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SubjectRe: [PATCH v15 3/6] x86/boot: Introduce efi_get_rsdp_addr() to find RSDP from EFI table
On Fri, Jan 11, 2019 at 11:32:25AM +0100, Borislav Petkov wrote:
>On Fri, Jan 11, 2019 at 09:23:53AM +0800, Chao Fan wrote:
>> Yes, 'table64' looks superfluous here, but after these lines, there is:
>> if (!IS_ENABLED(CONFIG_X86_64) && table64 >> 32) {
>> so the 'table64' is useful here for i386. 'table' is unsigned long, it
>> can't do the right shift. But the 'table64' who is u64 can do that right
>> shift.
>
>Have you actually tried fixing what I suggested or you're just talking?

Sure, I tried what I was talking.
I ever used 'unsigned long table' directly. But that caused warning:
CC arch/x86/boot/compressed/acpi.o
arch/x86/boot/compressed/acpi.c: In function ‘efi_get_rsdp_addr’:
arch/x86/boot/compressed/acpi.c:106:44: warning: right shift count >= width of type [-Wshift-count-overflow]
if (!IS_ENABLED(CONFIG_X86_64) && table >> 32) {
To avoid this warning, I add 'u64 table64' to do the right shift.

Well, the acpi_physicall_address defined as:

#if ACPI_MACHINE_WIDTH == 64
typedef u64 acpi_physical_address;
#elif ACPI_MACHINE_WIDTH == 32
#ifdef ACPI_32BIT_PHYSICAL_ADDRESS
typedef u32 acpi_physical_address;
#else /* ACPI_32BIT_PHYSICAL_ADDRESS */

/*
* It is reported that, after some calculations, the physical addresses
can
* wrap over the 32-bit boundary on 32-bit PAE environment.
* https://bugzilla.kernel.org/show_bug.cgi?id=87971
*/
typedef u64 acpi_physical_address;
#endif

'acpi_physical_address' could be define as u64 or u32.
In my guest machine, it's defined as u64, there is no problem as you
suggested.
I didn't find a machine where 'acpi_physical_address' defined as u32.
I thought if 'acpi_physical_address' defined as u32, it will meet
the same warning as using 'unsigned long'.
I tried to define 'table' as u32, that will also cause the warning.
So once acpi_physical_address is define as u32, that warning will
happen. We should make sure u64 to do the right shift.

Thanks,
Chao Fan

>
>---
>diff --git a/arch/x86/boot/compressed/acpi.c b/arch/x86/boot/compressed/acpi.c
>index e9dd84f459ed..0537d46fb21f 100644
>--- a/arch/x86/boot/compressed/acpi.c
>+++ b/arch/x86/boot/compressed/acpi.c
>@@ -77,21 +77,19 @@ static acpi_physical_address efi_get_rsdp_addr(void)
> sizeof(efi_config_table_32_t);
>
> for (i = 0; i < systab->nr_tables; i++) {
>+ acpi_physical_address table;
> void *config_tables;
>- unsigned long table;
> efi_guid_t guid;
>
> config_tables = (void *)(systab->tables + size * i);
> if (efi_64) {
> efi_config_table_64_t *tmp_table;
>- u64 table64;
>
> tmp_table = config_tables;
> guid = tmp_table->guid;
>- table64 = tmp_table->table;
>- table = table64;
>+ table = tmp_table->table;
>
>- if (!IS_ENABLED(CONFIG_X86_64) && table64 >> 32) {
>+ if (!IS_ENABLED(CONFIG_X86_64) && table >> 32) {
> debug_putstr("Error getting RSDP address: EFI config table located above 4GB.\n");
> return 0;
> }
>
>--
>Regards/Gruss,
> Boris.
>
>Good mailing practices for 400: avoid top-posting and trim the reply.
>
>


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