Currently the check also succeeds if the input path starts with a dot, whereas
we only want it to succeed for "." and "./". Tighten the check and add a test.
If we use CHASE_PARENT on a path ending in ".." then things are a bit
weird, because we the last path we look at is actually the *parent* and not
the *child* of the preceeding path. Hence we cannot just return the 2nd
to last fd we look at. We have to correct it, by going *two* levels up,
to get to the actual parent, and make sure CHASE_PARENT does what it
should.
Example: for the path /a/b/c chase() with CHASE_PARENT will return
/a/b/c as path, and the fd returned points to /a/b. All good. But now,
for the path /a/b/c/.. chase() with CHASE_PARENT would previously return
/a/b as path (which is OK) but the fd would point to /a/b/c, which is
*not* the parent of /a/b, after all! To get to the actual parent of
/a/b we have to go *two* levels up to get to /a.
Very confusing. But that's what we here for, no?
@mrc0mmand ran into this in https://github.com/systemd/systemd/pull/28891#issuecomment-1782833722
In chaseat() we call dir_fd_is_root() several places, and the final
result depends on it. If the root path specified to `chase()` is not
normalized but points to "/", e.g. "/../", assertions in `chaseat()` or
`chase()` may be triggered.
The function path_prefix_root_cwd() was introduced for prefixing the
result from chaseat() with root, but
- it is named slightly generic,
- the logic is different from what chase() does.
This makes the name more explanative and specific for the result of the
chaseat(), and make the logic consistent with chase().
Fixes https://github.com/systemd/systemd/pull/27199#issuecomment-1511387731.
Follow-up for #27199.
Usually, we pass the file descriptor of the root directory to chaseat()
when `--root=` is not specified. Previously, even in such case, the
result was relative, and we need to prefix the path with "/" when we
want to pass the path to other functions that do not support dir_fd, or
log or show the path. That's inconvenient.
When CHASE_MKDIR_0755 is specified without CHASE_NONEXISTENT and
CHASE_PARENT, then chase() succeeds only when the file specified by
the path already exists, and in that case, chase() does not create
any parent directories, and CHASE_MKDIR_0755 is meaningless.
Let's mention that CHASE_MKDIR_0755 needs to be specified with
CHASE_NONEXISTENT or CHASE_PARENT, and adds a assertion about that.
In that branch, 'root' is a non-root and absolute path.
Hence, delete_trailing_chars() does not make the path empty.
And, if the path contains redundant slashes at the end, that will be
dropped by path_simplify().
Previously, struct stat may not be correctly synced with the currently
opened fd, e.g. when a path contains symlink which points to an absolute
path.
This also rename variables for struct stat, to make them consistent with
the corresponding fd.
Chasing symlinks is a core function that's used in a lot of places
so it deservers a less verbose names so let's rename it to chase()
and chaseat().
We also slightly change the pattern used for the chaseat() helpers
so we get chase_and_openat() and similar.
