some warnings may set more specific taint flags.)

10. C if a staging driver has been loaded.

11. I if the kernel is working around a severe bug in the platformfirmware (BIOS or similar).

12. O if an externally-built (“out-of-tree”) module has been loaded.

13. E if an unsigned module has been loaded in a kernel supportingmodule signature.

14. L if a soft lockup has previously occurred on the system.

15. K if the kernel has been live patched.

16. X Auxiliary taint, defined for and used by Linux distributors.

17. T Kernel was build with the randstruct plugin, which can intentionallyproduce extremely unusual kernel structure layouts (even performancepathological ones), which is important to know when debugging. Set atbuild time.

18. N if an in-kernel test, such as a KUnit test, has been run.

19. J if userspace opened /dev/fwctl/* and performed a FWTCL_RPC_DEBUG_WRITEto use the devices debugging features. Device debugging features couldcause the device to malfunction in undefined ways.