Getting user-space stack backtraces in more probe contexts

[Frank Ch. Eigler]

roland wrote:

> [...]
> Now, to those three paths.
> 1. Work with what you got.
> [...]
>    This means, give the user unwinder some arch-specific code to
>    prime its state from a known-to-be-partial struct pt_regs.  [...]
>    [...]  But the bare guess is that it might well tend to cover
>    just recovering the PC and CFA (enough to keep doing a basic
>    backtrace) much more often than it covers all the registers [...]

I like it as a fallback / heuristic.  (Plus we should be able to fall
back to frame-pointer heuristics and/or the kernel's guesswork.)


> 2. Turtles all the way down!
> [...]
>    (The turtles are made of CFI.)  That is, unwind in kernel space all
>    the way back to the red line.  [...]
>    In 100% proper CFI these frames are marked as "signal frames" (it's
>    part of the "augmentation string"), so you can see those and then
>    check whether the "caller's PC" of that frame is < TASK_SIZE
>    [...]
>    All this requires is that all kernel code have CFI, that the CFI be
>    correct, and that you have that CFI.  Three small matters.  [...]

I like it.  This seems like the best first try.


> 3. Two phase with a safe point
>    This is the notion that Will mentioned, but there is a general and
>    optimal way to do it.  It's a classic "software interrupt" scheme:
>    at an arbitrary point, put down a marker [...]

I don't like it as much.  It's far more complex, plus I would like not
to sacrifice the ability to process backtraces as first class run-time
objects.


> 4. Pre-collect via syscall-entry tracing
> [...]

I like this, but doesn't appear to handle the interrupt / signal /
preemption type involuntary jumps into kernel space.


- FChE