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

[Aurelien Jarno]

From: Andreas Schwab <schwab@suse.de>

__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.
---
 ChangeLog                                      | 6 ++++++
 NEWS                                           | 6 ++++++
 ports/sysdeps/unix/sysv/linux/arm/setcontext.S | 7 +++++++
 3 files changed, 19 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index 3d2063b..30da116 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-08-15  Andreas Schwab  <schwab@suse.de>
+
+	[BZ #20435]
+	* ports/sysdeps/unix/sysv/linux/arm/setcontext.S (__startcontext):
+	Mark as .cantunwind.
+
 2015-02-24  Eric Rannaud  <e@nanocritical.com>
 
 	[BZ #17523]
diff --git a/NEWS b/NEWS
index 473e35a..a1bb834 100644
--- a/NEWS
+++ b/NEWS
@@ -96,6 +96,12 @@ Version 2.19.1
 * The Sun RPC UDP client could exhaust all available stack space when
   flooded with crafted ICMP and UDP messages.  Reported by Aldy Hernandez'
   alloca plugin for GCC.  (CVE-2016-4429)
+
+* 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)
 
 Version 2.19
 
diff --git a/ports/sysdeps/unix/sysv/linux/arm/setcontext.S b/ports/sysdeps/unix/sysv/linux/arm/setcontext.S
index 7b9b511..7da16a9 100644
--- a/ports/sysdeps/unix/sysv/linux/arm/setcontext.S
+++ b/ports/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
-- 
2.8.1