Re: [PATCH v11 2/5] x86/boot: Add bios_get_rsdp_addr() to search RSDP in memory

Borislav Petkov
2018-11-12T15:27:44Z

On Mon, Nov 12, 2018 at 05:46:42PM +0800, Chao Fan wrote:
> Imitate ACPI code to search RSDP pointer from memory.
> Walk memory and check the signature until get the RSDP signature.
> Based on acpi_tb_scan_memory_for_rsdp() and acpi_find_root_pointer().
> If didn't get RSDP from EFI table, will run this function.

That's some very strange english. Please improve.

> Used for later patch to dig out SRAT table and get the memory
> information. And figure out the immovable memory regions
> to avoid KASLR extracts kernel on movable memory, slove the
^^^^^^

Please introduce a spellchecker into your patch creation workflow.

> conflict between KASLR and movable_node feature.

Btw, this paragraph could be used for a CONFIG_ item you could define
for your particular use case. Because right now you have funnies like:

+#if (defined CONFIG_MEMORY_HOTREMOVE) && (defined CONFIG_RANDOMIZE_BASE)
+vmlinux-objs-$(CONFIG_RANDOMIZE_BASE) += $(obj)/acpitb.o
+#endif

where CONFIG_RANDOMIZE_BASE is repeated for no good reason.

But we'll see - need to get to the end of your patch series first.

> Signed-off-by: Chao Fan <fanc.fnst@cn.fujitsu.com>
> ---
> arch/x86/boot/compressed/acpitb.c | 106 ++++++++++++++++++++++++++++++
> 1 file changed, 106 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/acpitb.c b/arch/x86/boot/compressed/acpitb.c
> index 56b54b0e0889..50fa65cf824d 100644
> --- a/arch/x86/boot/compressed/acpitb.c
> +++ b/arch/x86/boot/compressed/acpitb.c
> @@ -94,3 +94,109 @@ static void efi_get_rsdp_addr(acpi_physical_address *rsdp_addr)
> }
> #endif
> }
> +
> +static u8 compute_checksum(u8 *buffer, u32 length)
> +{
> + u8 sum = 0;
> + u8 *end = buffer + length;
> +
> + while (buffer < end)
> + sum = (u8)(sum + *(buffer++));

What's that cast for?

Ah, this is the version in acpi_tb_checksum(). Well, I'd write this
simply as:

sum += *(buffer++);

> +
> + return sum;
> +}
> +
> +/*
> + * Used to search a block of memory for the RSDP signature.
> + * Return Pointer to the RSDP if found, otherwise NULL.

"Returns pointer... "

> + * Based on acpi_tb_scan_memory_for_rsdp().
> + */
> +static u8 *scan_mem_for_rsdp(u8 *start, u32 length)
> +{
> + struct acpi_table_rsdp *rsdp;
> + u8 *end;
> + u8 *rover;

rover?

> +
> + end = start + length;
> +
> + /* Search from given start address for the requested length */
> + for (rover = start; rover < end; rover += ACPI_RSDP_SCAN_STEP) {
> + /*
> + * The RSDP signature and checksum must both be correct
> + * Note: Sometimes there exists more than one RSDP in memory;
> + * the valid RSDP has a valid checksum, all others have an
> + * invalid checksum.
> + */
> + rsdp = (struct acpi_table_rsdp *)rover;
> +
> + /* Nope, BAD Signature */
> + if (!ACPI_VALIDATE_RSDP_SIG(rsdp->signature))
> + continue;
> +
> + /* Check the standard checksum */
> + if (compute_checksum((u8 *) rsdp, ACPI_RSDP_CHECKSUM_LENGTH))
> + continue;
> +
> + /* Check extended checksum if table version >= 2 */
> + if ((rsdp->revision >= 2) &&
> + (compute_checksum((u8 *) rsdp, ACPI_RSDP_XCHECKSUM_LENGTH)))
> + continue;
> +
> + /* Sig and checksum valid, we have found a real RSDP */
> + return rover;
> + }
> + return NULL;
> +}
> +
> +/*
> + * Used to search RSDP physical address.
> + * Based on acpi_find_root_pointer(). Since only use physical address
> + * in this period, so there is no need to do the memory map jobs.

You mean: "All addresses used here are physical."?

"memory map jobs"?

Please be more careful when writing comments which are going to be read
by other people. "jobs" means a lot of things and you don't want "jobs"
in that context here.

> + */
> +static void bios_get_rsdp_addr(acpi_physical_address *rsdp_addr)

Same remark as before: the function is void and you're returning through
its parameter. Make it return acpi_physical_address instead.

> +{
> + struct acpi_table_rsdp *rsdp;
> + u8 *table_ptr;
> + u8 *mem_rover;

rover?

> + u32 address;
> +
> + /*
> + * Get the location of the Extended BIOS Data Area (EBDA)
> + * Since we use physical address directely, so

It is "directly" - what about that spellchecker?

> + * acpi_os_map_memory() and acpi_os_unmap_memory() are
> + * not needed here.

Why do you even need to say that here?

> + */
> + table_ptr = (u8 *)ACPI_EBDA_PTR_LOCATION;
> + *(u32 *)(void *)&address = *(u16 *)(void *)table_ptr;
> + address <<= 4;
> + table_ptr = (u8 *)address;

arch/x86/boot/compressed/acpitb.c: In function ‘bios_get_rsdp_addr’:
arch/x86/boot/compressed/acpitb.c:172:14: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
table_ptr = (u8 *)address;
^

Also, that is some crazy casting here and I think you could use
unsigned longs here for all the address arithmetic and cast to
acpi_physical_address only at the end.

> +
> + /*
> + * Search EBDA paragraphs (EBDA is required to be a minimum of
> + * 1K length)
> + */
> + if (address > 0x400) {
> + mem_rover = scan_mem_for_rsdp(table_ptr, ACPI_EBDA_WINDOW_SIZE);
> +

Superfluous new line.

> + if (mem_rover) {
> + address += (u32)ACPI_PTR_DIFF(mem_rover, table_ptr);
> + *rsdp_addr = (acpi_physical_address)address;
> + return;
> + }
> + }
> +
> + table_ptr = (u8 *)ACPI_HI_RSDP_WINDOW_BASE;
> + mem_rover = scan_mem_for_rsdp(table_ptr, ACPI_HI_RSDP_WINDOW_SIZE);
> +
> + /*
> + * Search upper memory: 16-byte boundaries in E0000h-FFFFFh
> + * Since we use physical address directely, so
> + * acpi_os_map_memory() and acpi_os_unmap_memory() are
> + * not needed here.
> + */

And this comment needs to be repeated here because... ?

> + if (mem_rover) {
> + address = (u32)(ACPI_HI_RSDP_WINDOW_BASE +
> + ACPI_PTR_DIFF(mem_rover, table_ptr));
> + *rsdp_addr = (acpi_physical_address)address;
> + }
> +}
> --

--
Regards/Gruss,
Boris.

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