[PATCH] x86: rust: Disable entry padding with Rust

[Matthew Maurer <mmaurer@google.com>]

rustc-1.73.0 used by Linux does not support entry padding. Mark entry
padding support as explicitly incompatible with Rust.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---

This is temporary - upstream in the Rust compiler, I have:

* A MCP[1] to enable the feature as an unstable flag
* A RFC[2] to stabilize it and add the attribute
* An initial implementation[3] for the MCP and RFC once accepted

Once at least unstable support is merged, we can detect this in a future
patch.

Even once those are merged and Linux is using a newer rustc, we will
still want a separate HAVE_ENTRY_PADDING config, as Rust's gcc backend
will not initially support it.

While this is a temporary patch, I still think it's worth carrying
because it prevents us from selecting build configurations that will
fail at runtime.

[1]: https://github.com/rust-lang/compiler-team/issues/704
[2]: https://github.com/rust-lang/rfcs/pull/3543
[3]: https://github.com/maurer/rust/tree/patchable-function-entries
 arch/x86/Kconfig | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 18b9fb7df95b..a9d48709f7db 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2452,6 +2452,9 @@ config CC_HAS_RETURN_THUNK
 config CC_HAS_ENTRY_PADDING
 	def_bool $(cc-option,-fpatchable-function-entry=16,16)
 
+config HAVE_ENTRY_PADDING
+	def_bool CC_HAS_ENTRY_PADDING && !RUST
+
 config FUNCTION_PADDING_CFI
 	int
 	default 59 if FUNCTION_ALIGNMENT_64B
@@ -2469,17 +2472,17 @@ config FUNCTION_PADDING_BYTES
 
 config CALL_PADDING
 	def_bool n
-	depends on CC_HAS_ENTRY_PADDING && OBJTOOL
+	depends on HAVE_ENTRY_PADDING && OBJTOOL
 	select FUNCTION_ALIGNMENT_16B
 
 config FINEIBT
 	def_bool y
-	depends on X86_KERNEL_IBT && CFI_CLANG && RETPOLINE
+	depends on X86_KERNEL_IBT && CFI_CLANG && RETPOLINE && HAVE_ENTRY_PADDING
 	select CALL_PADDING
 
 config HAVE_CALL_THUNKS
 	def_bool y
-	depends on CC_HAS_ENTRY_PADDING && RETHUNK && OBJTOOL
+	depends on HAVE_ENTRY_PADDING && RETHUNK && OBJTOOL
 
 config CALL_THUNKS
 	def_bool n
-- 
2.43.0.472.g3155946c3a-goog