Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable

[Madhavan T. Venkataraman]

On 3/23/21 9:57 AM, Mark Rutland wrote:
> On Tue, Mar 23, 2021 at 09:15:36AM -0500, Madhavan T. Venkataraman wrote:
>> Hi Mark,
>>
>> I have a general question. When exceptions are nested, how does it work? Let us consider 2 cases:
>>
>> 1. Exception in a page fault handler itself. In this case, I guess one more pt_regs will get
>>    established in the task stack for the second exception.
> 
> Generally (ignoring SDEI and stack overflow exceptions) the regs will be
> placed on the stack that was in use when the exception occurred, e.g.
> 
>   task -> task
>   irq -> irq
>   overflow -> overflow
> 
> For SDEI and stack overflow, we'll place the regs on the relevant SDEI
> or overflow stack, e.g.
> 
>   task -> overflow
>   irq -> overflow
> 
>   task -> sdei
>   irq -> sdei
> 
> I tried to explain the nesting rules in:
> 
>   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kernel/stacktrace.c?h=v5.11#n59
>   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/arm64/kernel/stacktrace.c?h=v5.11&id=592700f094be229b5c9cc1192d5cea46eb4c7afc
> 
>> 2. Exception in an interrupt handler. Here the interrupt handler is running on the IRQ stack.
>>    Will the pt_regs get created on the IRQ stack?
> 
> For an interrupt the regs will be placed on the stack that was in use
> when the interrupt was taken. The kernel switches to the IRQ stack
> *after* stacking the registers. e.g.
> 
>   task -> task // subsequently switches to IRQ stack
>   irq -> irq
> 
>> Also, is there a maximum nesting for exceptions?
> 
> In practice, yes, but the specific number isn't a constant, so in the
> unwind code we have to act as if there is no limit other than stack
> sizing.
> 
> We try to prevent cerain exceptions from nesting (e.g. debug exceptions
> cannot nest), but there are still several level sof nesting, and some
> exceptions which can be nested safely (like faults). For example, it's
> possible to have a chain:
> 
>  syscall -> fault -> interrupt -> fault -> pNMI -> fault -> SError -> fault -> watchpoint -> fault -> overflow -> fault -> BRK
> 
> ... and potentially longer than that.
> 
> The practical limit is the size of all the stacks, and the unwinder's 
> stack monotonicity checks ensure that an unwind will terminate.
> 

Thanks for explaining the nesting. It is now clear to me.

So, my next question is - can we define a practical limit for the nesting so that any nesting beyond that
is fatal? The reason I ask is - if there is a max, then we can allocate an array of stack frames out of
band for the special frames so they are not part of the stack and will not likely get corrupted.

Also, we don't have to do any special detection. If the number of out of band frames used is one or more
then we have exceptions and the stack trace is unreliable.

Thanks.

Madhavan