Verify fallback VESA mode before attempting to set it

The mode must be listed in the list of supported modes and it
must have the correct attributes (e.g., supported, having linear
frame buffer).

kernel/arch/ia32/src/boot/vesa_real.inc

@@ -363,7 +363,30 @@ vesa_init_real2:
 	no_mode:
 
 		/* No prefered mode found */
+		push %di
+		mov $e_vesa_init - vesa_init, %di
+		mov 2 + VESA_MODE_LIST_PTR_OFFSET(%di), %si
+		mov %si, %gs
+		mov VESA_MODE_LIST_PTR_OFFSET(%di), %si
+		pop %di
+
+		/* Check if list of supported modes contains fallback mode */
+	find_fallback_mode:
+
+		mov %gs:(%si), %cx
+
+		cmp $0x111, %cx
+		je fallback_mode_listed
+
+		cmp $VESA_END_OF_MODES, %cx
+		je text_mode
 
+		inc %si
+		inc %si
+		jmp find_fallback_mode /* 16-bit relative jump */
+
+	fallback_mode_listed:
+		/* Make sure fallback mode is really supported */
 		mov $0x111, %cx
 		push %di
 		push %cx
@@ -374,7 +397,13 @@ vesa_init_real2:
 		pop %di
 		cmp $VESA_OK, %ax
 		jnz text_mode
-		jz set_mode  /* force relative jump */
+
+		/* Verify mode attributes */
+		and $0x99, %ax
+		cmp $0x99, %ax
+		jne text_mode
+
+		jmp set_mode  /* 16-bit relative jump */
 
 	text_mode: