x86, build: Dynamically find entry points in compressed startup code

We have historically hard-coded entry points in head.S just so it's easy
to build the executable/bzImage headers with references to them.

Unfortunately, this leads to boot loaders abusing these "known" addresses
even when they are *explicitly* told that they "should look at the ELF
header to find this address, as it may change in the future". And even
when the address in question *has* actually been changed in the past,
without fanfare or thought to compatibility.

Thus we have bootloaders doing stunningly broken things like jumping
to offset 0x200 in the kernel startup code in 64-bit mode, *hoping*
that startup_64 is still there (it has moved at least once
before). And hoping that it's actually a 64-bit kernel despite the
fact that we don't give them any indication of that fact.

This patch should hopefully remove the temptation to abuse internal
addresses in future, where sternly worded comments have not sufficed.
Instead of having hard-coded addresses and saying "please don't abuse
these", we actually pull the addresses out of the ELF payload into
zoffset.h, and make build.c shove them back into the right places in
the bzImage header.

Rather than including zoffset.h into build.c and thus having to rebuild
the tool for every kernel build, we parse it instead. The parsing code
is small and simple.

This patch doesn't actually move any of the interesting entry points, so
any offending bootloader will still continue to "work" after this patch
is applied. For some version of "work" which includes jumping into the
compressed payload and crashing, if the bzImage it's given is a 32-bit
kernel. No change there then.

[ hpa: some of the issues in the description are addressed or
  retconned by the 2.12 boot protocol.  This patch has been edited to
  only remove fixed addresses that were *not* thus retconned. ]

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Link: http://lkml.kernel.org/r/1358513837.2397.247.camel@shinybook.infradead.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Matt Fleming <matt.fleming@intel.com>

2 files changed, 7 insertions, 7 deletions

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index ccb2f4a5f7bd..1e3184f6072f 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -35,11 +35,11 @@ ENTRY(startup_32)
 #ifdef CONFIG_EFI_STUB
         jmp     preferred_addr
 
-        .balign 0x10
         /*
          * We don't need the return address, so set up the stack so
-         * efi_main() can find its arugments.
+         * efi_main() can find its arguments.
          */
+ENTRY(efi_pe_entry)
         add     $0x4, %esp
 
         call    make_boot_params
@@ -52,7 +52,7 @@ ENTRY(startup_32)
         pushl   %ecx
         sub     $0x4, %esp
 
-        .org 0x30,0x90
+ENTRY(efi_stub_entry)
         add     $0x4, %esp
         call    efi_main
         cmpl    $0, %eax
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 2c4b171eec33..f5d1aaa0dec8 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -201,12 +201,12 @@ ENTRY(startup_64)
          */
 #ifdef CONFIG_EFI_STUB
         /*
-         * The entry point for the PE/COFF executable is 0x210, so only
-         * legacy boot loaders will execute this jmp.
+         * The entry point for the PE/COFF executable is efi_pe_entry, so
+         * only legacy boot loaders will execute this jmp.
          */
         jmp     preferred_addr
 
-        .org 0x210
+ENTRY(efi_pe_entry)
         mov     %rcx, %rdi
         mov     %rdx, %rsi
         pushq   %rdi
@@ -218,7 +218,7 @@ ENTRY(startup_64)
         popq    %rsi
         popq    %rdi
 
-        .org 0x230,0x90
+ENTRY(efi_stub_entry)
         call    efi_main
         movq    %rax,%rsi
         cmpq    $0,%rax