[PATCH] riscv: Prevent early kernel panic in instrumented apply_early_boot_alternatives

[Changbin Du]

Under FTRACE=y, DYNAMIC_FTRACE=n, and RISCV_ALTERNATIVE_EARLY=n, the kernel
panics upon returning from _mcount() in the early boot path. This occurs
during _start_kernel() -> setup_vm() -> apply_early_boot_alternatives().

The CC_FLAGS_FTRACE is only removed from alternative.c when
CONFIG_RISCV_ALTERNATIVE_EARLY=y. Therefore, no function calls should be
made to alternative.c during early boot in this configuration.

Fixes: a35707c3d850 ("riscv: add memory-type errata for T-Head")
Signed-off-by: Changbin Du <changbin.du at huawei.com>
---
 arch/riscv/include/asm/alternative.h | 6 ++++++
 arch/riscv/kernel/alternative.c      | 4 ++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/alternative.h b/arch/riscv/include/asm/alternative.h
index 3c2b59b25017..c670b0cc55f4 100644
--- a/arch/riscv/include/asm/alternative.h
+++ b/arch/riscv/include/asm/alternative.h
@@ -31,7 +31,13 @@
 #define ALT_ALT_PTR(a)			__ALT_PTR(a, alt_offset)
 
 void __init apply_boot_alternatives(void);
+
+# ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
 void __init apply_early_boot_alternatives(void);
+# else
+static inline void apply_early_boot_alternatives(void) { }
+# endif
+
 void apply_module_alternatives(void *start, size_t length);
 
 void riscv_alternative_fix_offsets(void *alt_ptr, unsigned int len,
diff --git a/arch/riscv/kernel/alternative.c b/arch/riscv/kernel/alternative.c
index 7eb3cb1215c6..5406c3301627 100644
--- a/arch/riscv/kernel/alternative.c
+++ b/arch/riscv/kernel/alternative.c
@@ -205,6 +205,7 @@ void __init apply_boot_alternatives(void)
 	apply_vdso_alternatives();
 }
 
+#ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
 /*
  * apply_early_boot_alternatives() is called from setup_vm() with MMU-off.
  *
@@ -219,12 +220,11 @@ void __init apply_boot_alternatives(void)
  */
 void __init apply_early_boot_alternatives(void)
 {
-#ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
 	_apply_alternatives((struct alt_entry *)__alt_start,
 			    (struct alt_entry *)__alt_end,
 			    RISCV_ALTERNATIVES_EARLY_BOOT);
-#endif
 }
+#endif
 
 #ifdef CONFIG_MODULES
 void apply_module_alternatives(void *start, size_t length)
-- 
2.43.0