ACPI: tables: avoid relocations for table signature array

On architectures that implement KASLR using the ELF native RELA relocation
format (such as arm64), every absolute reference in the code incurs an
overhead of 24 bytes in the .rela section. So storing a 41 element array
of 4 character signature strings using an array of pointer-to-char incurs
an 8x overhead (32 bytes per entry => ~1500 bytes), and given the fixed
length of the entries, and the fact that the array is only used locally,
it is much better to use an array of arrays here, which gets rid of the
overhead entirely.

While at it, make it __initconst, as it is never referenced except from
__init code.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/acpi/tables.c b/drivers/acpi/tables.c
index 0e905c3d16451c2685ff1978dd820c243c10ff68..ada36bc40c2b4ba5b11f16d50326abfda8bb4e61 100644 (file)
--- a/drivers/acpi/tables.c
+++ b/drivers/acpi/tables.c
@@ -490,7 +490,7 @@ static u8 __init acpi_table_checksum(u8 *buffer, u32 length)
 }
 
 /* All but ACPI_SIG_RSDP and ACPI_SIG_FACS: */
-static const char * const table_sigs[] = {
+static const char table_sigs[][ACPI_NAMESEG_SIZE] __initconst = {
        ACPI_SIG_BERT, ACPI_SIG_BGRT, ACPI_SIG_CPEP, ACPI_SIG_ECDT,
        ACPI_SIG_EINJ, ACPI_SIG_ERST, ACPI_SIG_HEST, ACPI_SIG_MADT,
        ACPI_SIG_MSCT, ACPI_SIG_SBST, ACPI_SIG_SLIT, ACPI_SIG_SRAT,
@@ -501,7 +501,7 @@ static const char * const table_sigs[] = {
        ACPI_SIG_WDDT, ACPI_SIG_WDRT, ACPI_SIG_DSDT, ACPI_SIG_FADT,
        ACPI_SIG_PSDT, ACPI_SIG_RSDT, ACPI_SIG_XSDT, ACPI_SIG_SSDT,
        ACPI_SIG_IORT, ACPI_SIG_NFIT, ACPI_SIG_HMAT, ACPI_SIG_PPTT,
-       ACPI_SIG_NHLT, NULL };
+       ACPI_SIG_NHLT };
 
 #define ACPI_HEADER_SIZE sizeof(struct acpi_table_header)
 
@@ -548,11 +548,11 @@ void __init acpi_table_upgrade(void)
 
                table = file.data;
 
-               for (sig = 0; table_sigs[sig]; sig++)
+               for (sig = 0; sig < ARRAY_SIZE(table_sigs); sig++)
                        if (!memcmp(table->signature, table_sigs[sig], 4))
                                break;
 
-               if (!table_sigs[sig]) {
+               if (sig >= ARRAY_SIZE(table_sigs)) {
                        pr_err("ACPI OVERRIDE: Unknown signature [%s%s]\n",
                                cpio_path, file.name);
                        continue;