[2.22 COMMITTED] arm: mark __startcontext as .cantunwind (bug 20435)

[Florian Weimer]

__startcontext marks the bottom of the call stack of the contexts created
by makecontext.

(cherry picked from commit 9e2ff6c9cc54c0b4402b8d49e4abe7000fde7617)

Also includes the NEWS update, cherry-picked from commits
056dd72af83f5459ce6d545a49dea6dba7d635dc and
4d047efdbc55b0d68947cde682e5363d16a66294.

2016-08-15  Andreas Schwab  <schwab@suse.de>

	[BZ #20435]
	* sysdeps/unix/sysv/linux/arm/setcontext.S (__startcontext): Mark
	as .cantunwind.

diff --git a/NEWS b/NEWS
index 923468d..a19f656 100644
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,12 @@ using `glibc' in the "product" field.
 
 Version 2.22.1
 
+* On ARM EABI (32-bit), generating a backtrace for execution contexts which
+  have been created with makecontext could fail to terminate due to a
+  missing .cantunwind annotation.  This has been observed to lead to a hang
+  (denial of service) in some Go applications compiled with gccgo.  Reported
+  by Andreas Schwab.  (CVE-2016-6323)
+
 * A stack-based buffer overflow was found in libresolv when invoked from
   libnss_dns, allowing specially crafted DNS responses to seize control
   of execution flow in the DNS client.  The buffer overflow occurs in
diff --git a/sysdeps/unix/sysv/linux/arm/setcontext.S b/sysdeps/unix/sysv/linux/arm/setcontext.S
index 24c7294..926b65a 100644
--- a/sysdeps/unix/sysv/linux/arm/setcontext.S
+++ b/sysdeps/unix/sysv/linux/arm/setcontext.S
@@ -86,12 +86,19 @@ weak_alias(__setcontext, setcontext)
 
 	/* Called when a makecontext() context returns.  Start the
 	   context in R4 or fall through to exit().  */
+	/* Unwind descriptors are looked up based on PC - 2, so we have to
+	   make sure to mark the instruction preceding the __startcontext
+	   label as .cantunwind.  */
+	.fnstart
+	.cantunwind
+	nop
 ENTRY(__startcontext)
 	movs    r0, r4
 	bne     PLTJMP(__setcontext)
 
 	@ New context was 0 - exit
 	b       PLTJMP(HIDDEN_JUMPTARGET(exit))
+	.fnend
 END(__startcontext)
 
 #ifdef PIC