fsck: detect very large tree pathnames

In general, Git tries not to arbitrarily limit what it will store, and
there are currently no limits at all on the size of the path we find in
a tree. In theory you could have one that is gigabytes long.

But in practice this freedom is not really helping anybody, and is
potentially harmful:

  1. Most operating systems have much lower limits for the size of a
     single pathname component (e.g., on Linux you'll generally get
     ENAMETOOLONG for anything over 255 bytes). And while you _can_ use
     Git in a way that never touches the filesystem (manipulating the
     index and trees directly), it's still probably not a good idea to
     have gigantic tree names. Many operations load and traverse them,
     so any clever Git-as-a-database scheme is likely to perform poorly
     in that case.

  2. We still have a lot of code which assumes strings are reasonably
     sized, and I won't be at all surprised if you can trigger some
     interesting integer overflows with gigantic pathnames. Stopping
     malicious trees from entering the repository provides an extra line
     of defense, protecting downstream code.

This patch implements an fsck check so that such trees can be rejected
by transfer.fsckObjects. I've picked a reasonably high maximum depth
here (4096) that hopefully should not bother anybody in practice. I've
also made it configurable, as an escape hatch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/fsck-msgids.txt b/Documentation/fsck-msgids.txt
index 12eae8a222..09b0aecbf8 100644
--- a/Documentation/fsck-msgids.txt
+++ b/Documentation/fsck-msgids.txt
@@ -103,6 +103,13 @@
 `hasDotgit`::
 	(WARN) A tree contains an entry named `.git`.
 
+`largePathname`::
+	(WARN) A tree contains an entry with a very long path name. If
+	the value of `fsck.largePathname` contains a colon, that value
+	is used as the maximum allowable length (e.g., "warn:10" would
+	complain about any path component of 11 or more bytes). The
+	default value is 4096.
+
 `mailmapSymlink`::
 	(INFO) `.mailmap` is a symlink.