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

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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: H. Peter Anvin <hpa@zytor.com>
This commit is contained in:
H. Peter Anvin 2007-10-25 16:11:33 -07:00
parent c9927c2bf4
commit 6b6815c6d5
2 changed files with 46 additions and 20 deletions
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4
arch/x86/boot/boot.h
View File

@ -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;

62
arch/x86/boot/header.S
View File

@ -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