1490d7d82d
Since commit fcc07e980b (is_promisor_object(): free tree buffer after
parsing, 2021-04-13), we'll always free the buffers attached to a
"struct tree" after searching them for promisor links. But there's an
important case where we don't want to do so: if somebody else is already
using the tree!
This can happen during a "rev-list --missing=allow-promisor" traversal
in a partial clone that is missing one or more trees or blobs. The
backtrace for the free looks like this:
#1 free_tree_buffer tree.c:147
#2 add_promisor_object packfile.c:2250
#3 for_each_object_in_pack packfile.c:2190
#4 for_each_packed_object packfile.c:2215
#5 is_promisor_object packfile.c:2272
#6 finish_object__ma builtin/rev-list.c:245
#7 finish_object builtin/rev-list.c:261
#8 show_object builtin/rev-list.c:274
#9 process_blob list-objects.c:63
#10 process_tree_contents list-objects.c:145
#11 process_tree list-objects.c:201
#12 traverse_trees_and_blobs list-objects.c:344
[...]
We're in the middle of walking through the entries of a tree object via
process_tree_contents(). We see a blob (or it could even be another tree
entry) that we don't have, so we call is_promisor_object() to check it.
That function loops over all of the objects in the promisor packfile,
including the tree we're currently walking. When we're done with it
there, we free the tree buffer. But as we return to the walk in
process_tree_contents(), it's still holding on to a pointer to that
buffer, via its tree_desc iterator, and it accesses the freed memory.
Even a trivial use of "--missing=allow-promisor" triggers this problem,
as the included test demonstrates (it's just a vanilla --blob:none
clone).
We can detect this case by only freeing the tree buffer if it was
allocated on our behalf. This is a little tricky since that happens
inside parse_object(), and it doesn't tell us whether the object was
already parsed, or whether it allocated the buffer itself. But by
checking for an already-parsed tree beforehand, we can distinguish the
two cases.
That feels a little hacky, and does incur an extra lookup in the
object-hash table. But that cost is fairly minimal compared to actually
loading objects (and since we're iterating the whole pack here, we're
likely to be loading most objects, rather than reusing cached results).
It may also be a good direction for this function in general, as there
are other possible optimizations that rely on doing some analysis before
parsing:
- we could detect blobs and avoid reading their contents; they can't
link to other objects, but parse_object() doesn't know that we don't
care about checking their hashes.
- we could avoid allocating object structs entirely for most objects
(since we really only need them in the oidset), which would save
some memory.
- promisor commits could use the commit-graph rather than loading the
object from disk
This commit doesn't do any of those optimizations, but I think it argues
that this direction is reasonable, rather than relying on parse_object()
and trying to teach it to give us more information about whether it
parsed.
The included test fails reliably under SANITIZE=address just when
running "rev-list --missing=allow-promisor". Checking the output isn't
strictly necessary to detect the bug, but it seems like a reasonable
addition given the general lack of coverage for "allow-promisor" in the
test suite.
Reported-by: Andrew Olsen <andrew.olsen@koordinates.com>
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). To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org. The mailing list archives are available at https://lore.kernel.org/git/, http://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