[PATCH] nptl: Open libgcc.so with RTLD_NOW during pthread_cancel [BZ #22636]

[fweimer@redhat.com (Florian Weimer)]

Disabling lazy binding reduces stack usage during unwinding.

Note that RTLD_NOW only makes a difference if libgcc.so has not
already been loaded, so this is only a partial fix.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit f993b8754080ac7572b692870e926d8b493db16c)

2018-01-10  Florian Weimer  <fweimer@redhat.com>

	[BZ #22636]
	* sysdeps/nptl/unwind-forcedunwind.c (pthread_cancel_init): Open
	libgcc.so with RTLD_NOW, to avoid lazy binding during unwind.

diff --git a/NEWS b/NEWS
index 3a719e81e3..afa170f86e 100644
--- a/NEWS
+++ b/NEWS
@@ -100,6 +100,7 @@ The following bugs are resolved with this release:
   [22322] libc: [mips64] wrong bits/long-double.h installed
   [22325] glibc: Memory leak in glob with GLOB_TILDE (CVE-2017-15671)
   [22375] malloc returns pointer from tcache instead of NULL (CVE-2017-17426)
+  [22636] PTHREAD_STACK_MIN is too small on x86-64
   [22627] $ORIGIN in $LD_LIBRARY_PATH is substituted twice
   [22637] nptl: Fix stack guard size accounting
   [22679] getcwd(3) can succeed without returning an absolute path
diff --git a/sysdeps/nptl/unwind-forcedunwind.c b/sysdeps/nptl/unwind-forcedunwind.c
index 33a1975f5e..ace58b2468 100644
--- a/sysdeps/nptl/unwind-forcedunwind.c
+++ b/sysdeps/nptl/unwind-forcedunwind.c
@@ -49,7 +49,7 @@ pthread_cancel_init (void)
       return;
     }
 
-  handle = __libc_dlopen (LIBGCC_S_SO);
+  handle = __libc_dlopen_mode (LIBGCC_S_SO, RTLD_NOW | __RTLD_DLOPEN);
 
   if (handle == NULL
       || (resume = __libc_dlsym (handle, "_Unwind_Resume")) == NULL