Re: [PATCH v5 00/32] virtually mapped stacks and thread_info cleanup

[Christian Borntraeger]

On 07/11/2016 10:53 PM, Andy Lutomirski wrote:
> Hi all-
> 
> Since the dawn of time, a kernel stack overflow has been a real PITA
> to debug, has caused nondeterministic crashes some time after the
> actual overflow, and has generally been easy to exploit for root.
> 
> With this series, arches can enable HAVE_ARCH_VMAP_STACK.  Arches
> that enable it (just x86 for now) get virtually mapped stacks with
> guard pages.  This causes reliable faults when the stack overflows.
> 
> If the arch implements it well, we get a nice OOPS on stack overflow
> (as opposed to panicing directly or otherwise exploding badly).  On
> x86, the OOPS is nice, has a usable call trace, and the overflowing
> task is killed cleanly.
> 
> This series (starting with v4) also extensively cleans up
> thread_info.  thread_info has been partially redundant with
> thread_struct for a long time -- both are places for arch code to
> add additional per-task variables.  thread_struct is much cleaner:
> it's always in task_struct, and there's nothing particularly magical
> about it.  So this series contains a bunch of cleanups on x86 to
> move almost everything from thread_info to thread_struct (which,
> even by itself, deletes more code than it adds) and to remove x86's
> dependence on thread_info's position on the stack.  Then it opts x86
> into a new config option THREAD_INFO_IN_TASK to get rid of
> arch-specific thread_info entirely and simply embed a defanged
> thread_info (containing only flags) and 'int cpu' into task_struct.
> 
> Once thread_info stops being magical, there's another benefit: we
> can free the thread stack as soon as the task is dead (without
> waiting for RCU) and then, if vmapped stacks are in use, cache the
> entire stack for reuse on the same cpu.
> 
> This seems to be an overall speedup of about 0.5-1 Âs per
> pthread_create/join in a simple test -- a percpu cache of vmalloced
> stacks appears to be a bit faster than a high-order stack
> allocation, at least when the cache hits.  (I expect that workloads
> with a low cache hit rate are likely to be dominated by other
> effects anyway.)
> 
> This does not address interrupt stacks.
> 
> It's worth noting that s390 has an arch-specific gcc feature that
> detects stack overflows by adjusting function prologues.  Arches
> with features like that may wish to avoid using vmapped stacks to
> minimize the performance hit.

Yes, might not need this for stack overflow detection. What might 
be interesting is the thread_info/thread_struct change, if we can
strip down thread_info.(CONFIG_THREAD_INFO_IN_TASK). Would it actually
make sense to separate these two changes to see what performance
impact  CONFIG_THREAD_INFO_IN_TASK has on its own?