linux-kernel - Re: [PATCH v1 1/3] objtool/LoongArch: Fix fall through warning about efi_boot

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Message-ID: <a59b3eaa-133d-88bf-f1f3-41328d023c4a@loongson.cn>
Date: Mon, 1 Sep 2025 16:31:36 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Huacai Chen <chenhuacai@...nel.org>, Josh Poimboeuf
 <jpoimboe@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/3] objtool/LoongArch: Fix fall through warning about
 efi_boot_kernel()

On 2025/9/1 下午4:16, Peter Zijlstra wrote:
> On Mon, Sep 01, 2025 at 03:21:54PM +0800, Tiezhu Yang wrote:
>> 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.
>>
>> At the beginning, try to do something to make efi_boot_kernel() ends with
>> an unconditional jump instruction, but it is not a proper way.
>>
>> After more analysis, one simple way is to ignore these EFISTUB functions
>> in validate_branch() of objtool since they are useless for stack unwinder.
>>
> 
> This is drivers/firmware/efi/libstub/loongarch.c:efi_boot_kernel(),
> right?
> 
> Why not simply do something like:
> 
> diff --git a/drivers/firmware/efi/libstub/loongarch.c b/drivers/firmware/efi/libstub/loongarch.c
> index 3782d0a187d1..082611a5f1f0 100644
> --- a/drivers/firmware/efi/libstub/loongarch.c
> +++ b/drivers/firmware/efi/libstub/loongarch.c
> @@ -81,4 +81,5 @@ 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);
> +	BUG();
>   }

At the beginning, I did the above change, but no effect.

The first thing is to remove the attribute __noreturn for
real_kernel_entry(), otherwise the compiler can not generate
instructions after that.

But there is an argument in the previous RFC [1]:

"From my point of view this is incorrect, this function is indeed a
noreturn function, and this modification makes LoongArch different to
other architectures."

Josh suggested to do something so that the EFI stub code isn't linked 
into vmlinux.o [2], it needs to modify the link process and seems too
complicated and expensive for this warning to some extent.

So I did this change for objtool.

[1] 
https://lore.kernel.org/loongarch/CAAhV-H5wW_04NHQ7z+SCPb6-T5Hc__n+x=ykg-u9vn4b4GXuww@mail.gmail.com/
[2] 
https://lore.kernel.org/loongarch/xyrcgkl7ud5pgh4h5yjyejz646bc22fnnwxahaoafqvnqintf3@mdhtfaybai67/

Thanks,
Tiezhu