[PATCH] 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: Junio C Hamano <redacted>
---

 Junio C Hamano [off-list ref] writes:

 > About the CE_NAMEMASK limitation (and currently we do not check
 > it, so I think we would be screwed when a pathname that is
 > longer than (CE_NAMEMASK+1) and still fits under PATH_MAX is
 > given), I think we do not have to limit the maximum pathname
 > length.  Instead we can teach create_ce_flags() and ce_namelen()
 > that a name longer than 2k (or 4k) has the NAMEMASK bits that
 > are all 1 and ce->name[] must be counted if so (with an obvious
 > optimization to start counting at byte position 2k or 4k in
 > ce_namelen()).

 This should fix it.  Passes tests including the new test that
 the existing code fails.

 cache.h          |   17 +++++++++++++++--
 t/t0000-basic.sh |   18 ++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/cache.h b/cache.h
index 39331c2..ad53acc 100644
--- a/cache.h
+++ b/cache.h

@@ -114,8 +114,21 @@ struct cache_entry {
 #define CE_VALID     (0x8000)
 #define CE_STAGESHIFT 12
 
-#define create_ce_flags(len, stage) htons((len) | ((stage) << CE_STAGESHIFT))
-#define ce_namelen(ce) (CE_NAMEMASK & ntohs((ce)->ce_flags))
+static inline unsigned create_ce_flags(size_t len, unsigned stage)
+{
+	if (len >= CE_NAMEMASK)
+		len = CE_NAMEMASK;
+	return htons(len | (stage << CE_STAGESHIFT));
+}
+
+static inline size_t ce_namelen(const struct cache_entry *ce)
+{
+	size_t len = ntohs((ce)->ce_flags) & CE_NAMEMASK;
+	if (len < CE_NAMEMASK) /* likely */
+		return len;
+	return strlen(ce->name + CE_NAMEMASK) + CE_NAMEMASK;
+}
+
 #define ce_size(ce) cache_entry_size(ce_namelen(ce))
 #define ce_stage(ce) ((CE_STAGEMASK & ntohs((ce)->ce_flags)) >> CE_STAGESHIFT)
 

diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index 4e49d59..40551a3 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh

@@ -297,4 +297,22 @@ 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
+'
+
 test_done