CC_HAS_AUTO_VAR_INIT_ZERO requires a supported set of compiler options
distinct from those needed by CC_HAS_AUTO_VAR_INIT_PATTERN, Fix up
the Kconfig dependency for INIT_STACK_ALL_ZERO to test for the former
instead of the latter, as these are the options passed by the top-level
Makefile.
Cc: Kees Cook <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Gustavo A. R. Silva <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
Signed-off-by: Will Deacon <[email protected]>
---
I just noticed this while reading the code and I suspect it doesn't really
matter in practice.
security/Kconfig.hardening | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 90cbaff86e13..341e2fdcba94 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -29,7 +29,7 @@ choice
prompt "Initialize kernel stack variables at function entry"
default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
- default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
+ default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
default INIT_STACK_NONE
help
This option enables initialization of stack variables at
--
2.33.0.309.g3052b89438-goog
On 9/14/2021 3:28 AM, Will Deacon wrote:
> CC_HAS_AUTO_VAR_INIT_ZERO requires a supported set of compiler options
> distinct from those needed by CC_HAS_AUTO_VAR_INIT_PATTERN, Fix up
> the Kconfig dependency for INIT_STACK_ALL_ZERO to test for the former
> instead of the latter, as these are the options passed by the top-level
> Makefile.
>
> Cc: Kees Cook <[email protected]>
> Cc: Nathan Chancellor <[email protected]>
> Cc: Nick Desaulniers <[email protected]>
> Cc: Gustavo A. R. Silva <[email protected]>
> Cc: Greg Kroah-Hartman <[email protected]>
> Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> Signed-off-by: Will Deacon <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
One comment below.
> ---
>
> I just noticed this while reading the code and I suspect it doesn't really
> matter in practice.
>
> security/Kconfig.hardening | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 90cbaff86e13..341e2fdcba94 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -29,7 +29,7 @@ choice
> prompt "Initialize kernel stack variables at function entry"
> default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
> default INIT_STACK_NONE
> help
> This option enables initialization of stack variables at
>
While I think this change is correct in and of itself,
CONFIG_INIT_STACK_ALL_ZERO is broken with GCC 12.x, as
CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO won't be set even though GCC now
supports -ftrivial-auto-var-init=zero because GCC does not implement the
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
flag for obvious reasons ;) the cc-option call probably needs to be
adjusted.
Cheers,
Nathan
On Tue, Sep 14, 2021 at 08:58:12AM -0700, Nathan Chancellor wrote:
> On 9/14/2021 3:28 AM, Will Deacon wrote:
> > CC_HAS_AUTO_VAR_INIT_ZERO requires a supported set of compiler options
> > distinct from those needed by CC_HAS_AUTO_VAR_INIT_PATTERN, Fix up
> > the Kconfig dependency for INIT_STACK_ALL_ZERO to test for the former
> > instead of the latter, as these are the options passed by the top-level
> > Makefile.
> >
> > Cc: Kees Cook <[email protected]>
> > Cc: Nathan Chancellor <[email protected]>
> > Cc: Nick Desaulniers <[email protected]>
> > Cc: Gustavo A. R. Silva <[email protected]>
> > Cc: Greg Kroah-Hartman <[email protected]>
> > Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> > Signed-off-by: Will Deacon <[email protected]>
>
> Reviewed-by: Nathan Chancellor <[email protected]>
>
> One comment below.
>
> > ---
> >
> > I just noticed this while reading the code and I suspect it doesn't really
> > matter in practice.
> >
> > security/Kconfig.hardening | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> > index 90cbaff86e13..341e2fdcba94 100644
> > --- a/security/Kconfig.hardening
> > +++ b/security/Kconfig.hardening
> > @@ -29,7 +29,7 @@ choice
> > prompt "Initialize kernel stack variables at function entry"
> > default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> > default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> > - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> > + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
> > default INIT_STACK_NONE
> > help
> > This option enables initialization of stack variables at
> >
>
> While I think this change is correct in and of itself,
> CONFIG_INIT_STACK_ALL_ZERO is broken with GCC 12.x, as
> CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO won't be set even though GCC now supports
> -ftrivial-auto-var-init=zero because GCC does not implement the
> -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> flag for obvious reasons ;) the cc-option call probably needs to be
> adjusted.
GCC silently ignores the -enable flag, so things actually work correctly
as-is. But, yes, it makes the command line long and doesn't make sense.
