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: H. Peter Anvin <hpa@zytor.com>

2 files changed, 46 insertions, 20 deletions

diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
index 5f9a2e72a731..887874f4b49f 100644
--- a/arch/x86/boot/boot.h
+++ b/arch/x86/boot/boot.h
@@ -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;
diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 8353c81c41c0..6ef5a060fa11 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -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