[PATCH 12/28] efi/x86: Avoid using code32_start

Ard Biesheuvel
2020-03-08T08:09:52Z

From: Arvind Sankar <nivedita@alum.mit.edu>

code32_start is meant for 16-bit real-mode bootloaders to inform the
kernel where the 32-bit protected mode code starts. Nothing in the
protected mode kernel except the EFI stub uses it.

efi_main currently returns boot_params, with code32_start set inside it
to tell efi_stub_entry where startup_32 is located. Since it was invoked
by efi_stub_entry in the first place, boot_params is already known.
Return the address of startup_32 instead.

This will allow a 64-bit kernel to live above 4Gb, for example, and it's
cleaner.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200301230436.2246909-5-nivedita@alum.mit.edu
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/compressed/head_32.S | 3 +--
arch/x86/boot/compressed/head_64.S | 4 ++--
arch/x86/kernel/asm-offsets.c | 1 -
drivers/firmware/efi/libstub/x86-stub.c | 10 +++++-----
4 files changed, 8 insertions(+), 10 deletions(-)

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 356060c5332c..9ffc9454fd22 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -157,9 +157,8 @@ SYM_FUNC_END(startup_32)
SYM_FUNC_START(efi32_stub_entry)
SYM_FUNC_START_ALIAS(efi_stub_entry)
add $0x4, %esp
+ movl 8(%esp), %esi /* save boot_params pointer */
call efi_main
- movl %eax, %esi
- movl BP_code32_start(%esi), %eax
leal startup_32(%eax), %eax
jmp *%eax
SYM_FUNC_END(efi32_stub_entry)
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index b74a012a6fea..08351d16ccc0 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -472,9 +472,9 @@ SYM_CODE_END(startup_64)
SYM_FUNC_START(efi64_stub_entry)
SYM_FUNC_START_ALIAS(efi_stub_entry)
and $~0xf, %rsp /* realign the stack */
+ movq %rdx, %rbx /* save boot_params pointer */
call efi_main
- movq %rax,%rsi
- movl BP_code32_start(%esi), %eax
+ movq %rbx,%rsi
leaq startup_64(%rax), %rax
jmp *%rax
SYM_FUNC_END(efi64_stub_entry)
diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index 5c7ee3df4d0b..3ca07ad552ae 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -88,7 +88,6 @@ static void __used common(void)
OFFSET(BP_kernel_alignment, boot_params, hdr.kernel_alignment);
OFFSET(BP_init_size, boot_params, hdr.init_size);
OFFSET(BP_pref_address, boot_params, hdr.pref_address);
- OFFSET(BP_code32_start, boot_params, hdr.code32_start);

BLANK();
DEFINE(PTREGS_SIZE, sizeof(struct pt_regs));
diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index 9db98839d7b4..7f3e97c2aad3 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -703,10 +703,11 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle)
}

/*
- * On success we return a pointer to a boot_params structure, and NULL
- * on failure.
+ * On success, we return the address of startup_32, which has potentially been
+ * relocated by efi_relocate_kernel.
+ * On failure, we exit to the firmware via efi_exit instead of returning.
*/
-struct boot_params *efi_main(efi_handle_t handle,
+unsigned long efi_main(efi_handle_t handle,
efi_system_table_t *sys_table_arg,
struct boot_params *boot_params)
{
@@ -736,7 +737,6 @@ struct boot_params *efi_main(efi_handle_t handle,
goto fail;
}
}
- hdr->code32_start = (u32)bzimage_addr;

/*
* efi_pe_entry() may have been called before efi_main(), in which
@@ -799,7 +799,7 @@ struct boot_params *efi_main(efi_handle_t handle,
goto fail;
}

- return boot_params;
+ return bzimage_addr;
fail:
efi_printk("efi_main() failed!\n");

--
2.17.1