Fix big left-shifts of unsigned char

torvalds · gitster · commit 48fb7deb5bbd · 2009-06-18T09:22:46.000-07:00
Shifting 'unsigned char' or 'unsigned short' left can result in sign
extension errors, since the C integer promotion rules means that the
unsigned char/short will get implicitly promoted to a signed 'int' due to
the shift (or due to other operations).

This normally doesn't matter, but if you shift things up sufficiently, it
will now set the sign bit in 'int', and a subsequent cast to a bigger type
(eg 'long' or 'unsigned long') will now sign-extend the value despite the
original expression being unsigned.

One example of this would be something like

	unsigned long size;
	unsigned char c;

	size += c &lt;&lt; 24;

where despite all the variables being unsigned, 'c &lt;&lt; 24' ends up being a
signed entity, and will get sign-extended when then doing the addition in
an 'unsigned long' type.

Since git uses 'unsigned char' pointers extensively, we actually have this
bug in a couple of places.

I may have missed some, but this is the result of looking at

	git grep '[^0-9 	][ 	]*&lt;&lt;[ 	][a-z]' -- '*.c' '*.h'
	git grep '&lt;&lt;[   ]*24'

which catches at least the common byte cases (shifting variables by a
variable amount, and shifting by 24 bits).

I also grepped for just 'unsigned char' variables in general, and
converted the ones that most obviously ended up getting implicitly cast
immediately anyway (eg hash_name(), encode_85()).

In addition to just avoiding 'unsigned char', this patch also tries to use
a common idiom for the delta header size thing. We had three different
variations on it: "&amp; 0x7fUL" in one place (getting the sign extension
right), and "&amp; ~0x80" and "&amp; 0x7f" in two other places (not getting it
right). Apart from making them all just avoid using "unsigned char" at
all, I also unified them to then use a simple "&amp; 0x7f".

I considered making a sparse extension which warns about doing implicit
casts from unsigned types to signed types, but it gets rather complex very
quickly, so this is just a hack.

Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
Signed-off-by: Junio C Hamano &lt;gitster@pobox.com&gt;
diff --git a/attr.c b/attr.c
@@ -35,8 +35,7 @@ static struct git_attr *(git_attr_hash[HASHSIZE]);
 
 static unsigned hash_name(const char *name, int namelen)
 {
-	unsigned val = 0;
-	unsigned char c;
+	unsigned val = 0, c;
 
 	while (namelen--) {
 		c = *name++;
diff --git a/base85.c b/base85.c
@@ -91,7 +91,7 @@ void encode_85(char *buf, const unsigned char *data, int bytes)
 		unsigned acc = 0;
 		int cnt;
 		for (cnt = 24; cnt >= 0; cnt -= 8) {
-			int ch = *data++;
+			unsigned ch = *data++;
 			acc |= ch << cnt;
 			if (--bytes == 0)
 				break;
diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
@@ -653,8 +653,7 @@ static void rehash_objects(void)
 
 static unsigned name_hash(const char *name)
 {
-	unsigned char c;
-	unsigned hash = 0;
+	unsigned c, hash = 0;
 
 	if (!name)
 		return 0;
diff --git a/builtin-unpack-objects.c b/builtin-unpack-objects.c
@@ -422,8 +422,8 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 static void unpack_one(unsigned nr)
 {
 	unsigned shift;
-	unsigned char *pack, c;
-	unsigned long size;
+	unsigned char *pack;
+	unsigned long size, c;
 	enum object_type type;
 
 	obj_list[nr].offset = consumed_bytes;
diff --git a/delta.h b/delta.h
@@ -90,12 +90,11 @@ static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
 					       const unsigned char *top)
 {
 	const unsigned char *data = *datap;
-	unsigned char cmd;
-	unsigned long size = 0;
+	unsigned long cmd, size = 0;
 	int i = 0;
 	do {
 		cmd = *data++;
-		size |= (cmd & ~0x80) << i;
+		size |= (cmd & 0x7f) << i;
 		i += 7;
 	} while (cmd & 0x80 && data < top);
 	*datap = data;
diff --git a/index-pack.c b/index-pack.c
@@ -293,8 +293,8 @@ static void *unpack_entry_data(unsigned long offset, unsigned long size)
 
 static void *unpack_raw_entry(struct object_entry *obj, union delta_base *delta_base)
 {
-	unsigned char *p, c;
-	unsigned long size;
+	unsigned char *p;
+	unsigned long size, c;
 	off_t base_offset;
 	unsigned shift;
 	void *data;
@@ -312,7 +312,7 @@ static void *unpack_raw_entry(struct object_entry *obj, union delta_base *delta_
 		p = fill(1);
 		c = *p;
 		use(1);
-		size += (c & 0x7fUL) << shift;
+		size += (c & 0x7f) << shift;
 		shift += 7;
 	}
 	obj->size = size;
diff --git a/patch-delta.c b/patch-delta.c
@@ -44,7 +44,7 @@ void *patch_delta(const void *src_buf, unsigned long src_size,
 			if (cmd & 0x01) cp_off = *data++;
 			if (cmd & 0x02) cp_off |= (*data++ << 8);
 			if (cmd & 0x04) cp_off |= (*data++ << 16);
-			if (cmd & 0x08) cp_off |= (*data++ << 24);
+			if (cmd & 0x08) cp_off |= ((unsigned) *data++ << 24);
 			if (cmd & 0x10) cp_size = *data++;
 			if (cmd & 0x20) cp_size |= (*data++ << 8);
 			if (cmd & 0x40) cp_size |= (*data++ << 16);
diff --git a/sha1_file.c b/sha1_file.c
@@ -1162,8 +1162,7 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 		unsigned long len, enum object_type *type, unsigned long *sizep)
 {
 	unsigned shift;
-	unsigned char c;
-	unsigned long size;
+	unsigned long size, c;
 	unsigned long used = 0;
 
 	c = buf[used++];

Original file line number	Diff line number	Diff line change
`@@ -35,8 +35,7 @@ static struct git_attr *(git_attr_hash[HASHSIZE]);`
`35`	`35`
`36`	`36`	`static unsigned hash_name(const char *name, int namelen)`
`37`	`37`	`{`
`38`		`- unsigned val = 0;`
`39`		`- unsigned char c;`
	`38`	`+ unsigned val = 0, c;`
`40`	`39`
`41`	`40`	`while (namelen--) {`
`42`	`41`	`c = *name++;`
Original file line number	Diff line number	Diff line change
`@@ -653,8 +653,7 @@ static void rehash_objects(void)`
`653`	`653`
`654`	`654`	`static unsigned name_hash(const char *name)`
`655`	`655`	`{`
`656`		`- unsigned char c;`
`657`		`- unsigned hash = 0;`
	`656`	`+ unsigned c, hash = 0;`
`658`	`657`
`659`	`658`	`if (!name)`
`660`	`659`	`return 0;`
Original file line number	Diff line number	Diff line change
`@@ -1162,8 +1162,7 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,`
`1162`	`1162`	`unsigned long len, enum object_type type, unsigned long sizep)`
`1163`	`1163`	`{`
`1164`	`1164`	`unsigned shift;`
`1165`		`- unsigned char c;`
`1166`		`- unsigned long size;`
	`1165`	`+ unsigned long size, c;`
`1167`	`1166`	`unsigned long used = 0;`
`1168`	`1167`
`1169`	`1168`	`c = buf[used++];`