How about we do this instead:
diff --git a/Makefile b/Makefile
index 34a0afc3a8eb..34439deac939 100644
--- a/Makefile
+++ b/Makefile
@@ -831,12 +831,11 @@ endif
# Initialize all stack variables with a zero value.
ifdef CONFIG_INIT_STACK_ALL_ZERO
-# Future support for zero initialization is still being debated, see
-# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being
-# renamed or dropped.
KBUILD_CFLAGS += -ftrivial-auto-var-init=zero
+ifdef CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
endif
+endif
# While VLAs have been removed, GCC produces unreachable stack probes
# for the randomize_kstack_offset feature. Disable it for all compilers.
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 90cbaff86e13..beea81df3081 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -22,14 +22,22 @@ menu "Memory initialization"
config CC_HAS_AUTO_VAR_INIT_PATTERN
def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
+config CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE
+ def_bool $(cc-option,-ftrivial-auto-var-init=zero)
+
+config CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
+ # https://bugs.llvm.org/show_bug.cgi?id=45497
+ def_bool !CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE && \
+ $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
+
config CC_HAS_AUTO_VAR_INIT_ZERO
- def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
+ def_bool CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE || CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
choice
prompt "Initialize kernel stack variables at function entry"
default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
- default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
+ default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
default INIT_STACK_NONE
help
This option enables initialization of stack variables at
--
Kees Cook
`On Tue, Sep 14, 2021 at 10:21 AM Kees Cook <[email protected]> wrote:
>
> On Tue, Sep 14, 2021 at 08:58:12AM -0700, Nathan Chancellor wrote:
> > On 9/14/2021 3:28 AM, Will Deacon wrote:
> > > CC_HAS_AUTO_VAR_INIT_ZERO requires a supported set of compiler options
> > > distinct from those needed by CC_HAS_AUTO_VAR_INIT_PATTERN, Fix up
> > > the Kconfig dependency for INIT_STACK_ALL_ZERO to test for the former
> > > instead of the latter, as these are the options passed by the top-level
> > > Makefile.
> > >
> > > Cc: Kees Cook <[email protected]>
> > > Cc: Nathan Chancellor <[email protected]>
> > > Cc: Nick Desaulniers <[email protected]>
> > > Cc: Gustavo A. R. Silva <[email protected]>
> > > Cc: Greg Kroah-Hartman <[email protected]>
> > > Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> > > Signed-off-by: Will Deacon <[email protected]>
> >
> > Reviewed-by: Nathan Chancellor <[email protected]>
> >
> > One comment below.
> >
> > > ---
> > >
> > > I just noticed this while reading the code and I suspect it doesn't really
> > > matter in practice.
> > >
> > > security/Kconfig.hardening | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> > > index 90cbaff86e13..341e2fdcba94 100644
> > > --- a/security/Kconfig.hardening
> > > +++ b/security/Kconfig.hardening
> > > @@ -29,7 +29,7 @@ choice
> > > prompt "Initialize kernel stack variables at function entry"
> > > default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> > > default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> > > - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> > > + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
> > > default INIT_STACK_NONE
> > > help
> > > This option enables initialization of stack variables at
> > >
> >
> > While I think this change is correct in and of itself,
> > CONFIG_INIT_STACK_ALL_ZERO is broken with GCC 12.x, as
> > CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO won't be set even though GCC now supports
> > -ftrivial-auto-var-init=zero because GCC does not implement the
> > -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> > flag for obvious reasons ;) the cc-option call probably needs to be
> > adjusted.
>
> GCC silently ignores the -enable flag, so things actually work correctly
> as-is.
So then would that mean that CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
evaluates to true then, in your patch below?
Rather than create 2 new kconfigs with 1 new invocation of the
compiler via cc-option, how about just adding an `ifdef
CONFIG_CC_IS_CLANG` guard around adding the obnoxious flag to
`KBUILD_CFLAGS` in the top level Makefile?
> But, yes, it makes the command line long and doesn't make sense.
> How about we do this instead:
>
> diff --git a/Makefile b/Makefile
> index 34a0afc3a8eb..34439deac939 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -831,12 +831,11 @@ endif
>
> # Initialize all stack variables with a zero value.
> ifdef CONFIG_INIT_STACK_ALL_ZERO
> -# Future support for zero initialization is still being debated, see
> -# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being
> -# renamed or dropped.
> KBUILD_CFLAGS += -ftrivial-auto-var-init=zero
> +ifdef CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
> KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> endif
> +endif
>
> # While VLAs have been removed, GCC produces unreachable stack probes
> # for the randomize_kstack_offset feature. Disable it for all compilers.
