Git with broken hash generation to generate collisions between object IDs. Don't use this!
https://undefinedbehavior.de/posts/commit-vandalism/
3704fed5ea
This commit adds a new test case that demonstrates a bug in the
split-index code that is triggered under certain circumstances when the
FSMonitor is enabled, and its symptom manifests in the form of one of
the following error messages:
BUG: fsmonitor.c:20: fsmonitor_dirty has more entries than the index (2 > 1)
BUG: unpack-trees.c:776: pos <n> doesn't point to the first entry of <dir>/ in index
error: invalid path ''
error: The following untracked working tree files would be overwritten by reset:
initial.t
Which of these error messages appears depends on timing-dependent
conditions.
Technically the root cause lies with a bug in the split-index code that
has nothing to do with FSMonitor, but for the sake of this new test case
it was the easiest way to trigger the bug.
The bug is this: Under specific conditions, Git needs to skip writing
the "link" extension (which is the index extension containing the
information pertaining to the split-index). To do that, the `base_oid`
attribute of the `split_index` structure in the in-memory index is
zeroed out, and `do_write_index()` specifically checks for a "null"
`base_oid` to understand that the "link" extension should not be
written. However, this violates the consistency of the in-memory index
structure, but that does not cause problems in most cases because the
process exits without using the in-memory index structure anymore,
anyway.
But: _When_ the in-memory index is still used (which is the case e.g. in
`git rebase`), subsequent writes of `the_index` are at risk of writing
out a bogus index file, one that _should_ have a "link" extension but
does not. In many cases, the `SPLIT_INDEX_ORDERED` flag _happens_ to be
set for subsequent writes, forcing the shared index to be written, which
re-initializes `base_oid` to a non-bogus state, and all is good.
When it is _not_ set, however, all kinds of mayhem ensue, resulting in
above-mentioned error messages, and often enough putting worktrees in a
totally broken state where the only recourse is to manually delete the
`index` and the `index.lock` files and then call `git reset` manually.
Not something to ask users to do.
The reason why it is comparatively easy to trigger the bug with
FSMonitor is that there is _another_ bug in the FSMonitor code:
`mark_fsmonitor_valid()` sets `cache_changed` to 1, i.e. treating that
variable as a Boolean. But it is a bit field, and 1 happens to be the
`SOMETHING_CHANGED` bit that forces the "link" extension to be skipped
when writing the index, among other things.
"Comparatively easy" is a relative term in this context, for sure. The
essence of how the new test case triggers the bug is as following:
1. The `git rebase` invocation will first reset the worktree to
a commit that contains only the `one.t` file, and then execute a
rebase script that starts with the following commands (commit hashes
skipped):
label onto
reset initial
pick two
label two
reset two
pick three
[...]
2. Before executing the `label` command, a split index is written, as
well as the shared index.
3. The `reset initial` command in the rebase script writes out a new
split index but skips writing the shared index, as intended.
4. The `pick two` command updates the worktree and refreshes the index,
marking the `two.t` entry as valid via the FSMonitor, which sets the
`SOMETHING_CHANGED` bit in `cache_changed`, which in turn causes the
`base_oid` attribute to be zeroed out and a full (non-split) index
to be written (making sure _not_ to write the "link" extension).
5. Now, the `reset two` command will leave the worktree alone, but
still write out a new split index, not writing the shared index
(because `base_oid` is still zeroed out, and there is no index entry
update requiring it to be written, either).
6. When it is turn to run `pick three`, the index is read, but it is
too short: It only contains a single entry when there should be two,
because the "link" extension is missing from the written-out index
file.
There are three bugs at play, actually, which will be fixed over the
course of the next commits:
- The `base_oid` attribute should not be zeroed out to indicate when
the "link" extension should not be written, as it puts the in-memory
index structure into an inconsistent state.
- The FSMonitor should not overwrite bits in `cache_changed`.
- The `unpack_trees()` function tries to reuse the `split_index`
structure from the source index, if any, but does not propagate the
`SPLIT_INDEX_ORDERED` flag.
While a fix for the second bug would let this test case pass, there are
other conditions where the `SOMETHING_CHANGED` bit is set. Therefore,
the bug that most crucially needs to be fixed is the first one.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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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).
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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