Re: [edk2-devel] About EDK2 supports Self Modifying Code

["Pedro Falcato" <pedro.falcato@gmail.com>]

On Tue, Aug 15, 2023 at 9:20 AM Chao Li <lichao@loongson.cn> wrote:
>
> Hi Andrew,
>
> Yes, you are right, I also think that SMC is a bit flawed in terms of security, but can we use some security mechanism to protect the SMC, like encryption and decryption? Sorry, I'm not consider mature enough about SMC security.

There isn't any. Actual use cases in something like a kernel are
heavily vetted and read-protected as soon as possible.

>
> I can tell you real problem, there are some CSR instructions in LoongArch64 that can only accept immediate value, for example: `csrrd $a0, 0x1`, the 0x1 is the selection of CSR register number, it can't use the registers to select. This operation should be in the MdePkg base library.
>
> I know that .c or .h files in MdePkg shouldn't depend on a single compiler feature, so I can't use the GNU AT&T style inline ASM function(AT&T style inline supports input parameters being immedite value, use "i" option). In this case, I think using SMC can handle this, that is use register transfer the CSR registers selection, and dynamically modify CSR instructions during execution phase with reference to transfer register value, this way is depend on the .text section or target memory is executable and writable.

FYI, poking instructions willy-nilly is unsafe and unreliable (except
on x86 due to kludges, but then it's slow).

>
> The problem of immediate values can only be handled by preprocessing stage or using SMC, otherwise I can only write a lot of similar functions and use `switch case` to call them. This method will cause the program size to expand a lot.
>
> So, I think I have following choice:
>
> Choice 1:
>
> Use AT&T style inline function, and create a file named: CsrOperationGcc.c, and other future compiler feature-dependent files will be named: CsrOperationClang.c, CsrOperationXlang.c and so on.

If you're going to use inline assembly, just expose them directly? I
don't see the problem there, I don't expect loongarch to be picked up
by visual studio any time soon.

>
>
> Choice 2:
>
> Use SMC.
>
>
> Choice 3:
>
> Write a lot of similar CSR functions.

You /could/ use a GAS macro.

.macro csr_write csr
.global CsrWrite\csr
CsrWrite\csr:
    csrw a0, \csr
    ret

(this is riscv pseudo-asm but I know your arch is similar enough)

-- 
Pedro


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