From: Sören Tempel <soe...@soeren-tempel.net> 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 <soe...@soeren-tempel.net> gcc/ChangeLog: * common/config/s390/s390-common.c (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. --- This version of the patch fixes a few codingstyle violations pointed out to me by Richard Sandiford, it does not include any functional changes compared to previous versions of this patch. gcc/common/config/s390/s390-common.cc | 14 ++++++++++---- gcc/config/i386/gnu-user-common.h | 5 +++-- gcc/config/i386/gnu.h | 5 ++++- 3 files changed, 17 insertions(+), 7 deletions(-) diff --git a/gcc/common/config/s390/s390-common.cc b/gcc/common/config/s390/s390-common.cc index 6ed2f89f3d0..547b0826f93 100644 --- a/gcc/common/config/s390/s390-common.cc +++ b/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 diff --git a/gcc/config/i386/gnu-user-common.h b/gcc/config/i386/gnu-user-common.h index 23b54c5be52..7525f788a9c 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 + targets glibc. */ +#if HAVE_GAS_CFI_PERSONALITY_DIRECTIVE && OPTION_GLIBC #define TARGET_CAN_SPLIT_STACK #endif diff --git a/gcc/config/i386/gnu.h b/gcc/config/i386/gnu.h index 401e60c9a02..daa505a5d45 100644 --- a/gcc/config/i386/gnu.h +++ b/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
