Invalid program counters and unwinding

[Nathan Sidwell]

On 06/26/2018 07:01 AM, Florian Weimer wrote:
> On 06/26/2018 12:56 PM, Nathan Sidwell wrote:
>> On 06/26/2018 05:26 AM, Florian Weimer wrote:
>>
>>> So it looks to me that the caller of _Unwind_Find_FDE needs to ensure 
>>> that the PC is a valid element of the call stack.  Is this a correct 
>>> assumption?
>>
>> I thought this was an (implicit?) requirement. You're unwinding a 
>> stack to deliver an exception up it.  Are there use cases where that 
>> is not the case?
> 
> We have something approaching this scenario.
> 
> pthread_cancel in glibc unwinds the stack using DWARF information until 
> encounters a frame without unwind information, when it switches to 
> longjmp to get past that obstacle.

This is a long jump to the originating pthread function at the end of 
the stack, right?  We not only get past the obstacle, but any and all 
DWARF frames on top of it.  (just for my understanding)

> However, at the point of transition from a valid DWARF frame into the 
> wilderness (without unwind data), we should still have accurate 
> information on the caller's PC, so _Unwind_Find_FDE will reliably fail 
> to find any unwind data for it.  It's not a random pointer somewhere 
> else, so I think even the pthread_cancel case is fully supported.

That sounds right.  It's a PC that you could return to if you weren't 
trying to unwind the stack.

nathan

-- 
Nathan Sidwell