[PATCH v3 1/7] scripts/sorttable: Make the arm64 before_func offset configurable

[Jose Fernandez (Anthropic)]

sorttable hardcodes before_func to 8 bytes on arm64 (two prefix NOPs).
Make it configurable from CONFIG_ARM64_FUNCTION_PREFIX_NOPS instead.
kCFI kernels with CALL_OPS need three NOPs. The symbol does not exist
yet, so no functional change.

Reviewed-by: Ben Cressey <ben at cressey.dev>
Reviewed-by: Florent Revest (Anthropic) <florent.revest at linux.dev>
Acked-by: Alexei Starovoitov <ast at kernel.org>
Assisted-by: LLM
Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez at linux.dev>
---
 scripts/Makefile    | 4 ++++
 scripts/sorttable.c | 7 ++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile b/scripts/Makefile
index d7fbb7ca098b..7ef1c84cf79d 100644
--- a/scripts/Makefile
+++ b/scripts/Makefile
@@ -54,6 +54,10 @@ endif
 
 ifdef CONFIG_BUILDTIME_MCOUNT_SORT
 HOSTCFLAGS_sorttable.o += -DMCOUNT_SORT_ENABLED
+# 0 or unset (no prefix NOPs) leaves sorttable's built-in default in place.
+ifneq ($(filter-out 0,$(CONFIG_ARM64_FUNCTION_PREFIX_NOPS)),)
+HOSTCFLAGS_sorttable.o += -DFUNCTION_PREFIX_BYTES=$(shell expr 4 \* $(CONFIG_ARM64_FUNCTION_PREFIX_NOPS))
+endif
 endif
 
 hostprogs += gen_init_cpio
diff --git a/scripts/sorttable.c b/scripts/sorttable.c
index 88e49a5c4251..2e951f2f60e8 100644
--- a/scripts/sorttable.c
+++ b/scripts/sorttable.c
@@ -281,6 +281,11 @@ static int add_field(uint64_t addr, uint64_t size)
 /* Used for when mcount/fentry is before the function entry */
 static int before_func;
 
+/* How far before the function entry a patchable entry may be recorded */
+#ifndef FUNCTION_PREFIX_BYTES
+#define FUNCTION_PREFIX_BYTES	8
+#endif
+
 /* Only return match if the address lies inside the function size */
 static int cmp_func_addr(const void *K, const void *A)
 {
@@ -912,7 +917,7 @@ static int do_file(char const *const fname, void *addr)
 		/* fallthrough */
 	case EM_RISCV:
 		/* arm64 and RISC-V place patchable entries before the function. */
-		before_func = 8;
+		before_func = FUNCTION_PREFIX_BYTES;
 #else
 	case EM_AARCH64:
 	case EM_RISCV:

-- 
2.54.0