Re: [PATCH v3] kernel: add panic_on_taint

From: Luis Chamberlain
Date: Mon May 11 2020 - 14:24:59 EST


On Sat, May 09, 2020 at 09:57:37AM -0400, Rafael Aquini wrote:
> +Trigger Kdump on add_taint()
> +============================
> +
> +The kernel parameter, panic_on_taint, calls panic() from within add_taint(),
> +whenever the value set in this bitmask matches with the bit flag being set
> +by add_taint(). This will cause a kdump to occur at the panic() call.
> +In cases where a user wants to specify this during runtime,
> +/proc/sys/kernel/panic_on_taint can be set to a respective bitmask value
> +to achieve the same behaviour.
> +
> Contact
> =======
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 7bc83f3d9bdf..4a69fe49a70d 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3404,6 +3404,21 @@
> panic_on_warn panic() instead of WARN(). Useful to cause kdump
> on a WARN().
>
> + panic_on_taint= [KNL] conditionally panic() in add_taint()
> + Format: <str>
> + Specifies, as a string, the TAINT flag set that will
> + compose a bitmask for calling panic() when the kernel
> + gets tainted.
> + See Documentation/admin-guide/tainted-kernels.rst for
> + details on the taint flags that users can pick to
> + compose the bitmask to assign to panic_on_taint.
> + When the string is prefixed with a '-' the bitmask
> + set in panic_on_taint will be mutually exclusive
> + with the sysctl knob kernel.tainted, and any attempt
> + to write to that sysctl will fail with -EINVAL for
> + any taint value that masks with the flags set for
> + this option.

This talks about using a string, but that it sets a bitmask. Its not
very clear that one must use the string representation from each taint
flag. Also, I don't think to use the character representation as we
limit ourselves to the alphabet and quirky what-should-be-arbitrary
characters that represent the taint flags. The taint flag character
representation is juse useful for human reading of a panic, but I think
because of the limitation of the mask with the alphabet this was not
such a great idea long term.

So, I don't think we should keep on extending the alphabet use case, a
simple digit representation would suffice. I think this means we'd need
two params one for exclusive and one for the value of the taint.

Using a hex value or number also lets us make the input value shorter.

If a kernel boots with panic-on-taint flag not yet supported, we don't
complain, therefore getting a false sense of security that we will panic
with a not yet supported taint flag. I think we should pr_warn() or
fail to boot when that happens.

Luis