Commit d7868550 authored by Miguel Ojeda's avatar Miguel Ojeda
Browse files

x86/rust: support MITIGATION_RETHUNK 

The Rust compiler added support for `-Zfunction-return=thunk-extern` [1]
in 1.76.0 [2], i.e. the equivalent of `-mfunction-return=thunk-extern`.
Thus add support for `MITIGATION_RETHUNK`.

Without this, `objtool` would warn if enabled for Rust and already warns
under IBT builds, e.g.:

    samples/rust/rust_print.o: warning: objtool:
    _R...init+0xa5c: 'naked' return found in RETHUNK build

Link: https://github.com/rust-lang/rust/issues/116853 [1]
Link: https://github.com/rust-lang/rust/pull/116892

 [2]
Reviewed-by: default avatarGary Guo <gary@garyguo.net>
Tested-by: default avatarAlice Ryhl <aliceryhl@google.com>
Tested-by: default avatarBenno Lossin <benno.lossin@proton.me>
Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://github.com/Rust-for-Linux/linux/issues/945
Link: https://lore.kernel.org/r/20240725183325.122827-4-ojeda@kernel.org


Signed-off-by: default avatarMiguel Ojeda <ojeda@kernel.org>
parent 284a3ac4

arch/x86/Makefile

+5 −0
Original line number Diff line number Diff line
@@ -24,11 +24,15 @@ RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch-cs-prefix) 


ifdef CONFIG_MITIGATION_RETHUNK

RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern

RETHUNK_RUSTFLAGS	:= -Zfunction-return=thunk-extern

RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)

RETPOLINE_RUSTFLAGS	+= $(RETHUNK_RUSTFLAGS)

endif



export RETHUNK_CFLAGS

export RETHUNK_RUSTFLAGS

export RETPOLINE_CFLAGS

export RETPOLINE_RUSTFLAGS

export RETPOLINE_VDSO_CFLAGS



# For gcc stack alignment is specified with -mpreferred-stack-boundary,
@@ -218,6 +222,7 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables 
# Avoid indirect branches in kernel to deal with Spectre

ifdef CONFIG_MITIGATION_RETPOLINE

  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)

  KBUILD_RUSTFLAGS += $(RETPOLINE_RUSTFLAGS)

  # Additionally, avoid generating expensive indirect jumps which

  # are subject to retpolines for small number of switch cases.

  # LLVM turns off jump table generation by default when under