Re: s390/defconfigs: set CONFIG_INIT_STACK_NONE=y

[Kees Cook <keescook@chromium.org>]

On Fri, May 26, 2023 at 03:42:56PM +0200, Heiko Carstens wrote:
> Hi Kees,
> 
> > I had this[1] patch pointed out to me, but I couldn't find any discussion
> > about it on public lists. Can you give me some background on this? There
> > haven't been any general workloads identified where this has been
> > a problem, so I'm curious why this was seen as globally an issue on
> > s390. The expectation was to use __uninitialized on any variables where
> > this was noticed as a performance issue, and where the memory safety of
> > the variable could be proven. Turning it off by default seems like
> > rather too much, but perhaps there is something unique to s390 I don't
> > know about. :)
> 
> This was the result of some micro benchmarks being reported "too slow".
> Actually our syscall entry/exit path got naturally slower since we switched
> to generic entry; now we are trying to improve things a bit again.
> 
> There is also this RFC from Sven, which tries to inline some of the
> generic system call functions, in order to avoid function calls:
> https://lore.kernel.org/lkml/20230516133810.171487-1-svens@linux.ibm.com/
> 
> I stumbled upon CONFIG_INIT_STACK_NONE only by accident when wondering why
> the compiler would generate quite some instructions which aren't necessary,
> just to zero variables. For the getpid() system call this makes a runtime
> difference of ~3%, which is quite a bit.

Hm, that does seem high. It implies there are large variable that are
being passed by reference, perhaps in the syscall path? I had similar
problems a while back on x86 but due to stack-protector seeing the
register arrays and thinking they needed protection. I had to explicitly
turn that off for the entry code, since they're provably safe. :)

> What is the overhead on other architectures?

It's been in the noise for real workloads.

> That said: I was also unaware of __uninitialized. But on the other hand,
> there is no sign of __uninitialized in the kernel, nor could I find
> anything that could match in compiler_attributes.h.
> Am I missing something here?

No, nothing missed -- there just have been no workloads identified yet
where it's needed.

> Thanks for bringing this up, I guess if there is some annotation available,
> we can revisit at least the architecture specific entry code, and maybe
> figure out how to avoid most of the extra runtime, but still keep the
> feature enabled.
> 
> (Adding Sven, since I will be offline the next two weeks).

Yeah, if you find a place where it's needed, please add the compiler
attribute and put it to use. It'll give people an example use-case for
it. :)

-- 
Kees Cook