[PATCH] Add __private_ss to s390 struct tcbhead.

[Carlos O'Donell]

On 01/02/2016 11:34 AM, Marcin Kościelnicki wrote:
> Preparation for gcc -fsplit-stack support (gcc bug #68191).  The new
> field is basically identical to the one on x86.  Its TCB offset needs
> to be constant, as it'll be hardcoded in gcc.
> 
> 	* sysdeps/s390/nptl/tls.h: Add __private_ss to struct tcbhead.

What happens if you run newly compiled code with split-stack support on
a glibc that doesn't include this space allocated in tcbhead_t? You get
a write beyond the tcbhead_t into some other data? Depending on the thread
memory layout that could be a guard page or static TLS data?

This is the same problem we saw in POWER when adding a field in tcbhead_t
for fast-architecture access.

The only way to do this compatibly is to add a versioned symbol that
the compiler references to in order to prevent new binaries from
running on old glibc and crashing or worse silently corrupting data.

To reiterate: How do you plan to handle compatibility for this new feature?

If you did an analysis of nptl/allocatestack.c and showed that there were
still alignment bytes left, that might be one way to work around this without
needing a versioned reference.

Cheers,
Carlos.

> ---
>  ChangeLog               | 4 ++++
>  sysdeps/s390/nptl/tls.h | 4 ++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/ChangeLog b/ChangeLog
> index 8c339d2..e39e2ce 100644
> --- a/ChangeLog
> +++ b/ChangeLog
> @@ -1,3 +1,7 @@
> +2015-01-02  Marcin Kościelnicki  <koriakin@0x04.net>
> +
> +	* sysdeps/s390/nptl/tls.h: Add __private_ss to struct tcbhead.
> +
>  2016-01-01  Mike Frysinger  <vapier@gentoo.org>
>  
>  	[BZ #15421]
> diff --git a/sysdeps/s390/nptl/tls.h b/sysdeps/s390/nptl/tls.h
> index 28a2119..0e61154 100644
> --- a/sysdeps/s390/nptl/tls.h
> +++ b/sysdeps/s390/nptl/tls.h
> @@ -53,7 +53,11 @@ typedef struct
>    int gscope_flag;
>  #ifndef __ASSUME_PRIVATE_FUTEX
>    int private_futex;
> +#else
> +  int __glibc_reserved1;
>  #endif
> +  /* GCC split stack support.  */
> +  void *__private_ss;
>  } tcbhead_t;
>  
>  # ifndef __s390x__
>