[tip: efi/core] efi/x86: Only copy the compressed kernel image in efi_relocate_kernel()

[tip-bot2 for Arvind Sankar]

The following commit has been merged into the efi/core branch of tip:

Commit-ID:     688eb28211abdf82a3f51e8997f1c8137947227d
Gitweb:        https://git.kernel.org/tip/688eb28211abdf82a3f51e8997f1c8137947227d
Author:        Arvind Sankar <nivedita@xxxxxxxxxxxx>
AuthorDate:    Sun, 11 Oct 2020 10:20:12 -04:00
Committer:     Ard Biesheuvel <ardb@xxxxxxxxxx>
CommitterDate: Mon, 26 Oct 2020 08:06:36 +01:00

efi/x86: Only copy the compressed kernel image in efi_relocate_kernel()

The image_size argument to efi_relocate_kernel() is currently specified
as init_size, but this is unnecessarily large. The compressed kernel is
much smaller, in fact, its image only extends up to the start of _bss,
since at this point, the .bss section is still uninitialized.

Depending on compression level, this can reduce the amount of data
copied by 4-5x.

Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201011142012.96493-1-nivedita@xxxxxxxxxxxx
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 drivers/firmware/efi/libstub/x86-stub.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 3672539..f14c4ff 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -715,8 +715,11 @@ unsigned long efi_main(efi_handle_t handle,
 	    (IS_ENABLED(CONFIG_X86_32) && buffer_end > KERNEL_IMAGE_SIZE)    ||
 	    (IS_ENABLED(CONFIG_X86_64) && buffer_end > MAXMEM_X86_64_4LEVEL) ||
 	    (image_offset == 0)) {
+		extern char _bss[];
+
 		status = efi_relocate_kernel(&bzimage_addr,
-					     hdr->init_size, hdr->init_size,
+					     (unsigned long)_bss - bzimage_addr,
+					     hdr->init_size,
 					     hdr->pref_address,
 					     hdr->kernel_alignment,
 					     LOAD_PHYSICAL_ADDR);