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SubjectRe: [PATCH v4 2/2] s390/livepatch: Implement reliable stack tracing for the consistency model
On Fri, 29 Nov 2019, Vasily Gorbik wrote:

> From: Miroslav Benes <mbenes@suse.cz>
>
> The livepatch consistency model requires reliable stack tracing
> architecture support in order to work properly. In order to achieve
> this, two main issues have to be solved. First, reliable and consistent
> call chain backtracing has to be ensured. Second, the unwinder needs to
> be able to detect stack corruptions and return errors.
>
> The "zSeries ELF Application Binary Interface Supplement" says:
>
> "The stack pointer points to the first word of the lowest allocated
> stack frame. If the "back chain" is implemented this word will point to
> the previously allocated stack frame (towards higher addresses), except
> for the first stack frame, which shall have a back chain of zero (NULL).
> The stack shall grow downwards, in other words towards lower addresses."
>
> "back chain" is optional. GCC option -mbackchain enables it. Quoting
> Martin Schwidefsky [1]:
>
> "The compiler is called with the -mbackchain option, all normal C
> function will store the backchain in the function prologue. All
> functions written in assembler code should do the same, if you find one
> that does not we should fix that. The end result is that a task that
> *voluntarily* called schedule() should have a proper backchain at all
> times.
>
> Dependent on the use case this may or may not be enough. Asynchronous
> interrupts may stop the CPU at the beginning of a function, if kernel
> preemption is enabled we can end up with a broken backchain. The
> production kernels for IBM Z are all compiled *without* kernel
> preemption. So yes, we might get away without the objtool support.
>
> On a side-note, we do have a line item to implement the ORC unwinder for
> the kernel, that includes the objtool support. Once we have that we can
> drop the -mbackchain option for the kernel build. That gives us a nice
> little performance benefit. I hope that the change from backchain to the
> ORC unwinder will not be too hard to implement in the livepatch tools."
>
> Since -mbackchain is enabled by default when the kernel is compiled, the
> call chain backtracing should be currently ensured and objtool should
> not be necessary for livepatch purposes.
>
> Regarding the second issue, stack corruptions and non-reliable states
> have to be recognized by the unwinder. Mainly it means to detect
> preemption or page faults, the end of the task stack must be reached,
> return addresses must be valid text addresses and hacks like function
> graph tracing and kretprobes must be properly detected.
>
> Unwinding a running task's stack is not a problem, because there is a
> livepatch requirement that every checked task is blocked, except for the
> current task. Due to that, we can consider a task's kernel/thread stack
> only and skip the other stacks.
>
> [1] 20180912121106.31ffa97c@mschwideX1 [not archived on lore.kernel.org]
>
> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Signed-off-by: Miroslav Benes <mbenes@suse.cz>

M

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