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SubjectRe: [tip:x86/boot] x86/boot: Early parse RSDP and save it in boot_params
On Mon, Feb 11, 2019 at 10:17:36AM +0000, Ard Biesheuvel wrote:
> That still does not explain how 'table' can assume a value > 4 GB
> after assigning the contents of a u32 to it.

See efi_get_rsdp_addr() in tip/master and especially that systable address
computation:

#ifdef CONFIG_X86_64
systab = (efi_system_table_t *)(ei->efi_systab | ((__u64)ei->efi_systab_hi<<32));
#else
if (ei->efi_systab_hi || ei->efi_memmap_hi) {
debug_putstr("Error getting RSDP address: EFI system table located above 4GB.\n");
return 0;
}
systab = (efi_system_table_t *)ei->efi_systab;
#endif

...

config_tables = (void *)(systab->tables + size * i);

It is hard to debug that early but I managed to singlestep it last night
to this deref:

} else {
efi_config_table_32_t *tmp_table;

tmp_table = config_tables;
guid = tmp_table->guid;
^^^^^^^^^^^^^^

table = tmp_table->table;

which in asm is:

# arch/x86/boot/compressed/acpi.c:114: guid = tmp_table->guid;
#NO_APP
movq (%rdi), %rax # MEM[(struct efi_config_table_32_t *)config_tables_37].guid, guid
movq 8(%rdi), %rsi # MEM[(struct efi_config_table_32_t *)config_tables_37].guid, guid
# arch/x86/boot/compressed/acpi.c:115: table = tmp_table->table;
movl 16(%rdi), %r10d # MEM[(struct efi_config_table_32_t *)config_tables_37].table, table

and %rdi has 0x630646870 so either we got the wrong address or qemu
mapped it above 4G...

It is only an observation for now though...

Thx.

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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