[COMMITED 2.21 2.20 2.19] S390: Fix "backtrace() returns infinitely deep stack frames with makecontext()" [BZ #18508].

[Stefan Liebler <stli at linux dot vnet dot ibm dot com>]

Hi,

I've applied the "S390: Fix "backtrace() returns infinitely deep stack 
frames with makecontext()" [BZ #18508]." patch to release branch 2.19 - 
2.21.

On s390/s390x backtrace(buffer, size) returns the series of called 
functions until
"makecontext_ret" and additional entries (up to "size") with 
"makecontext_ret".
GDB-backtrace is also warning:
"Backtrace stopped: previous frame identical to this frame (corrupt stack?)"

To reproduce this scenario you have to setup a new context with 
makecontext()
and activate it with setcontext(). See e.g. cf() function in testcase 
stdlib/tst-makecontext.c.
Or see bug in libgo "Bug 66303 - runtime.Caller() returns infinitely 
deep stack frames
on s390x " (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66303).

This patch omits the cfi_startproc/cfi_endproc directives in ENTRY/END 
macro of
__makecontext_ret. Thus no frame information is generated in .eh_frame 
and backtrace
stops after __makecontext_ret. There is also no .eh_frame info for _start or
thread_start functions.

ChangeLog:

	[BZ #18508]
	* stdlib/Makefile ($(objpfx)tst-makecontext3):
	Depend on $(libdl).
	* stdlib/tst-makecontext.c (cf): Test if _Unwind_Backtrace
	is not called infinitely times.
	(backtrace_helper): New function.
	(trace_arg): New struct.
	(st1): Enlarge stack size.
	* sysdeps/unix/sysv/linux/s390/s390-32/__makecontext_ret.S:
	(__makecontext_ret): Omit cfi_startproc and cfi_endproc.
	* sysdeps/unix/sysv/linux/s390/s390-64/__makecontext_ret.S:
	Likewise.

(cherry picked from commit 890b7a4b33d482b5c768ab47d70758b80227e9bc)