x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments

Linux Kernel / Linux Kernel - Nick Desaulniers [google.com] - 11 August 2022 01:30 UTC

Users of GNU ld (BFD) from binutils 2.39+ will observe multiple instances of a new warning when linking kernels in the form:

ld: warning: arch/x86/boot/pmjump.o: missing .note.GNU-stack section implies executable stack ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker ld: warning: arch/x86/boot/compressed/vmlinux has a LOAD segment with RWX permissions

Generally, we would like to avoid the stack being executable. Because there could be a need for the stack to be executable, assembler sources have to opt-in to this security feature via explicit creation of the .note.GNU-stack feature (which compilers create by default) or command line flag --noexecstack. Or we can simply tell the linker the production of such sections is irrelevant and to link the stack as

LLVM's LLD linker defaults to -z noexecstack, so this flag isn't strictly necessary when linking with LLD, only BFD, but it doesn't hurt to be explicit here for all linkers IMO. --no-warn-rwx-segments is currently BFD specific and only available in the current latest release, so it's wrapped in an ld-option check.

While the kernel makes extensive usage of ELF sections, it doesn't use permissions from ELF segments.

Link: https://lore.kernel.org/linux-block/3af4127a-f453-4cf7-f133-a181cce06f73@kernel.dk/ Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=ba951afb99912da01a6e8434126b8fac7aa75107 Link: https://github.com/llvm/llvm-project/issues/57009

ffcf9c5700e4 x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments
arch/x86/boot/Makefile | 2 +-
arch/x86/boot/compressed/Makefile | 4 ++++
arch/x86/entry/vdso/Makefile | 2 +-
3 files changed, 6 insertions(+), 2 deletions(-)

Upstream: git.kernel.org

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