[v2,1/5] Makefile: add -fno-builtin-stpcpy

Commit Message

Nick Desaulniers Aug. 19, 2020, 7:16 p.m. UTC

LLVM implemented a recent "libcall optimization" that lowers calls to
`sprintf(dest, "%s", str)` where the return value is used to
`stpcpy(dest, str) - dest`. This generally avoids the machinery involved
in parsing format strings. This optimization was introduced into
clang-12. Because the kernel does not provide an implementation of
stpcpy, we observe linkage failures for almost all targets when building
with ToT clang.

The interface is unsafe as it does not perform any bounds checking.
Disable this "libcall optimization" via `-fno-builtin-stpcpy`.

Cc: [e-mail] # 4.4
Link: https://bugs.llvm.org/show_bug.cgi?id=47162
Link: https://github.com/ClangBuiltLinux/linux/issues/1126
Link: https://reviews.llvm.org/D85963
Reported-by: Sami Tolvanen <[e-mail]>
Suggested-by: Dávid Bolvanský <[e-mail]>
Suggested-by: Kees Cook <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
---
 Makefile | 1 +
 1 file changed, 1 insertion(+)

Comments

Nathan Chancellor Aug. 20, 2020, 3:33 a.m. UTC | #1

On Wed, Aug 19, 2020 at 12:16:50PM -0700, Nick Desaulniers wrote:
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings. This optimization was introduced into
> clang-12. Because the kernel does not provide an implementation of
> stpcpy, we observe linkage failures for almost all targets when building
> with ToT clang.
> 
> The interface is unsafe as it does not perform any bounds checking.
> Disable this "libcall optimization" via `-fno-builtin-stpcpy`.
> 
> Cc: [e-mail] # 4.4
> Link: https://bugs.llvm.org/show_bug.cgi?id=47162
> Link: https://github.com/ClangBuiltLinux/linux/issues/1126
> Link: https://reviews.llvm.org/D85963
> Reported-by: Sami Tolvanen <[e-mail]>
> Suggested-by: Dávid Bolvanský <[e-mail]>
> Suggested-by: Kees Cook <[e-mail]>
> Reviewed-by: Kees Cook <[e-mail]>
> Signed-off-by: Nick Desaulniers <[e-mail]>

Reviewed-by: Nathan Chancellor <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/Makefile b/Makefile
> index 9cac6fde3479..e523dc8d30e0 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -578,6 +578,7 @@ ifneq ($(LLVM_IAS),1)
>  CLANG_FLAGS	+= -no-integrated-as
>  endif
>  CLANG_FLAGS	+= -Werror=unknown-warning-option
> +CLANG_FLAGS	+= -fno-builtin-stpcpy
>  KBUILD_CFLAGS	+= $(CLANG_FLAGS)
>  KBUILD_AFLAGS	+= $(CLANG_FLAGS)
>  export CLANG_FLAGS
> -- 
> 2.28.0.297.g1956fa8f8d-goog
>

Patch

diff --git a/Makefile b/Makefile
index 9cac6fde3479..e523dc8d30e0 100644
--- a/Makefile
+++ b/Makefile
@@ -578,6 +578,7 @@  ifneq ($(LLVM_IAS),1)
 CLANG_FLAGS	+= -no-integrated-as
 endif
 CLANG_FLAGS	+= -Werror=unknown-warning-option
+CLANG_FLAGS	+= -fno-builtin-stpcpy
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS