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From Fox Chen:

  The Path lookup is a very complex subject in VFS. The path-lookup
  document provides a very detailed guidance to help people understand how
  path lookup works in the kernel. This document was originally written
  based on three lwn articles five years ago. As times goes by, some of the
  content is outdated. This patchset is intended to update the document to
  make it more relevant to current codebase.
This commit is contained in:
Jonathan Corbet 2021-06-18 11:37:35 -06:00
commit 98cf495184
1 changed files with 81 additions and 105 deletions
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Documentation/filesystems/path-lookup.rst
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@ -448,15 +448,17 @@ described. If it finds a ``LAST_NORM`` component it first calls
filesystem to revalidate the result if it is that sort of filesystem.
If that doesn't get a good result, it calls "``lookup_slow()``" which
takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
to find a definitive answer. Each of these will call
``follow_managed()`` (as described below) to handle any mount points.
to find a definitive answer.
In the absence of symbolic links, ``walk_component()`` creates a new
``struct path`` containing a counted reference to the new dentry and a
reference to the new ``vfsmount`` which is only counted if it is
different from the previous ``vfsmount``. It then calls
``path_to_nameidata()`` to install the new ``struct path`` in the
``struct nameidata`` and drop the unneeded references.
As the last step of walk_component(), step_into() will be called either
directly from walk_component() or from handle_dots(). It calls
handle_mounts(), to check and handle mount points, in which a new
``struct path`` is created containing a counted reference to the new dentry and
a reference to the new ``vfsmount`` which is only counted if it is
different from the previous ``vfsmount``. Then if there is
a symbolic link, step_into() calls pick_link() to deal with it,
otherwise it installs the new ``struct path`` in the ``struct nameidata``, and
drops the unneeded references.
This "hand-over-hand" sequencing of getting a reference to the new
dentry before dropping the reference to the previous dentry may
@ -470,8 +472,8 @@ Handling the final component
``nd->last_type`` to refer to the final component of the path. It does
not call ``walk_component()`` that last time. Handling that final
component remains for the caller to sort out. Those callers are
``path_lookupat()``, ``path_parentat()``, ``path_mountpoint()`` and
``path_openat()`` each of which handles the differing requirements of
path_lookupat(), path_parentat() and
path_openat() each of which handles the differing requirements of
different system calls.
``path_parentat()`` is clearly the simplest - it just wraps a little bit
@ -486,20 +488,18 @@ perform their operation.
object is wanted such as by ``stat()`` or ``chmod()``. It essentially just
calls ``walk_component()`` on the final component through a call to
``lookup_last()``. ``path_lookupat()`` returns just the final dentry.
``path_mountpoint()`` handles the special case of unmounting which must
not try to revalidate the mounted filesystem. It effectively
contains, through a call to ``mountpoint_last()``, an alternate
implementation of ``lookup_slow()`` which skips that step. This is
important when unmounting a filesystem that is inaccessible, such as
It is worth noting that when flag ``LOOKUP_MOUNTPOINT`` is set,
path_lookupat() will unset LOOKUP_JUMPED in nameidata so that in the
subsequent path traversal d_weak_revalidate() won't be called.
This is important when unmounting a filesystem that is inaccessible, such as
one provided by a dead NFS server.
Finally ``path_openat()`` is used for the ``open()`` system call; it
contains, in support functions starting with "``do_last()``", all the
contains, in support functions starting with "open_last_lookups()", all the
complexity needed to handle the different subtleties of O_CREAT (with
or without O_EXCL), final "``/``" characters, and trailing symbolic
links. We will revisit this in the final part of this series, which
focuses on those symbolic links. "``do_last()``" will sometimes, but
focuses on those symbolic links. "open_last_lookups()" will sometimes, but
not always, take ``i_rwsem``, depending on what it finds.
Each of these, or the functions which call them, need to be alert to
@ -535,8 +535,7 @@ covered in greater detail in autofs.txt in the Linux documentation
tree, but a few notes specifically related to path lookup are in order
here.
The Linux VFS has a concept of "managed" dentries which is reflected
in function names such as "``follow_managed()``". There are three
The Linux VFS has a concept of "managed" dentries. There are three
potentially interesting things about these dentries corresponding
to three different flags that might be set in ``dentry->d_flags``:
@ -652,10 +651,10 @@ RCU-walk finds it cannot stop gracefully, it simply gives up and
restarts from the top with REF-walk.
This pattern of "try RCU-walk, if that fails try REF-walk" can be
clearly seen in functions like ``filename_lookup()``,
``filename_parentat()``, ``filename_mountpoint()``,
``do_filp_open()``, and ``do_file_open_root()``. These five
correspond roughly to the four ``path_*()`` functions we met earlier,
clearly seen in functions like filename_lookup(),
filename_parentat(),
do_filp_open(), and do_file_open_root(). These four
correspond roughly to the three ``path_*()`` functions we met earlier,
each of which calls ``link_path_walk()``. The ``path_*()`` functions are
called using different mode flags until a mode is found which works.
