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


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 " (

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.


	[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:

(cherry picked from commit 890b7a4b33d482b5c768ab47d70758b80227e9bc)

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