linux-kernel - [RFC PATCH 1/2] objtool/LoongArch: Fix fall through warning about efi_boot

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Message-ID: <20250826064631.9617-2-yangtiezhu@loongson.cn>
Date: Tue, 26 Aug 2025 14:46:30 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nathan Chancellor <nathan@...nel.org>
Cc: loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH 1/2] objtool/LoongArch: Fix fall through warning about efi_boot_kernel()

When compiling with LLVM and CONFIG_LTO_CLANG is set, there exists
the following objtool warning:

  vmlinux.o: warning: objtool: __efistub_efi_boot_kernel()
  falls through to next function __efistub_exit_boot_func()

This is because efi_boot_kernel() doesn't end with a return instruction
or an unconditional jump, then objtool has determined that the function
can fall through into the next function.

Actually, efi_boot_kernel()'s last instruction is "jirl $ra, $a3, 0", it
is a call to a noreturn function pointer real_kernel_entry() which points
to the symbol kernel_entry() in arch/loongarch/kernel/head.S.

drivers/firmware/efi/libstub/loongarch.c:

typedef void __noreturn (*kernel_entry_t)(bool efi, unsigned long cmdline,
					  unsigned long systab);

efi_status_t efi_boot_kernel(void *handle, efi_loaded_image_t *image,
			     unsigned long kernel_addr, char *cmdline_ptr)
{
	kernel_entry_t real_kernel_entry;
	...
	real_kernel_entry = (void *)kernel_entry_address(kernel_addr, image);

	real_kernel_entry(true, (unsigned long)cmdline_ptr,
			  (unsigned long)efi_system_table);
}

According to the description of tools/objtool/Documentation/objtool.txt,
in order to silence this warning, at the beginning just add the noreturn
real_kernel_entry() to objtool's hard-coded global_noreturns array, but
there is no effect, because it is not a valid symbol.

There exists an alternative way to silence this warning, the first thing
is to remove the attribute __noreturn for real_kernel_entry(), otherwise
the compiler can not generate instructions after that, and then just add
"while (1);" at the end of efi_boot_kernel(), so that efi_boot_kernel()
ends with an unconditional jump instruction "b".

Note that at the end of efi_boot_kernel(), using unreachable() has no
effect because it can still generate fall-through code, using BUG() is
not proper because it will generate the following ld.lld warning:

  vmlinux.o:(.init__bug_table) is being placed in '.init__bug_table'

Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
---
 drivers/firmware/efi/libstub/loongarch.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/loongarch.c b/drivers/firmware/efi/libstub/loongarch.c
index 3782d0a187d1..e309fd78fca7 100644
--- a/drivers/firmware/efi/libstub/loongarch.c
+++ b/drivers/firmware/efi/libstub/loongarch.c
@@ -10,7 +10,7 @@
 #include "efistub.h"
 #include "loongarch-stub.h"

-typedef void __noreturn (*kernel_entry_t)(bool efi, unsigned long cmdline,
+typedef void (*kernel_entry_t)(bool efi, unsigned long cmdline,
 					  unsigned long systab);

 efi_status_t check_platform_features(void)
@@ -81,4 +81,7 @@ efi_status_t efi_boot_kernel(void *handle, efi_loaded_image_t *image,

 	real_kernel_entry(true, (unsigned long)cmdline_ptr,
 			  (unsigned long)efi_system_table);
+
+	/* We should never get here */
+	while (1);
 }
-- 
2.42.0