[PATCH v3 7/7] arm64: ftrace: Allow CALL_OPS on kCFI kernels built with Rust

["Jose Fernandez (Anthropic)" <jose.fernandez@linux.dev>]

Pass -Zpatchable-function-entry=M,M,.discard.patchable_function_entries
to rustc when CFI and CALL_OPS are both enabled, so Rust functions get
the same prefix NOPs as C functions. The section argument needs
Rust 1.98 [1]. Probe for it, and let ARM64_CFI_PERMITS_CALL_OPS
accept RUST=y when the probe passes.

Filter -fpatchable-function-entry out of bindgen's C flags on every
architecture, not only on arm64 with CFI, since removing the flag does
not change the ABI.

[1] https://github.com/rust-lang/rust/pull/157445

Suggested-by: Ben Cressey <ben@cressey.dev>
Reviewed-by: Ben Cressey <ben@cressey.dev>
Reviewed-by: Florent Revest (Anthropic) <florent.revest@linux.dev>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Assisted-by: LLM
Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
---
 arch/arm64/Kconfig  | 7 ++++++-
 arch/arm64/Makefile | 3 +++
 rust/Makefile       | 7 ++++++-
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index e8cb44d881d0..e64d053e1854 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -277,11 +277,16 @@ config CC_HAS_PATCHABLE_FUNCTION_ENTRY_SECTION
 	# https://github.com/llvm/llvm-project/pull/131230
 	def_bool $(cc-option,-fpatchable-function-entry=1$(comma)1$(comma).discard.patchable_function_entries)
 
+config RUSTC_HAS_PATCHABLE_FUNCTION_ENTRY_SECTION
+	# The section argument needs Rust 1.98.0 or later:
+	# https://github.com/rust-lang/rust/pull/157445
+	def_bool $(rustc-option,-Zpatchable-function-entry=1$(comma)1$(comma).discard.patchable_function_entries)
+
 config ARM64_CFI_PERMITS_CALL_OPS
 	def_bool y
 	depends on CFI
 	depends on CC_HAS_PATCHABLE_FUNCTION_ENTRY_SECTION
-	depends on !RUST
+	depends on !RUST || RUSTC_HAS_PATCHABLE_FUNCTION_ENTRY_SECTION
 
 config ARM64_FUNCTION_PREFIX_NOPS
 	int
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 9fbfe5ab3724..b47733b585b5 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -147,6 +147,9 @@ ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS),y)
 			 -DCC_USING_PATCHABLE_FUNCTION_PREFIX=$(prefix_nops)
     KBUILD_CFLAGS += $(CFI_PREFIX_CFLAGS)
     export CFI_PREFIX_CFLAGS
+    # Rust functions are not patched by ftrace, but they are called indirectly
+    # from C and make indirect calls themselves, so they need the same prefix.
+    KBUILD_RUSTFLAGS += -Zpatchable-function-entry=$(prefix_nops),$(prefix_nops),.discard.patchable_function_entries
   endif
 else ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_ARGS),y)
   KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
diff --git a/rust/Makefile b/rust/Makefile
index 0ac9ab28cc2b..686964c826f7 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -511,7 +511,12 @@ endif
 # prototypes for functions like `memcpy` -- if this flag is not passed,
 # `bindgen`-generated prototypes use `c_ulong` or `c_uint` depending on
 # architecture instead of generating `usize`.
-bindgen_c_flags_final = $(bindgen_c_flags_lto) -fno-builtin -D__BINDGEN__
+#
+# Do not pass -fpatchable-function-entry to bindgen: its libclang may not
+# support the section argument (Clang >= 21), and removing the flag does not
+# change the ABI.
+bindgen_c_flags_final = $(filter-out -fpatchable-function-entry=%, \
+	$(bindgen_c_flags_lto)) -fno-builtin -D__BINDGEN__
 
 # `--rust-target` points to our minimum supported Rust version.
 quiet_cmd_bindgen = BINDGEN $@

-- 
2.54.0