[prefix] Avoid using base memory for temporary decompression area

In the unlikely (but observable) event that INT 15,88 returns less
memory above 1MB than is required for the temporary decompression
area, ignore it and use the 1MB point anyway.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/arch/i386/prefix/libprefix.S b/src/arch/i386/prefix/libprefix.S
index 76f45586247ccd1f7661137ae1ce0daa69a29f3e..77151a67e0bb75a63d468f29d0f93244cf6ed3bb 100644 (file)
--- a/src/arch/i386/prefix/libprefix.S
+++ b/src/arch/i386/prefix/libprefix.S
@@ -753,21 +753,25 @@ payload_death_message:
        lcall   *init_libkir_vector
 #else
        /* Find a suitable decompression temporary area, if none specified */
+       pushl   %eax
        testl   %ebp, %ebp
        jnz     1f
        /* Use INT 15,88 to find the highest available address via INT
         * 15,88.  This limits us to around 64MB, which should avoid
         * all of the POST-time memory map failure modes.
         */
-       pushl   %eax
        movb    $0x88, %ah
        int     $0x15
        movw    %ax, %bp
        addl    $0x400, %ebp
        subl    $_textdata_memsz_kb, %ebp
        shll    $10, %ebp
-       popl    %eax
-1:
+       /* Sanity check: if we have ended up below 1MB, use 1MB */
+       cmpl    $0x100000, %ebp
+       jae     1f
+       movl    $0x100000, %ebp
+1:     popl    %eax
+
        /* Install .text and .data to temporary area in high memory,
         * prior to reading the E820 memory map and relocating
         * properly.