Re: [PATCH 3/6] xen/efi: Make efi-boot.h compile with -Wwrite-strings

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/11/2023 6:03 pm, Andrew Cooper wrote:
> On 21/11/2023 8:40 am, Jan Beulich wrote:
>> On 20.11.2023 23:49, Andrew Cooper wrote:
>>> GCC complains:
>>>
>>>   In file included from arch/arm/efi/boot.c:700:
>>>   arch/arm/efi/efi-boot.h: In function 'efi_arch_handle_cmdline':
>>>   arch/arm/efi/efi-boot.h:482:16: error: assignment discards 'const' 
>>> qualifier from pointer target type [-Werror=discarded-qualifiers]
>>>     482 |         name.s = "xen";
>>>         |                ^
>>>
>>> There's no easy option.  .rodata is really read-only, so the fact Xen 
>>> doesn't
>>> crash means these strings aren't written to.
>> And the consuming sites confirm this being the case. Hence ...
>>
>>> Lie to the compiler using a union.
>> ... to at least slightly limit the lying, how about ...
>>
>>> --- a/xen/arch/arm/efi/efi-boot.h
>>> +++ b/xen/arch/arm/efi/efi-boot.h
>>> @@ -479,7 +479,7 @@ static void __init efi_arch_handle_cmdline(CHAR16 
>>> *image_name,
>>>          w2s(&name);
>>>      }
>>>      else
>>> -        name.s = "xen";
>>> +        name.cs = "xen"; /* TODO, find a better way of doing this. */
>>>  
>>>      prop_len = 0;
>>>      prop_len += snprintf(buf + prop_len,
>> ... you also switch to using name.cs down below here and ...
>>
>>> --- a/xen/arch/x86/efi/efi-boot.h
>>> +++ b/xen/arch/x86/efi/efi-boot.h
>>> @@ -324,7 +324,8 @@ static void __init efi_arch_handle_cmdline(CHAR16 
>>> *image_name,
>>>          w2s(&name);
>>>      }
>>>      else
>>> -        name.s = "xen";
>>> +        name.cs = "xen"; /* TODO, find a better way of doing this. */
>>> +
>>>      place_string(&mbi.cmdline, name.s);
>> ... here?
>>
>> An alternative would be to introduce 'char xen[4] = "xen";' in both
>> cases, and use them instead of plain string literals.
> They'd have to be static, or dynamically allocated or they'll end up out
> of scope, wont they?
>
> I have to admit I find this logic very hard to follow.
>
> Furthermore, I note:
>
> mbi.boot_loader_name = (long)"EFI";
>
> which is exactly the kind of pointer which is liable to end up being
> edited via kextra in the other patch.

Hang on.  place_string()'ing here is just to prepend a piece of data we
go to other lengths to strip and ignore later in boot.

On x86 we can just delete it and make our lives simpler.  I hope the
same is true on ARM too.

~Andrew