[2.27 COMMITTED] sparc32: Add nop before __startcontext to stop unwinding [BZ #22919]

[2.27 COMMITTED] sparc32: Add nop before __startcontext to stop unwinding [BZ #22919]

[Aurelien Jarno]

On sparc32 tst-makecontext fails, as backtrace called within a context
created by makecontext to yield infinite backtrace.

Fix that the same way than nios2 by adding a nop just before
__startcontext. This is needed as otherwise FDE lookup just repeatedly
finds __setcontext's FDE in an infinite loop, due to the convention of
using 'address - 1' for FDE lookup.

Changelog:
	[BZ #22919]
	* sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S (__startcontext):
	Add nop before __startcontext, add explaining comments.

(cherry picked from commit 9aa5c222b9e0409143410a02b6364a3b25dbf028)
---
 ChangeLog                                          |  6 ++++++
 sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S | 13 +++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index 494f98b51b3..3ae0ecd07a3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2018-03-09  Aurelien Jarno  <aurelien@aurel32.net>
+
+	[BZ #22919]
+	* sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S (__startcontext):
+	Add nop before __startcontext, add explaining comments.
+
 2018-03-07  Adhemerval Zanella  <adhemerval.zanella@linaro.org>
 
 	[BZ #22926]
diff --git a/sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S b/sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S
index 695f172c634..283aef1b304 100644
--- a/sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S
+++ b/sysdeps/unix/sysv/linux/sparc/sparc32/setcontext.S
@@ -95,6 +95,19 @@ END(__setcontext)
 
 weak_alias (__setcontext, setcontext)
 
+/* We add an NOP here to separate between __setcontext/__startcontext. The
+   wanted behavior that happens is: when unwinding from a function called
+   inside a makecontext() context, FDE lookup will use '&__startcontext - 1',
+   then returns NULL for no FDE found, and immediately ends the unwind, in
+   a normal fashion.
+
+   If this NOP word does not exist, FDE lookup just repeatedly finds
+   __setcontext's FDE in an infinite loop, due to the convention of using
+   'address - 1' for FDE lookup. Modifiying/deleting the below
+   __startcontext's FDE has no help on this.  */
+
+	nop
+
 /* This is the helper code which gets called if a function which is
    registered with 'makecontext' returns.  In this case we have to
    install the context listed in the uc_link element of the context
-- 
2.16.1