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 90cbaff86e13..beea81df3081 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -22,14 +22,22 @@ menu "Memory initialization"
> config CC_HAS_AUTO_VAR_INIT_PATTERN
> def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
>
> +config CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE
> + def_bool $(cc-option,-ftrivial-auto-var-init=zero)
> +
> +config CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
> + # https://bugs.llvm.org/show_bug.cgi?id=45497
> + def_bool !CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE && \
> + $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
> +
> config CC_HAS_AUTO_VAR_INIT_ZERO
> - def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
> + def_bool CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE || CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
>
> choice
> prompt "Initialize kernel stack variables at function entry"
> default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
> default INIT_STACK_NONE
> help
> This option enables initialization of stack variables at
>
>
>
> --
> Kees Cook
--
Thanks,
~Nick Desaulniers
On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:
> `On Tue, Sep 14, 2021 at 10:21 AM Kees Cook <[email protected]> wrote:
> >
> > On Tue, Sep 14, 2021 at 08:58:12AM -0700, Nathan Chancellor wrote:
> > > On 9/14/2021 3:28 AM, Will Deacon wrote:
> > > > CC_HAS_AUTO_VAR_INIT_ZERO requires a supported set of compiler options
> > > > distinct from those needed by CC_HAS_AUTO_VAR_INIT_PATTERN, Fix up
> > > > the Kconfig dependency for INIT_STACK_ALL_ZERO to test for the former
> > > > instead of the latter, as these are the options passed by the top-level
> > > > Makefile.
> > > >
> > > > Cc: Kees Cook <[email protected]>
> > > > Cc: Nathan Chancellor <[email protected]>
> > > > Cc: Nick Desaulniers <[email protected]>
> > > > Cc: Gustavo A. R. Silva <[email protected]>
> > > > Cc: Greg Kroah-Hartman <[email protected]>
> > > > Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> > > > Signed-off-by: Will Deacon <[email protected]>
> > >
> > > Reviewed-by: Nathan Chancellor <[email protected]>
> > >
> > > One comment below.
> > >
> > > > ---
> > > >
> > > > I just noticed this while reading the code and I suspect it doesn't really
> > > > matter in practice.
> > > >
> > > > security/Kconfig.hardening | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> > > > index 90cbaff86e13..341e2fdcba94 100644
> > > > --- a/security/Kconfig.hardening
> > > > +++ b/security/Kconfig.hardening
> > > > @@ -29,7 +29,7 @@ choice
> > > > prompt "Initialize kernel stack variables at function entry"
> > > > default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> > > > default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> > > > - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> > > > + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
> > > > default INIT_STACK_NONE
> > > > help
> > > > This option enables initialization of stack variables at
> > > >
> > >
> > > While I think this change is correct in and of itself,
> > > CONFIG_INIT_STACK_ALL_ZERO is broken with GCC 12.x, as
> > > CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO won't be set even though GCC now supports
> > > -ftrivial-auto-var-init=zero because GCC does not implement the
> > > -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> > > flag for obvious reasons ;) the cc-option call probably needs to be
> > > adjusted.
> >
> > GCC silently ignores the -enable flag, so things actually work correctly
> > as-is.
>
> So then would that mean that CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
> evaluates to true then, in your patch below?
No, I exclude it based on the results from
CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE.
here:
> > + def_bool !CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE && \
> Rather than create 2 new kconfigs with 1 new invocation of the
> compiler via cc-option, how about just adding an `ifdef
> CONFIG_CC_IS_CLANG` guard around adding the obnoxious flag to
> `KBUILD_CFLAGS` in the top level Makefile?
That is a bit more sensible, yes. :) Let me try that...
--
Kees Cook
On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:
> Rather than create 2 new kconfigs with 1 new invocation of the
> compiler via cc-option, how about just adding an `ifdef
> CONFIG_CC_IS_CLANG` guard around adding the obnoxious flag to
> `KBUILD_CFLAGS` in the top level Makefile?
v2:
diff --git a/Makefile b/Makefile
index 34a0afc3a8eb..72d165ffabdb 100644
--- a/Makefile
+++ b/Makefile
@@ -831,12 +831,12 @@ endif
# Initialize all stack variables with a zero value.
ifdef CONFIG_INIT_STACK_ALL_ZERO
-# Future support for zero initialization is still being debated, see
-# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being
-# renamed or dropped.
