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