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    SubjectRe: [PATCH v5 01/15] stacktrace/x86: add function for detecting reliable stack traces
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    On Mon, 2017-02-13 at 19:42 -0600, Josh Poimboeuf wrote:
    > For live patching and possibly other use cases, a stack trace is only
    > useful if it can be assured that it's completely reliable. Add a new
    > save_stack_trace_tsk_reliable() function to achieve that.
    >
    > Note that if the target task isn't the current task, and the target task
    > is allowed to run, then it could be writing the stack while the unwinder
    > is reading it, resulting in possible corruption. So the caller of
    > save_stack_trace_tsk_reliable() must ensure that the task is either
    > 'current' or inactive.
    >
    > save_stack_trace_tsk_reliable() relies on the x86 unwinder's detection
    > of pt_regs on the stack. If the pt_regs are not user-mode registers
    > from a syscall, then they indicate an in-kernel interrupt or exception
    > (e.g. preemption or a page fault), in which case the stack is considered
    > unreliable due to the nature of frame pointers.
    >
    > It also relies on the x86 unwinder's detection of other issues, such as:
    >
    > - corrupted stack data
    > - stack grows the wrong way
    > - stack walk doesn't reach the bottom
    > - user didn't provide a large enough entries array
    >
    > Such issues are reported by checking unwind_error() and !unwind_done().
    >
    > Also add CONFIG_HAVE_RELIABLE_STACKTRACE so arch-independent code can
    > determine at build time whether the function is implemented.
    >
    > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
    > ---

    Could you comment on why we need a reliable trace for live-patching? Are
    we in any way reliant on the stack trace to patch something broken?

    Thanks,
    Balbir Singh.

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