Commit:     16252da654800461e0e1c32697cb59f4cda15aa9
Parent:     09f345da758fca1222b0971b65b2fddbdf78bb83
Author:     Jens Rottmann <[EMAIL PROTECTED]>
AuthorDate: Tue Nov 27 12:35:13 2007 +0100
Committer:  H. Peter Anvin <[EMAIL PROTECTED]>
CommitDate: Wed Nov 28 18:17:17 2007 -0800

    x86 setup: don't recalculate ss:esp unless really necessary
    In order to work around old LILO versions providing an invalid ss
    register, the current setup code always sets up a new stack,
    immediately following .bss and the heap. But this breaks LOADLIN.
    This rewrite of the workaround checks for an invalid stack (ss!=ds)
    first, and leaves ss:sp alone otherwise (apart from aligning esp).
    [hpa note: LOADLIN has a number of arbitrary hard-coded limits that
    are being pushed up against.  Without some major revision of LOADLIN
    itself it will not be sustainable keeping it alive.  This gives it
    another brief lease on life, however.  This patch also helps the
    cmdline truncation problem with old versions of SYSLINUX.]
    Signed-off-by: Jens Rottmann <JRottmann at LiPPERT-AT. de>
    Signed-off-by: H. Peter Anvin <[EMAIL PROTECTED]>
 arch/x86/boot/header.S |   41 ++++++++++++++++-------------------------
 1 files changed, 16 insertions(+), 25 deletions(-)

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 6ef5a06..4cc5b04 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -236,39 +236,30 @@ start_of_setup:
        movw    %ax, %es
-# 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.
+# Apparently some ancient versions of LILO invoked the kernel with %ss != %ds,
+# which happened to work by accident for the old code.  Recalculate the stack
+# pointer if %ss is invalid.  Otherwise leave it alone, LOADLIN sets up the
+# stack behind its own code, so we can't blindly put it directly past the heap.
-       # 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...
+       je      2f              # -> assume %sp is reasonably set
+       # Invalid %ss, make up a new stack
+       movw    $_end, %dx
+       testb   $CAN_USE_HEAP, loadflags
+       jz      1f
+       movw    heap_end_ptr, %dx
+1:     addw    $STACK_SIZE, %dx
+       jnc     2f
+       xorw    %dx, %dx        # Prevent wraparound
-       # Make sure the stack is at least minimum size.  Take a value
-       # of zero to mean "full segment."
+2:     # Now %dx should point to the end of our stack space
        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
+3:     movw    %ax, %ss
        movzwl  %dx, %esp       # Clear upper half of %esp
        sti                     # Now we should have a working stack
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