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Re: Invalid program counters and unwinding


On 06/26/2018 01:25 PM, Jakub Jelinek wrote:
On Tue, Jun 26, 2018 at 01:01:06PM +0200, 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.

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.

The usual ways to get bogus PCs in the frames is:
1) stack corruption
2) setcontext/swapcontext with uninitialized or corrupted ucontext_t
3) bogus unwind info (compiler or linker etc. bug)

But if that happens, all bets are off, and we could still get a crash with the current implementation.

And any approach which does not inhibit concurrent dlclose will only make things worse if there are such concurrent calls, which is perhaps an unusual combination.

At least for unwinding, I think we don't and shouldn't care, we assume only
valid programs.  For cases like _Unwind_Backtrace when used to print info in
case of fatal signal or stack corruption, it is more questionable, but at
least the current implmentation doesn't care either.

At least glibc no longer tries to print a backtrace from a corrupted stack.

Thanks,
Florian


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