Re: [PATCH v2 01/10] x86/boot: Drop pointless re-implementation of panic()

["Ard Biesheuvel" <ardb@kernel.org>]

On Wed, 9 Sep 2026, at 21:14, Borislav Petkov wrote:
> On Wed, Sep 09, 2026 at 01:55:32PM +0200, Ard Biesheuvel wrote:
>> From: Ard Biesheuvel <ardb@kernel.org>
>> 
>> The decompressor has its own implementation of panic(), which is based
>> on the vsnprintf() routine provided by the EFI stub.
>> 
>> Relying on the EFI stub from code that does not execute in the context
>> of the EFI boot services is a bad idea. It is also completely pointless
>> in this case, given that the only user of this version of panic() only
>> passes a compile time constant string, without any printf conversions.
>> 
>> So use error() instead of panic() in that case, and drop the panic()
>> implementation entirely. This is needed so that the EFI stub's
>> vsnprintf() can be modified in a manner that is incompatible with the
>> expectations of this caller.
>> 
>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>> ---
>>  arch/x86/boot/compressed/error.c | 19 -------------------
>>  arch/x86/boot/compressed/error.h |  1 -
>>  arch/x86/boot/compressed/mem.c   |  2 +-
>>  3 files changed, 1 insertion(+), 21 deletions(-)
>> 
>> diff --git a/arch/x86/boot/compressed/error.c b/arch/x86/boot/compressed/error.c
>> index 19a8251de506..ce5ed7d8265e 100644
>> --- a/arch/x86/boot/compressed/error.c
>> +++ b/arch/x86/boot/compressed/error.c
>> @@ -22,22 +22,3 @@ void error(char *m)
>>  	while (1)
>>  		asm("hlt");
>>  }
>> -
>> -/* EFI libstub  provides vsnprintf() */
>> -#ifdef CONFIG_EFI_STUB
>> -void panic(const char *fmt, ...)
>
> So this thing appeared magically in v8 of the TDX unaccepted memory patches
> and I don't think we questioned it back then.
>
> v7's tdx_accept_memory() does error():
>
> https://lore.kernel.org/all/20220614120231.48165-15-kirill.shutemov@linux.intel.com/
>
> and v8 started doing panic():
>
> +void tdx_accept_memory(phys_addr_t start, phys_addr_t end)
> +{
> +	if (!tdx_enc_status_changed_phys(start, end, true))
> +		panic("Accepting memory failed: %#llx-%#llx\n",  start, end);
> +}
>
> https://lore.kernel.org/all/20221207014933.8435-15-kirill.shutemov@linux.intel.com/
>
> and it switched to it being a vsnprintf() wrapper because it wanted to dump
> start and end perhaps.
>
> But then it ended up dropping the params in v13 and landed upstream with
> a single string as an argument.
>
> Anyway, adding Kiryl/Kirill for comment and leaving in the rest for reference.
>

Looking at that v13, it seems the panic() call was added to
arch/x86/coco/tdx/tdx-shared.c, which was shared between the kernel proper and
the decompressorat the time, and so a panic() implementation was needed in the
decompressor too. But that is no longer the case.