Re: [PATCH v5 01/15] stacktrace/x86: add function for detecting reliable stack traces

[Josh Poimboeuf]

On Tue, Mar 07, 2017 at 05:50:55PM +1100, Balbir Singh wrote:
> 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@xxxxxxxxxx>
> > ---
> 
> 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?

I tried to cover this comprehensively in patch 13/15 in
Documentation/livepatch/livepatch.txt.  Does that answer your questions?

-- 
Josh