Subject: Re: [PATCH 1/1] attr: add builtin objectmode values support

From: Junio C Hamano <[email-masked]>
Date: Tue, 12 Dec 2023 15:17:40 -0800

Joanna Wang <[email-masked]> writes:

>> Cumulatively, aside from the removal of the t/#t* file, here is what
>> I ended up with so far.
>
> I want to double check if I should followup here.
> I assumed that you had already applied these final fixes on my behalf,
> similar to my patch for enabling attr for `git-add`. But if I was wrong,
> I'm happy to send another update with all the fixes.

I've squashed the fix in, so no need to resend only to patch up what
I pointed out earlier.

The end result fails t0003 under GIT_TEST_PASSING_SANITIZE_LEAK
though.  As the synthetic attribute values are allocated without
being in the hashmap based on the value read from .gitattributes
files, somebody needs to hold pointers to them *and* we need to
avoid allocating unbounded number of them.

The attached is one possible way to plug the leak; I am not sure if
it is the best one, though.  One thing I like about the solution is
that the approach makes sure that the mode attributes we would ever
return are very tightly controlled and does not allow a buggy code
to come up with "mode" to be passed to this new helper function to
pass random and unsupported mode bits without triggering the BUG().

 attr.c | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git c/attr.c w/attr.c
index b03c20f768..679e42258c 100644
--- c/attr.c
+++ w/attr.c
@@ -1250,10 +1250,34 @@ static struct object_id *default_attr_source(void)
 	return &attr_source;
 }
 
+static const char *interned_mode_string(unsigned int mode)
+{
+	static struct {
+		unsigned int val;
+		char str[7];
+	} mode_string[] = {
+		{ .val = 0040000 },
+		{ .val = 0100644 },
+		{ .val = 0100755 },
+		{ .val = 0120000 },
+		{ .val = 0160000 },
+	};
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(mode_string); i++) {
+		if (mode_string[i].val != mode)
+			continue;
+		if (!*mode_string[i].str)
+			snprintf(mode_string[i].str, sizeof(mode_string[i].str),
+				 "%06o", mode);
+		return mode_string[i].str;
+	}
+	BUG("Unsupported mode 0%o", mode);
+}
+
 static const char *builtin_object_mode_attr(struct index_state *istate, const char *path)
 {
 	unsigned int mode;
-	struct strbuf sb = STRBUF_INIT;
 
 	if (direction == GIT_ATTR_CHECKIN) {
 		struct object_id oid;
@@ -1287,8 +1311,8 @@ static const char *builtin_object_mode_attr(struct index_state *istate, const ch
 		else
 			return ATTR__UNSET;
 	}
-	strbuf_addf(&sb, "%06o", mode);
-	return strbuf_detach(&sb, NULL);
+
+	return interned_mode_string(mode);
 }