Disable -fsplit-stack support on non-glibc targets

The -fsplit-stack option requires the pthread_t TCB definition in the
libc to provide certain struct fields at specific hardcoded offsets. As
far as I know, only glibc provides these fields at the required offsets.
Most notably, musl libc does not have these fields. However, since gcc
accesses the fields using a fixed offset, this does not cause a
compile-time error, but instead results in a silent memory corruption at
run-time with musl libc. For example, on s390x libgcc's
__stack_split_initialize CTOR will overwrite the cancel field in the
pthread_t TCB on musl.

The -fsplit-stack option is used within the gcc code base itself by
gcc-go (if available). On musl-based systems with split-stack support
(i.e. s390x or x86) this causes Go programs compiled with gcc-go to
misbehave at run-time.

This patch fixes gcc-go on musl by disabling -fsplit-stack in gcc itself
since it is not supported on non-glibc targets anyhow. This is achieved
by checking if gcc targets a glibc-based system. This check has been
added for x86 and s390x, the rs6000 config already checks for
TARGET_GLIBC_MAJOR. Other architectures do not have split-stack
support. With this patch applied, the gcc-go configure script will
detect that -fsplit-stack support is not available and will not use it.

See https://www.openwall.com/lists/musl/2012/10/16/12

This patch was written under the assumption that glibc is the only libc
implementation which supports the required fields at the required
offsets in the pthread_t TCB. The patch has been tested on Alpine Linux
Edge on the s390x and x86 architectures by bootstrapping Google's Go
implementation with gcc-go.

Signed-off-by: Sören Tempel <soeren@soeren-tempel.net>

gcc/ChangeLog:

	* common/config/s390/s390-common.cc (s390_supports_split_stack):
	Only support split-stack on glibc targets.
	* config/i386/gnu-user-common.h (STACK_CHECK_STATIC_BUILTIN): Ditto.
	* config/i386/gnu.h (defined): Ditto.

gcc/common/config/s390/s390-common.cc

@@ -116,13 +116,19 @@ s390_handle_option (struct gcc_options *opts ATTRIBUTE_UNUSED,
 
 /* -fsplit-stack uses a field in the TCB, available with glibc-2.23.
    We don't verify it, since earlier versions just have padding at
-   its place, which works just as well.  */
+   its place, which works just as well.  For other libc implementations
+   we disable the feature entirely to avoid corrupting the TCB.  */
 
 static bool
-s390_supports_split_stack (bool report ATTRIBUTE_UNUSED,
-			   struct gcc_options *opts ATTRIBUTE_UNUSED)
+s390_supports_split_stack (bool report,
+			   struct gcc_options *opts)
 {
-  return true;
+  if (opts->x_linux_libc == LIBC_GLIBC)
+    return true;
+
+  if (report)
+    error ("%<-fsplit-stack%> currently only supported on GNU/Linux");
+  return false;
 }
 
 #undef TARGET_DEFAULT_TARGET_FLAGS

gcc/config/i386/gnu-user-common.h

@@ -66,7 +66,8 @@ along with GCC; see the file COPYING3.  If not see
 #define STACK_CHECK_STATIC_BUILTIN 1
 
 /* We only build the -fsplit-stack support in libgcc if the
-   assembler has full support for the CFI directives.  */
-#if HAVE_GAS_CFI_PERSONALITY_DIRECTIVE
+   assembler has full support for the CFI directives and
+   targets glibc.  */
+#if HAVE_GAS_CFI_PERSONALITY_DIRECTIVE && OPTION_GLIBC
 #define TARGET_CAN_SPLIT_STACK
 #endif

gcc/config/i386/gnu.h

@@ -35,7 +35,10 @@ along with GCC.  If not, see <http://www.gnu.org/licenses/>.
    crti.o%s %{static:crtbeginT.o%s;shared|pie:crtbeginS.o%s;:crtbegin.o%s}"
 #endif
 
-#ifdef TARGET_LIBC_PROVIDES_SSP
+/* -fsplit-stack uses a field in the TCB at a fixed offset. This
+   field is only available for glibc.  Disable -fsplit-stack for
+   other libc implementations to avoid silent TCB corruptions.  */
+#if defined (TARGET_LIBC_PROVIDES_SSP) && OPTION_GLIBC
 
 /* i386 glibc provides __stack_chk_guard in %gs:0x14.  */
 #define TARGET_THREAD_SSP_OFFSET        0x14