Re: [PATCH] powerpc: Use sysconf (_SC_SIGSTKSZ) to set SIGSTKSZ and MINSIGSTKSZ.

[Florian Weimer <fweimer@redhat.com>]

* Manjunath Matti via Libc-alpha:

> diff --git a/sysdeps/unix/sysv/linux/powerpc/bits/sigstksz.h b/sysdeps/unix/sysv/linux/powerpc/bits/sigstksz.h
> new file mode 100644
> index 0000000000..399a49c1b0
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/powerpc/bits/sigstksz.h
> @@ -0,0 +1,33 @@

> +#ifndef _SIGNAL_H
> +# error "Never include <bits/sigstksz.h> directly; use <signal.h> instead."
> +#endif
> +
> +#if defined __USE_DYNAMIC_STACK_SIZE && __USE_DYNAMIC_STACK_SIZE
> +# include <unistd.h>
> +
> +/* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
> +# undef SIGSTKSZ
> +# define SIGSTKSZ sysconf (_SC_SIGSTKSZ)
> +
> +/* Minimum stack size for a signal handler: SIGSTKSZ/4.  */
> +# undef MINSIGSTKSZ
> +# define MINSIGSTKSZ (SIGSTKSZ >> 2)
> +#endif

I'm not sure if it is a good idea to hard-code that constant 4 here.
Similarly, it's not great to encode the constant 1 in the default case.

This is a subtle portability hazard because with these changes,
MINSIGSTKSZ is not enough on POWER to do anything on the signal stack
(because it's exactly the kernel supplied value).  On other
architectures, it's at least possible to do some minor stuff and call
e.g. siglongjmp (at least if lazy binding is not required).

The factor 4 in the implementation is somewhat x86-64-specific because
it accounts for the fact that we do a signal-like context switch
(pushing XSAVE data on the stack) in the lazy binding trampoline.

Thanks,
Florian