cc:ing the x86 and s390 maintainers
soeren--- via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> 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 addresses feedback by Andrew Pinski and uses
> OPTION_GLIBC as well as opts->x_linux_libc == LIBC_GLIBC to detect glibc
> targets (instead of relying on TARGET_GLIBC_MAJOR).
>
> gcc/common/config/s390/s390-common.c | 11 +++++++++--
> gcc/config/i386/gnu-user-common.h | 5 +++--
> gcc/config/i386/gnu.h | 6 +++++-
> 3 files changed, 17 insertions(+), 5 deletions(-)
Sorry for the slow review. The patch LGTM bar some minor formatting
nits below, but target maintainers should have the final say.
> diff --git a/gcc/common/config/s390/s390-common.c
> b/gcc/common/config/s390/s390-common.c
> index b6bc8501742..fc86e0bc5e7 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
GCC style is to use 2 spaces after a full stop. Same for the x86 part.
> + 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)
These parameters are no longer unused after the patch, so it'd be good
to remove the attributes.
> {
> - return true;
> + if (opts->x_linux_libc == LIBC_GLIBC) {
> + return true;
> + } else {
> + if (report)
> + error("%<-fsplit-stack%> currently only supported on GNU/Linux");
> + return false;
> + }
Normal GCC formatting would be something like:
if (opts->x_linux_libc == LIBC_GLIBC)
return true;
if (report)
error ("%<-fsplit-stack%> currently only supported on GNU/Linux");
return false;
Sorry for the fussy rules.
Thanks,
Richard
> }
>
> #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..6e13315b5a3 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 25fbc07f58c..adfe817201e 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) && OPTION_GLIBC
>
> /* i386 glibc provides __stack_chk_guard in %gs:0x14. */
> #define TARGET_THREAD_SSP_OFFSET 0x14
