[PATCH] efi_loader: add comments on efi_file_from_path() regarding alignment

[Heinrich Schuchardt]

On 11/10/22 14:31, Ilias Apalodimas wrote:
> UEFI specification requires pointers that are passed to protocol member
> functions to be aligned.  There's a u16_strdup in that function which
> doesn't make sense otherwise  Add a comment so no one removes it
> accidentally
> 
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> ---
>   lib/efi_loader/efi_file.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/lib/efi_loader/efi_file.c b/lib/efi_loader/efi_file.c
> index 8480ed3007c7..5c254ccdd64d 100644
> --- a/lib/efi_loader/efi_file.c
> +++ b/lib/efi_loader/efi_file.c
> @@ -1135,6 +1135,11 @@ struct efi_file_handle *efi_file_from_path(struct efi_device_path *fp)
>   			return NULL;
>   		}
>   
> +		/*
> +		 * UEFI specification requires pointers that are passed to
> +		 * protocol member functions to be aligned.  So memcpy it
> +		 * unconditionally
> +		 */
>   		filename = u16_strdup(fdp->str);

On ARM we enable unaligned access which is supported by the CPU. On 
RISC-V unaligned access is emulated by OpenSBI which is very slow. 
Therefore copying make sense.

u16_strdup() calls u16_strsize() which itself is not caring about 
alignment. So this u16_strdup does not resolve all alignment issues.

We could use the length field of the file path node to determine the 
length of the string to be copied and invoke

     malloc(fdp->length - 4).
     memcpy(,, fdp->length - 4).

This would be better performance wise on RISC-V.

Best regards

Heinrich

>   		if (!filename)
>   			return NULL;