Commit 6b6815c6 authored by H. Peter Anvin's avatar H. Peter Anvin
Browse files

x86 setup: handle boot loaders which set up the stack incorrectly 



Apparently some specific versions of LILO enter the kernel with a
stack pointer that doesn't match the rest of the segments.  Make our
best attempt at untangling the resulting mess.

Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
parent c9927c2b

arch/x86/boot/boot.h

+2 −2
Original line number Diff line number Diff line
@@ -17,6 +17,8 @@ 
#ifndef BOOT_BOOT_H

#define BOOT_BOOT_H



#define STACK_SIZE	512	/* Minimum number of bytes for stack */



#ifndef __ASSEMBLY__



#include <stdarg.h>
@@ -198,8 +200,6 @@ static inline int isdigit(int ch) 
}



/* Heap -- available for dynamic lists. */

#define STACK_SIZE	512	/* Minimum number of bytes for stack */



extern char _end[];

extern char *HEAP;

extern char *heap_end;

arch/x86/boot/header.S

+44 −18
Original line number Diff line number Diff line
@@ -173,7 +173,8 @@ ramdisk_size: .long 0 # its size in bytes 
bootsect_kludge:

		.long	0		# obsolete



heap_end_ptr:	.word	_end+1024	# (Header version 0x0201 or later)

heap_end_ptr:	.word	_end+STACK_SIZE-512

					# (Header version 0x0201 or later)

					# space from here (exclusive) down to

					# end of setup code can be used by setup

					# for local heap purposes.
@@ -230,28 +231,53 @@ start_of_setup: 
	int	$0x13

#endif



# We will have entered with %cs = %ds+0x20, normalize %cs so

# it is on par with the other segments.

	pushw	%ds

	pushw	$setup2

	lretw



setup2:

# Force %es = %ds

	movw	%ds, %ax

	movw	%ax, %es

	cld



# Stack paranoia: align the stack and make sure it is good

# for both 16- and 32-bit references.  In particular, if we

# were meant to have been using the full 16-bit segment, the

# caller might have set %sp to zero, which breaks %esp-based

# references.

	andw	$~3, %sp	# dword align (might as well...)

	jnz	1f

	movw	$0xfffc, %sp	# Make sure we're not zero

1:	movzwl	%sp, %esp	# Clear upper half of %esp

	sti

# Apparently some ancient versions of LILO invoked the kernel

# with %ss != %ds, which happened to work by accident for the

# old code.  If the CAN_USE_HEAP flag is set in loadflags, or

# %ss != %ds, then adjust the stack pointer.



	# Smallest possible stack we can tolerate

	movw	$(_end+STACK_SIZE), %cx



	movw	heap_end_ptr, %dx

	addw	$512, %dx

	jnc	1f

	xorw	%dx, %dx	# Wraparound - whole segment available

1:	testb	$CAN_USE_HEAP, loadflags

	jnz	2f



	# No CAN_USE_HEAP

	movw	%ss, %dx

	cmpw	%ax, %dx	# %ds == %ss?

	movw	%sp, %dx

	# If so, assume %sp is reasonably set, otherwise use

	# the smallest possible stack.

	jne	4f		# -> Smallest possible stack...



	# Make sure the stack is at least minimum size.  Take a value

	# of zero to mean "full segment."

2:

	andw	$~3, %dx	# dword align (might as well...)

	jnz	3f

	movw	$0xfffc, %dx	# Make sure we're not zero

3:	cmpw	%cx, %dx

	jnb	5f

4:	movw	%cx, %dx	# Minimum value we can possibly use

5:	movw	%ax, %ss

	movzwl	%dx, %esp	# Clear upper half of %esp

	sti			# Now we should have a working stack



# We will have entered with %cs = %ds+0x20, normalize %cs so

# it is on par with the other segments.

	pushw	%ds

	pushw	$6f

	lretw

6:



# Check signature at end of setup

	cmpl	$0x5a5aaa55, setup_sig