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SubjectRe: [PATCH 0/6] Apple device properties
On Mon, Aug 15, 2016 at 12:54:14PM +0100, Matt Fleming wrote:
> On Tue, 09 Aug, at 03:38:16PM, Lukas Wunner wrote:
> > @@ -208,7 +201,10 @@ struct efi_config {
> > __pure const struct efi_config *__efi_early(void);
> >
> > #define efi_call_early(f, ...) \
> > - __efi_early()->call(__efi_early()->f, __VA_ARGS__);
> > + __efi_early()->call(__efi_early()->is64 ? \
> > + ((efi_boot_services_64_t *)__efi_early()->boot_services)->f : \
> > + ((efi_boot_services_32_t *)__efi_early()->boot_services)->f, \
> > + __VA_ARGS__);
> >
>
> You cannot use pointers from the firmware directly in mixed mode
> because the kernel is compiled for 64-bits but the firmware is using
> 32-bit addresses, so dereferencing a pointer causes a 64-bit load.

Please behold the resulting binary code, which uses a 32-bit load,
not a 64-bit load (note the "mov edi, dword [ds:rax+0x2c]").

This is a call to AllocatePool *with* my patch:

0x22c1 mov rax, qword [ds:efi_early]
0x22c8 add rdx, 0x10 ; buffer size argument
0x22cc cmp byte [ds:rax+0x28], 0x0 ; !efi_early->is64 ?
0x22d0 mov r8, qword [ds:rax+0x20] ; efi_early->call()
0x22d4 mov rax, qword [ds:rax+0x10] ; efi_early->boot_services
0x22d8 je 0x2410
0x22de mov rdi, qword [ds:rax+0x40] ; allocate_pool (64 bit)
0x22e2 xor eax, eax
0x22e4 mov rcx, r13 ; buffer argument
0x22e7 mov esi, 0x2 ; EfiLoaderData argument
0x22ec call r8
...
0x2410 mov edi, dword [ds:rax+0x2c] ; allocate_pool (32 bit)
0x2413 jmp 0x22e2

The same *without* my patch:

0x1d41 mov r8, qword [ds:efi_early]
0x1d48 add r15, 0x40
0x1d4c mov rcx, qword [ss:rsp-0x10+arg_20] ; buffer argument
0x1d51 mov rdx, r15 ; buffer size argument
0x1d54 mov esi, 0x2 ; EfiLoaderData argument
0x1d59 mov rdi, qword [ds:r8+0x10] ; allocate_pool
0x1d5d call qword [ds:r8+0x58] ; efi_early->call

So it looks to me like my patch should work just fine on 32-bit,
even though I cannot verify it through testing.

The ARM folks afford invocation of arbitrary boot services, it just
seemed natural to me to allow the same for x86. The portion of the
stub code which is shared between arches cannot use more than the
8 boot services supported by x86 even though ARM would be capable
of using all of them.

Of course the binary code with my patch is longer, less readable,
and needs to follow multiple indirections and I can understand if
you would rather stay with the current approach for these reasons.

But I would like to understand the "cannot jump through pointers at
runtime" argument because the binary code looks to me like it should
work on 32 bit. I guess I must be missing something obvious?

Thanks,

Lukas

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