[PATCH RFC 7/8] arm64: ftrace: Use five prefix NOPs on ThinLTO kernels with BTI

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

Default CONFIG_ARM64_FUNCTION_PREFIX_NOPS to 5 on ThinLTO kernels with
BTI. Many functions there have a landing pad but no hash. That moves the
entry point 4 bytes. The 8-byte slot lands on the landing pad and ftrace
refuses the site. With five NOPs every function has a slot.

Commit d3359af21fc9e ("arm64: bti: Disable in-kernel BTI with recent
versions of Clang") keeps BTI off on clang 21 or later, so this default
does not trigger on a mainline build today. Josh Poimboeuf's series to
bring it back is under review [1].

With five NOPs the series costs 7.4% of .text on a defconfig-based
ThinLTO kernel with clang 21.1.4:

  Kernel                 || .text bytes | Delta       | Delta
  =======================++=============+=============+=======
  ThinLTO kCFI, base     ||  21,466,256 |             |
  ThinLTO kCFI, M = 5    ||  23,047,312 |  +1,581,056 | +7.4%
  -----------------------++-------------+-------------+-------

ftracetest results match with and without this series. The three
failures on a ThinLTO kernel are pre-existing. The BPF trampoline,
struct_ops and kprobe_multi selftests pass with CFI enforcing.

[1] https://lore.kernel.org/all/cover.1786768375.git.jpoimboe@kernel.org/

Reviewed-by: Ben Cressey <ben@cressey.dev>
Reviewed-by: Florent Revest (Anthropic) <florent.revest@linux.dev>
Assisted-by: LLM
Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
---
 arch/arm64/Kconfig | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 6e5fa009c11af..afa566e0fa86c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -290,6 +290,8 @@ config ARM64_CFI_PERMITS_CALL_OPS
 
 config ARM64_FUNCTION_PREFIX_NOPS
 	int
+	default 5 if DYNAMIC_FTRACE_WITH_CALL_OPS && CFI && \
+		     (LTO_CLANG_THIN || LTO_CLANG_THIN_DIST) && ARM64_BTI_KERNEL
 	default 3 if DYNAMIC_FTRACE_WITH_CALL_OPS && CFI
 	default 2 if DYNAMIC_FTRACE_WITH_CALL_OPS
 	default 0

-- 
2.52.0