6cd05e768b
When a client sends us a "want" or "have" line, we call parse_object()
to get an object struct. If the object is a tree, then the parsed state
means that tree->buffer points to the uncompressed contents of the tree.
But we don't really care about it. We only really need to parse commits
and tags; for trees and blobs, the important output is just a "struct
object" with the correct type.
But much worse, we do not ever free that tree buffer. It's not leaked in
the traditional sense, in that we still have a pointer to it from the
global object hash. But if the client requests many trees, we'll hold
all of their contents in memory at the same time.
Nobody really noticed because it's rare for clients to directly request
a tree. It might happen for a lightweight tag pointing straight at a
tree, or it might happen for a "tree:depth" partial clone filling in
missing trees.
But it's also possible for a malicious client to request a lot of trees,
causing upload-pack's memory to balloon. For example, without this
patch, requesting every tree in git.git like:
pktline() {
local msg="$*"
printf "%04x%s\n" $((1+4+${#msg})) "$msg"
}
want_trees() {
pktline command=fetch
printf 0001
git cat-file --batch-all-objects --batch-check='%(objectname) %(objecttype)' |
while read oid type; do
test "$type" = "tree" || continue
pktline want $oid
done
pktline done
printf 0000
}
want_trees | GIT_PROTOCOL=version=2 valgrind --tool=massif ./git upload-pack . >/dev/null
shows a peak heap usage of ~3.7GB. Which is just about the sum of the
sizes of all of the uncompressed trees. For linux.git, it's closer to
17GB.
So the obvious thing to do is to call free_tree_buffer() after we
realize that we've parsed a tree. We know that upload-pack won't need it
later. But let's push the logic into parse_object_with_flags(), telling
it to discard the tree buffer immediately. There are two reasons for
this. One, all of the relevant call-sites already call the with_options
variant to pass the SKIP_HASH flag. So it actually ends up as less code
than manually free-ing in each spot. And two, it enables an extra
optimization that I'll discuss below.
I've touched all of the sites that currently use SKIP_HASH in
upload-pack. That drops the peak heap of the upload-pack invocation
above from 3.7GB to ~24MB.
I've also modified the caller in get_reference(); a partial clone
benefits from its use in pack-objects for the reasons given in
0bc2557951 (upload-pack: skip parse-object re-hashing of "want" objects,
2022-09-06), where we were measuring blob requests. But note that the
results of get_reference() are used for traversing, as well; so we
really would _eventually_ use the tree contents. That makes this at
first glance a space/time tradeoff: we won't hold all of the trees in
memory at once, but we'll have to reload them each when it comes time to
traverse.
And here's where our extra optimization comes in. If the caller is not
going to immediately look at the tree contents, and it doesn't care
about checking the hash, then parse_object() can simply skip loading the
tree entirely, just like we do for blobs! And now it's not a space/time
tradeoff in get_reference() anymore. It's just a lazy-load: we're
delaying reading the tree contents until it's time to actually traverse
them one by one.
And of course for upload-pack, this optimization means we never load the
trees at all, saving lots of CPU time. Timing the "every tree from
git.git" request above shows upload-pack dropping from 32 seconds of CPU
to 19 (the remainder is mostly due to pack-objects actually sending the
pack; timing just the upload-pack portion shows we go from 13s to
~0.28s).
These are all highly gamed numbers, of course. For real-world
partial-clone requests we're saving only a small bit of time in
practice. But it does help harden upload-pack against malicious
denial-of-service attacks.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.
Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission and Documentation/CodingGuidelines).
Those wishing to help with error message, usage and informational message
string translations (localization l10) should see po/README.md
(a po file is a Portable Object file that holds the translations).
To subscribe to the list, send an email to git+subscribe@vger.kernel.org (see https://subspace.kernel.org/subscribing.html for details). The mailing list archives are available at https://lore.kernel.org/git/, https://marc.info/?l=git and other archival sites.
Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very first version. He described the tool as "the stupid content tracker" and the name as (depending on your mood):
- random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
- stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
- "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
- "goddamn idiotic truckload of sh*t": when it breaks