Subject: Re: [PATCH 1/2] avoid shifting signed integers 31 bits

From: Junio C Hamano <[email-masked]>
Date: Mon, 04 Jan 2016 09:52:10 -0800

Jeff King <[email-masked]> writes:

> On Thu, Dec 31, 2015 at 12:10:33PM +0700, Duy Nguyen wrote:
>
>> On Tue, Dec 29, 2015 at 1:35 PM, Jeff King <[email-masked]> wrote:
>> > We sometimes use 32-bit unsigned integers as bit-fields.
>> > It's fine to access the MSB, because it's unsigned. However,
>> > doing so as "1 << 31" is wrong, because the constant "1" is
>> > a signed int, and we shift into the sign bit, causing
>> > undefined behavior.
>> >
>> > We can fix this by using "1U" as the constant.
>> 
>> We have this in cache.h, should it be fixed as well?
>> 
>> /* CE_EXTENDED2 is for future extension */
>> #define CE_EXTENDED2         (1 << 31)
>
> Sort of. We don't actually use it, and since it's a macro, that means it
> never even hits the compiler proper itself. So it's not a bug, but it's
> a bug waiting to happen. :)
>
> -Peff

Let's squash an obvious change for that in to 1/2, then, before I
merge the series to 'next'.

Thanks.

-- >8 --
From: Jeff King <[email-masked]>
Date: Tue, 29 Dec 2015 01:35:46 -0500
Subject: [PATCH] avoid shifting signed integers 31 bits

We sometimes use 32-bit unsigned integers as bit-fields.
It's fine to access the MSB, because it's unsigned. However,
doing so as "1 << 31" is wrong, because the constant "1" is
a signed int, and we shift into the sign bit, causing
undefined behavior.

We can fix this by using "1U" as the constant.

Signed-off-by: Jeff King <[email-masked]>
Signed-off-by: Junio C Hamano <[email-masked]>
---
 builtin/receive-pack.c | 2 +-
 cache.h                | 2 +-
 diff.h                 | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index e6b93d0..e35ed40 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1597,7 +1597,7 @@ static void prepare_shallow_update(struct command *commands,
 				continue;
 			si->need_reachability_test[i]++;
 			for (k = 0; k < 32; k++)
-				if (si->used_shallow[i][j] & (1 << k))
+				if (si->used_shallow[i][j] & (1U << k))
 					si->shallow_ref[j * 32 + k]++;
 		}
 
diff --git a/cache.h b/cache.h
index 6f53962..9088843 100644
--- a/cache.h
+++ b/cache.h
@@ -214,7 +214,7 @@ struct cache_entry {
 #define CE_INTENT_TO_ADD     (1 << 29)
 #define CE_SKIP_WORKTREE     (1 << 30)
 /* CE_EXTENDED2 is for future extension */
-#define CE_EXTENDED2         (1 << 31)
+#define CE_EXTENDED2         (1U << 31)
 
 #define CE_EXTENDED_FLAGS (CE_INTENT_TO_ADD | CE_SKIP_WORKTREE)
 
diff --git a/diff.h b/diff.h
index f7208ad..893f446 100644
--- a/diff.h
+++ b/diff.h
@@ -91,7 +91,7 @@ typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data)
 #define DIFF_OPT_DIRSTAT_BY_LINE     (1 << 28)
 #define DIFF_OPT_FUNCCONTEXT         (1 << 29)
 #define DIFF_OPT_PICKAXE_IGNORE_CASE (1 << 30)
-#define DIFF_OPT_DEFAULT_FOLLOW_RENAMES (1 << 31)
+#define DIFF_OPT_DEFAULT_FOLLOW_RENAMES (1U << 31)
 
 #define DIFF_OPT_TST(opts, flag)    ((opts)->flags & DIFF_OPT_##flag)
 #define DIFF_OPT_TOUCHED(opts, flag)    ((opts)->touched_flags & DIFF_OPT_##flag)
-- 
2.7.0-rc3-132-g73ad441