arm64: warn on incorrect placement of the kernel by the bootloader

Commit cfa7ede ("arm64: set TEXT_OFFSET to 0x0 in preparation for
removing it entirely") results in boot failures when booting kernels that
are built without KASLR support on broken bootloaders that ignore the
TEXT_OFFSET value passed via the header, and use the default of 0x80000
instead.

To work around this, turn CONFIG_RELOCATABLE on by default, even if KASLR
itself (CONFIG_RANDOMIZE_BASE) is turned off, and require CONFIG_EXPERT
to be enabled to deviate from this. Then, emit a warning into the kernel
log if we are not booting via the EFI stub (which is permitted to deviate
from the placement restrictions) and the kernel base address is not placed
according to the rules as laid out in Documentation/arm64/booting.rst.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20200611124330.252163-1-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/Kconfig

@@ -1746,8 +1746,9 @@ config ARM64_DEBUG_PRIORITY_MASKING
 endif
 
 config RELOCATABLE
-	bool
+	bool "Build a relocatable kernel image" if EXPERT
 	select ARCH_HAS_RELR
+	default y
 	help
 	  This builds the kernel as a Position Independent Executable (PIE),
 	  which retains all relocation metadata required to relocate the

arch/arm64/kernel/setup.c

@@ -319,6 +319,10 @@ void __init setup_arch(char **cmdline_p)
 
 	xen_early_init();
 	efi_init();
+
+	if (!efi_enabled(EFI_BOOT) && ((u64)_text % MIN_KIMG_ALIGN) != 0)
+	     pr_warn(FW_BUG "Kernel image misaligned at boot, please fix your bootloader!");
+
 	arm64_memblock_init();
 
 	paging_init();