KBUILD_CFLAGS += -ftrivial-auto-var-init=zero
+ifdef CONFIG_CC_IS_CLANG
+# https://bugs.llvm.org/show_bug.cgi?id=45497
KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
endif
+endif
# While VLAs have been removed, GCC produces unreachable stack probes
# for the randomize_kstack_offset feature. Disable it for all compilers.
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 90cbaff86e13..ded17b8abce2 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN
def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
config CC_HAS_AUTO_VAR_INIT_ZERO
+ # GCC ignores the -enable flag, so we can test for the feature with
+ # a single invocation using the flag, but drop it as appropriate in
+ # the Makefile, depending on the presence of Clang.
def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
choice
prompt "Initialize kernel stack variables at function entry"
default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
- default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
+ default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
default INIT_STACK_NONE
help
This option enables initialization of stack variables at
--
Kees Cook
On Tue, Sep 14, 2021 at 12:14 PM Kees Cook <[email protected]> wrote:
>
> On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:
> > Rather than create 2 new kconfigs with 1 new invocation of the
> > compiler via cc-option, how about just adding an `ifdef
> > CONFIG_CC_IS_CLANG` guard around adding the obnoxious flag to
> > `KBUILD_CFLAGS` in the top level Makefile?
>
> v2:
LGTM
Reviewed-by: Nick Desaulniers <[email protected]>
I think the formal patch should have Will's Suggested-by on it.
>
> diff --git a/Makefile b/Makefile
> index 34a0afc3a8eb..72d165ffabdb 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -831,12 +831,12 @@ endif
>
> # Initialize all stack variables with a zero value.
> ifdef CONFIG_INIT_STACK_ALL_ZERO
> -# Future support for zero initialization is still being debated, see
> -# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being
> -# renamed or dropped.
> KBUILD_CFLAGS += -ftrivial-auto-var-init=zero
> +ifdef CONFIG_CC_IS_CLANG
> +# https://bugs.llvm.org/show_bug.cgi?id=45497
> KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> endif
> +endif
>
> # While VLAs have been removed, GCC produces unreachable stack probes
> # for the randomize_kstack_offset feature. Disable it for all compilers.
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 90cbaff86e13..ded17b8abce2 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN
> def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
>
> config CC_HAS_AUTO_VAR_INIT_ZERO
> + # GCC ignores the -enable flag, so we can test for the feature with
> + # a single invocation using the flag, but drop it as appropriate in
> + # the Makefile, depending on the presence of Clang.
> def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
>
> choice
> prompt "Initialize kernel stack variables at function entry"
> default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
> default INIT_STACK_NONE
> help
> This option enables initialization of stack variables at
>
> --
> Kees Cook
--
Thanks,
~Nick Desaulniers
On Tue, Sep 14, 2021 at 12:14:47PM -0700, Kees Cook wrote:
> On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:
> > Rather than create 2 new kconfigs with 1 new invocation of the
> > compiler via cc-option, how about just adding an `ifdef
> > CONFIG_CC_IS_CLANG` guard around adding the obnoxious flag to
> > `KBUILD_CFLAGS` in the top level Makefile?
>
> v2:
Thanks for clarifying! (since I did not actually test before making the
comment...)
Reviewed-by: Nathan Chancellor <[email protected]>
> diff --git a/Makefile b/Makefile
> index 34a0afc3a8eb..72d165ffabdb 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -831,12 +831,12 @@ endif
>
> # Initialize all stack variables with a zero value.
> ifdef CONFIG_INIT_STACK_ALL_ZERO
> -# Future support for zero initialization is still being debated, see
> -# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being
> -# renamed or dropped.
> KBUILD_CFLAGS += -ftrivial-auto-var-init=zero
> +ifdef CONFIG_CC_IS_CLANG
> +# https://bugs.llvm.org/show_bug.cgi?id=45497
> KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> endif
> +endif
>
> # While VLAs have been removed, GCC produces unreachable stack probes
> # for the randomize_kstack_offset feature. Disable it for all compilers.
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 90cbaff86e13..ded17b8abce2 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN
> def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
>
> config CC_HAS_AUTO_VAR_INIT_ZERO
> + # GCC ignores the -enable flag, so we can test for the feature with
> + # a single invocation using the flag, but drop it as appropriate in
> + # the Makefile, depending on the presence of Clang.
> def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
>
> choice
> prompt "Initialize kernel stack variables at function entry"
> default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
> default INIT_STACK_NONE
> help
> This option enables initialization of stack variables at
>
> --
> Kees Cook
>