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From 30e4c9be8d17c7806beaca9e78f6aff6ebae6463 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=B6ren=20Tempel?= <soeren+git@soeren-tempel.net>
Date: Wed, 15 Dec 2021 00:14:19 +0100
Subject: [PATCH] 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. 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 by gcc-go by
default. On musl based systems with split-stack support (e.g. 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 TARGET_GLIBC_MAJOR is defined to a non-zero value (it
defaults to zero on non-glibc systems). The check has been added for x86
and s390x, the rs6000 config already checks for TARGET_GLIBC_MAJOR. With
this patch applied, the gcc-go configure script will detect that
-fsplit-stack support is not available and will not use it.
This patch has been tested on Alpine Linux Edge on the s390x
architecture by bootstrapping Google's Go implementation with gcc-go.
See https://www.openwall.com/lists/musl/2012/10/16/12
---
gcc/common/config/s390/s390-common.c | 9 ++++++++-
gcc/config/i386/gnu-user-common.h | 5 +++--
gcc/config/i386/gnu.h | 6 +++++-
3 files changed, 16 insertions(+), 4 deletions(-)
diff --git a/gcc/common/config/s390/s390-common.c b/gcc/common/config/s390/s390-common.c
index b6bc8501742..afbd8d3fe66 100644
--- a/gcc/common/config/s390/s390-common.c
+++ b/gcc/common/config/s390/s390-common.c
@@ -116,13 +116,20 @@ 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)
{
+#if TARGET_GLIBC_MAJOR
return true;
+#else
+ if (report)
+ error("%<-fsplit-stack%> currently only supported on GNU/Linux");
+ return false;
+#endif
}
#undef TARGET_DEFAULT_TARGET_FLAGS
diff --git a/gcc/config/i386/gnu-user-common.h b/gcc/config/i386/gnu-user-common.h
index 00226f5a455..554e146dbbe 100644
--- a/gcc/config/i386/gnu-user-common.h
+++ b/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
+ uses glibc. */
+#if HAVE_GAS_CFI_PERSONALITY_DIRECTIVE && TARGET_GLIBC_MAJOR
#define TARGET_CAN_SPLIT_STACK
#endif
diff --git a/gcc/config/i386/gnu.h b/gcc/config/i386/gnu.h
index 25fbc07f58c..895a7369816 100644
--- a/gcc/config/i386/gnu.h
+++ b/gcc/config/i386/gnu.h
@@ -35,7 +35,11 @@ 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 implementation to avoid silent TCB corruptions. */
+
+#if defined (TARGET_LIBC_PROVIDES_SSP) && TARGET_GLIBC_MAJOR
/* i386 glibc provides __stack_chk_guard in %gs:0x14. */
#define TARGET_THREAD_SSP_OFFSET 0x14
--
2.34.1
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