disk: handle LBA I/O with zero sector count

Unlike basic_access, extended_access does not check for a zero
sector count.  However, this is a problem because for example
it would be interpreted as 256 when processing an ATA request.

Based on patch from: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/disk.c b/src/disk.c
index f2c6621..29714d6 100644
--- a/src/disk.c
+++ b/src/disk.c
@@ -135,6 +135,11 @@ extended_access(struct bregs *regs, struct drive_s *drive_g, u16 command)
 
     dop.buf_fl = SEGOFF_TO_FLATPTR(GET_INT13EXT(regs, data));
     dop.count = GET_INT13EXT(regs, count);
+    if (! dop.count) {
+        // Nothing to do.
+        disk_ret(regs, DISK_RET_SUCCESS);
+        return;
+    }
 
     int status = send_disk_op(&dop);