index: be careful when handling long names

We currently use lower 12-bit (masked with CE_NAMEMASK) in the
ce_flags field to store the length of the name in cache_entry,
without checking the length parameter given to
create_ce_flags().  This can make us store incorrect length.

Currently we are mostly protected by the fact that many
codepaths first copy the path in a variable of size PATH_MAX,
which typically is 4096 that happens to match the limit, but
that feels like a bug waiting to happen.  Besides, that would
not allow us to shorten the width of CE_NAMEMASK to use the bits
for new flags.

This redefines the meaning of the name length stored in the
cache_entry.  A name that does not fit is represented by storing
CE_NAMEMASK in the field, and the actual length needs to be
computed by actually counting the bytes in the name[] field.
This way, only the unusually long paths need to suffer.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

cache.h

@@ -131,8 +131,21 @@ struct cache_entry {
 #define CE_UPDATE    (0x10000)
 #define CE_REMOVE    (0x20000)
 
-#define create_ce_flags(len, stage) ((len) | ((stage) << CE_STAGESHIFT))
-#define ce_namelen(ce) (CE_NAMEMASK & (ce)->ce_flags)
+static inline unsigned create_ce_flags(size_t len, unsigned stage)
+{
+	if (len >= CE_NAMEMASK)
+		len = CE_NAMEMASK;
+	return (len | (stage << CE_STAGESHIFT));
+}
+
+static inline size_t ce_namelen(const struct cache_entry *ce)
+{
+	size_t len = ce->ce_flags & CE_NAMEMASK;
+	if (len < CE_NAMEMASK)
+		return len;
+	return strlen(ce->name + CE_NAMEMASK) + CE_NAMEMASK;
+}
+
 #define ce_size(ce) cache_entry_size(ce_namelen(ce))
 #define ondisk_ce_size(ce) ondisk_cache_entry_size(ce_namelen(ce))
 #define ce_stage(ce) ((CE_STAGEMASK & (ce)->ce_flags) >> CE_STAGESHIFT)

read-cache.c

@@ -928,6 +928,8 @@ int read_index(struct index_state *istate)
 
 static void convert_from_disk(struct ondisk_cache_entry *ondisk, struct cache_entry *ce)
 {
+	size_t len;
+
 	ce->ce_ctime = ntohl(ondisk->ctime.sec);
 	ce->ce_mtime = ntohl(ondisk->mtime.sec);
 	ce->ce_dev   = ntohl(ondisk->dev);
@@ -939,7 +941,15 @@ static void convert_from_disk(struct ondisk_cache_entry *ondisk, struct cache_en
 	/* On-disk flags are just 16 bits */
 	ce->ce_flags = ntohs(ondisk->flags);
 	hashcpy(ce->sha1, ondisk->sha1);
-	memcpy(ce->name, ondisk->name, ce_namelen(ce)+1);
+
+	len = ce->ce_flags & CE_NAMEMASK;
+	if (len == CE_NAMEMASK)
+		len = strlen(ondisk->name);
+	/*
+	 * NEEDSWORK: If the original index is crafted, this copy could
+	 * go unchecked.
+	 */
+	memcpy(ce->name, ondisk->name, len + 1);
 }
 
 /* remember to discard_cache() before reading a different cache! */

t/t0000-basic.sh

@@ -297,4 +297,24 @@ test_expect_success 'absolute path works as expected' '
 	test "$sym" = "$(test-absolute-path $dir2/syml)"
 '
 
+test_expect_success 'very long name in the index handled sanely' '
+
+	a=a && # 1
+	a=$a$a$a$a$a$a$a$a$a$a$a$a$a$a$a$a && # 16
+	a=$a$a$a$a$a$a$a$a$a$a$a$a$a$a$a$a && # 256
+	a=$a$a$a$a$a$a$a$a$a$a$a$a$a$a$a$a && # 4096
+	a=${a}q &&
+
+	>path4 &&
+	git update-index --add path4 &&
+	(
+		git ls-files -s path4 |
+		sed -e "s/	.*/	/" |
+		tr -d "\012"
+		echo "$a"
+	) | git update-index --index-info &&
+	len=$(git ls-files "a*" | wc -c) &&
+	test $len = 4098
+'
+
 test_done