[x86 setup] EDD: Fix the computation of the MBR sector buffer

Some BIOSes require that sector buffers not cross 64K
boundaries.  As a result, we compute a dynamic address on the
setup heap.  Unfortunately, this address computation was just
totally wrong.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/arch/i386/boot/edd.c b/arch/i386/boot/edd.c
index 25a282494f4c..77d92daf7923 100644
--- a/arch/i386/boot/edd.c
+++ b/arch/i386/boot/edd.c
@@ -72,17 +72,18 @@ static u32 read_mbr_sig(u8 devno, struct edd_info *ei)
         u32 mbrsig;
         u32 buf_base, mbr_base;
         extern char _end[];
-        static char mbr_buf[1024];
 
         sector_size = ei->params.bytes_per_sector;
         if (!sector_size)
                 sector_size = 512; /* Best available guess */
 
+        /* Produce a naturally aligned buffer on the heap */
         buf_base = (ds() << 4) + (u32)&_end;
         mbr_base = (buf_base+sector_size-1) & ~(sector_size-1);
-        mbrbuf_ptr = mbr_buf + (mbr_base-buf_base);
+        mbrbuf_ptr = _end + (mbr_base-buf_base);
         mbrbuf_end = mbrbuf_ptr + sector_size;
 
+        /* Make sure we actually have space on the heap... */
         if (!(boot_params.hdr.loadflags & CAN_USE_HEAP))
                 return 0;
         if (mbrbuf_end > (char *)(size_t)boot_params.hdr.heap_end_ptr)