efi/zboot: Add BSS padding before compression

We don't really care about the size of the decompressed image - what
matters is how much space needs to be allocated for the image to
execute, and this includes space for BSS that is not part of the
loadable image and so it is not accounted for in the decompressed size.

So let's add some zero padding to the end of the image: this compresses
well, and it ensures that BSS is accounted for, and as a bonus, it will
be zeroed before launching the image.

Since all architectures that implement support for EFI zboot carry this
value in the header in the same location, we can just grab it from the
binary that is being compressed.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: ardbiesheuvel

drivers/firmware/efi/libstub/Makefile.zboot

@@ -3,23 +3,40 @@
 # to be include'd by arch/$(ARCH)/boot/Makefile after setting
 # EFI_ZBOOT_PAYLOAD, EFI_ZBOOT_BFD_TARGET and EFI_ZBOOT_MACH_TYPE
 
+quiet_cmd_copy_and_pad = PAD     $@
+      cmd_copy_and_pad = cp $< $@ && \
+			 truncate -s $(shell hexdump -s16 -n4 -e '"%u"' $<) $@
+
+# Pad the file to the size of the uncompressed image in memory, including BSS
+$(obj)/vmlinux.bin: $(obj)/$(EFI_ZBOOT_PAYLOAD) FORCE
+	$(call if_changed,copy_and_pad)
+
 comp-type-$(CONFIG_KERNEL_GZIP)		:= gzip
 comp-type-$(CONFIG_KERNEL_LZ4)		:= lz4
 comp-type-$(CONFIG_KERNEL_LZMA)		:= lzma
 comp-type-$(CONFIG_KERNEL_LZO)		:= lzo
 comp-type-$(CONFIG_KERNEL_XZ)		:= xzkern
 comp-type-$(CONFIG_KERNEL_ZSTD)		:= zstd22
 
-# Copy the SizeOfHeaders, SizeOfCode and SizeOfImage fields from the payload to
-# the end of the compressed image. Note that this presupposes a PE header
-# offset of 64 bytes, which is what arm64, RISC-V and LoongArch use.
-quiet_cmd_compwithsize = $(quiet_cmd_$(comp-type-y))
-      cmd_compwithsize = $(cmd_$(comp-type-y)) && ( \
+# in GZIP, the appended le32 carrying the uncompressed size is part of the
+# format, but in other cases, we just append it at the end for convenience,
+# causing the original tools to complain when checking image integrity.
+# So disregard it when calculating the payload size in the zimage header.
+zboot-method-y                         := $(comp-type-y)_with_size
+zboot-size-len-y                       := 12
+
+zboot-method-$(CONFIG_KERNEL_GZIP)     := gzip
+zboot-size-len-$(CONFIG_KERNEL_GZIP)   := 8
+
+# Copy the SizeOfHeaders and SizeOfCode fields from the payload to the end of
+# the compressed image. Note that this presupposes a PE header offset of 64
+# bytes, which is what arm64, RISC-V and LoongArch use.
+quiet_cmd_compwithsize = $(quiet_cmd_$(zboot-method-y))
+      cmd_compwithsize = $(cmd_$(zboot-method-y)) && ( \
 			   dd status=none if=$< bs=4 count=1 skip=37 ; \
-			   dd status=none if=$< bs=4 count=1 skip=23 ; \
-			   dd status=none if=$< bs=4 count=1 skip=36 ) >> $@
+			   dd status=none if=$< bs=4 count=1 skip=23 ) >> $@
 
-$(obj)/vmlinuz: $(obj)/$(EFI_ZBOOT_PAYLOAD) FORCE
+$(obj)/vmlinuz: $(obj)/vmlinux.bin FORCE
 	$(call if_changed,compwithsize)
 
 OBJCOPYFLAGS_vmlinuz.o := -I binary -O $(EFI_ZBOOT_BFD_TARGET) \
@@ -29,6 +46,7 @@ $(obj)/vmlinuz.o: $(obj)/vmlinuz FORCE
 
 AFLAGS_zboot-header.o += -DMACHINE_TYPE=IMAGE_FILE_MACHINE_$(EFI_ZBOOT_MACH_TYPE) \
 			 -DZBOOT_EFI_PATH="\"$(realpath $(obj)/vmlinuz.efi.elf)\"" \
+			 -DZBOOT_SIZE_LEN=$(zboot-size-len-y) \
 			 -DCOMP_TYPE="\"$(comp-type-y)\""
 
 $(obj)/zboot-header.o: $(srctree)/drivers/firmware/efi/libstub/zboot-header.S FORCE
@@ -44,4 +62,4 @@ OBJCOPYFLAGS_vmlinuz.efi := -O binary
 $(obj)/vmlinuz.efi: $(obj)/vmlinuz.efi.elf FORCE
 	$(call if_changed,objcopy)
 
-targets += zboot-header.o vmlinuz vmlinuz.o vmlinuz.efi.elf vmlinuz.efi
+targets += zboot-header.o vmlinux.bin vmlinuz vmlinuz.o vmlinuz.efi.elf vmlinuz.efi

drivers/firmware/efi/libstub/zboot-header.S

@@ -17,7 +17,7 @@ __efistub_efi_zboot_header:
 	.long		MZ_MAGIC
 	.ascii		"zimg"					// image type
 	.long		__efistub__gzdata_start - .Ldoshdr	// payload offset
-	.long		__efistub__gzdata_size - 12		// payload size
+	.long		__efistub__gzdata_size - ZBOOT_SIZE_LEN	// payload size
 	.long		0, 0					// reserved
 	.asciz		COMP_TYPE				// compression type
 	.org		.Ldoshdr + 0x38

drivers/firmware/efi/libstub/zboot.c

@@ -91,12 +91,12 @@ efi_zboot_entry(efi_handle_t handle, efi_system_table_t *systab)
 	efi_info("Decompressing Linux Kernel...\n");
 
 	// SizeOfImage from the compressee's PE/COFF header
-	alloc_size = round_up(get_unaligned_le32(_gzdata_end - 4),
+	alloc_size = round_up(get_unaligned_le32(_gzdata_end - 12),
 			      EFI_ALLOC_ALIGN);
 
 	// SizeOfHeaders and SizeOfCode from the compressee's PE/COFF header
-	code_size = get_unaligned_le32(_gzdata_end - 8) +
-		    get_unaligned_le32(_gzdata_end - 12);
+	code_size = get_unaligned_le32(_gzdata_end - 4) +
+		    get_unaligned_le32(_gzdata_end - 8);
 
 	 // If the architecture has a preferred address for the image,
 	 // try that first.