Re: [PATCHv2] ARM64:Fix MINSIGSTKSZ and SIGSTKSZ

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 06 October 2015 11:31:34 Dave Martin wrote:
> On Tue, Oct 06, 2015 at 09:49:29AM +0200, Arnd Bergmann wrote:

> > * Can you explain in the changelog how the numbers were decided?
> >   I don't see any other architecture using 5kb and cannot see why
> >   it has to be this value rather than something else.
> 
> glibc quietly "fixed" this earlier this year, by inventing these numbers
> and putting them in the glibc headers. [1]

I saw the commit, but the changelog is not really useful.

> Except for a moribund architecture that will never be extended I
> think that the idea of MINSIGSTKSZ is badly flawed -- a #define
> for not-necessarily-quite-enough-stack-to-realistically-take-a-signal
> is a pretty useless concept even if the signal frame never grows, and
> it looks like it is little used in practice.

Right, even if we modified the constants in the kernel/glibc
headers at that point, it would remain broken for new kernels
and old user space.

> Since this bug hasn't been reported until now, I suspect that
> MINSIGSTKSZ is used very rarely or not at all by real userspace
> software.  I wonder whether we can get away with simply raising
> MINSIGSTKSZ to match SIGSTKSZ, since it's clear that any software
> using MINSIGSTKSZ was already broken.

I think it makes sense to stick with the traditional definition
of MINSIGSTKSZ == "the minimum amount that you will always need,
add whatever you require yourself" and SIGSTKSZ == "Should be
enough for a couple of function calls". If we want to be conservative
in the kernel, using 8192 and 32768, to stay with the x4 ratio
that everyone else uses would be my first pick, though I'm not
particularly attached to those values.

	Arnd

> [1] https://sourceware.org/ml/libc-alpha/2015-04/msg00033.html