[PATCH sparse] parse: shifting by full number of bits is undefined

[PATCH sparse] parse: shifting by full number of bits is undefined

[Jason A. Donenfeld]

The type checker wasn't identifying upper bounds for huge unsigned
64-bit numbers, because the right shift turned into a no-op:

zx2c4@thinkpad /tmp $ cat sparse.c
enum { sparse_does_not_like_this = 0x8000000000000003ULL };
zx2c4@thinkpad /tmp $ sparse sparse.c
sparse.c:1:36: warning: cast truncates bits from constant value (8000000000000003 becomes 3)

This works around the issue by detecting when we're going to shift by
the size of the variable and treat that as always zero.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 parse.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/parse.c b/parse.c
index 02a55a7..02d0615 100644
--- a/parse.c
+++ b/parse.c
@@ -788,7 +788,7 @@ static int type_is_ok(struct symbol *type, Num *upper, Num *lower)
 
 	if (!is_unsigned)
 		shift--;
-	if (upper->x == 0 && upper->y >> shift)
+	if (upper->x == 0 && (shift < (sizeof(upper->y) << 3)) && upper->y >> shift)
 		return 0;
 	if (lower->x == 0 || (!is_unsigned && (~lower->y >> shift) == 0))
 		return 1;
-- 
2.19.1

Re: [PATCH sparse] parse: shifting by full number of bits is undefined

[Luc Van Oostenryck]

On Fri, Oct 26, 2018 at 05:17:00AM +0200, Jason A. Donenfeld wrote:
> The type checker wasn't identifying upper bounds for huge unsigned
> 64-bit numbers, because the right shift turned into a no-op:
> 
> zx2c4@thinkpad /tmp $ cat sparse.c
> enum { sparse_does_not_like_this = 0x8000000000000003ULL };
> zx2c4@thinkpad /tmp $ sparse sparse.c
> sparse.c:1:36: warning: cast truncates bits from constant value (8000000000000003 becomes 3)

Hi Jason,

This is already fixed in the development tree:
   git://github.com/lucvoo/sparse.git
together with some other fixes & changes regarding enums.

I hope to be able to push this to the official tree real soon now.


Kind regards,
-- Luc