They are first called with ``LOOKUP_RCU`` set to request "RCU-walk". If
@ -993,8 +992,8 @@ is 4096. There are a number of reasons for this limit; not letting the
kernel spend too much time on just one path is one of them. With
symbolic links you can effectively generate much longer paths so some
sort of limit is needed for the same reason. Linux imposes a limit of
at most 40 symlinks in any one path lookup. It previously imposed a
further limit of eight on the maximum depth of recursion, but that was
at most 40 (MAXSYMLINKS) symlinks in any one path lookup. It previously imposed
a further limit of eight on the maximum depth of recursion, but that was
raised to 40 when a separate stack was implemented, so there is now
just the one limit.
@ -1061,42 +1060,26 @@ filesystem cannot successfully get a reference in RCU-walk mode, it
must return ``-ECHILD`` and ``unlazy_walk()`` will be called to return to
REF-walk mode in which the filesystem is allowed to sleep.
The place for all this to happen is the ``i_op->follow_link()`` inode
method. In the present mainline code this is never actually called in
RCU-walk mode as the rewrite is not quite complete. It is likely that
in a future release this method will be passed an ``inode`` pointer when
called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
validated pointer. Much like the ``i_op->permission()`` method we
looked at previously, ``->follow_link()`` would need to be careful that
The place for all this to happen is the ``i_op->get_link()`` inode
method. This is called both in RCU-walk and REF-walk. In RCU-walk the
``dentry*`` argument is NULL, ``->get_link()`` can return -ECHILD to drop out of
RCU-walk. Much like the ``i_op->permission()`` method we
looked at previously, ``->get_link()`` would need to be careful that
all the data structures it references are safe to be accessed while
holding no counted reference, only the RCU lock. Though getting a
reference with ``->follow_link()`` is not yet done in RCU-walk mode, the
code is ready to release the reference when that does happen.
This need to drop the reference to a symlink adds significant
complexity. It requires a reference to the inode so that the
``i_op->put_link()`` inode operation can be called. In REF-walk, that
reference is kept implicitly through a reference to the dentry, so
keeping the ``struct path`` of the symlink is easiest. For RCU-walk,
the pointer to the inode is kept separately. To allow switching from
RCU-walk back to REF-walk in the middle of processing nested symlinks
we also need the seq number for the dentry so we can confirm that
switching back was safe.
Finally, when providing a reference to a symlink, the filesystem also
provides an opaque "cookie" that must be passed to ``->put_link()`` so that it
knows what to free. This might be the allocated memory area, or a
pointer to the ``struct page`` in the page cache, or something else
completely. Only the filesystem knows what it is.
holding no counted reference, only the RCU lock. A callback
``struct delayed_called`` will be passed to ``->get_link()``:
file systems can set their own put_link function and argument through
set_delayed_call(). Later on, when VFS wants to put link, it will call
do_delayed_call() to invoke that callback function with the argument.
In order for the reference to each symlink to be dropped when the walk completes,
whether in RCU-walk or REF-walk, the symlink stack needs to contain,
along with the path remnants:
- the ``struct path`` to provide a reference to the inode in REF-walk
- the ``struct inode *`` to provide a reference to the inode in RCU-walk
- the ``struct path`` to provide a reference to the previous path
- the ``const char *`` to provide a reference to the to previous name
- the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
- the ``cookie`` that tells ``->put_path()`` what to put.
- the ``struct delayed_call`` for later invocation.
This means that each entry in the symlink stack needs to hold five
pointers and an integer instead of just one pointer (the path
@ -1120,12 +1103,10 @@ doesn't need to notice. Getting this ``name`` variable on and off the
stack is very straightforward; pushing and popping the references is
a little more complex.
When a symlink is found, ``walk_component()`` returns the value ``1``
(``0`` is returned for any other sort of success, and a negative number
is, as usual, an error indicator). This causes ``get_link()`` to be
called; it then gets the link from the filesystem. Providing that
operation is successful, the old path ``name`` is placed on the stack,
and the new value is used as the ``name`` for a while. When the end of
When a symlink is found, walk_component() calls pick_link() via step_into()
which returns the link from the filesystem.
Providing that operation is successful, the old path ``name`` is placed on the
stack, and the new value is used as the ``name`` for a while. When the end of
the path is found (i.e. ``*name`` is ``'\0'``) the old ``name`` is restored
off the stack and path walking continues.
@ -1142,23 +1123,23 @@ stack in ``walk_component()`` immediately when the symlink is found;
old symlink as it walks that last component. So it is quite
convenient for ``walk_component()`` to release the old symlink and pop
the references just before pushing the reference information for the
new symlink. It is guided in this by two flags; ``WALK_GET``, which
gives it permission to follow a symlink if it finds one, and
``WALK_PUT``, which tells it to release the current symlink after it has been
followed. ``WALK_PUT`` is tested first, leading to a call to
``put_link()``. ``WALK_GET`` is tested subsequently (by
``should_follow_link()``) leading to a call to ``pick_link()`` which sets
up the stack frame.
new symlink. It is guided in this by three flags: ``WALK_NOFOLLOW`` which
forbids it from following a symlink if it finds one, ``WALK_MORE``
which indicates that it is yet too early to release the
current symlink, and ``WALK_TRAILING`` which indicates that it is on the final
component of the lookup, so we will check userspace flag ``LOOKUP_FOLLOW`` to
decide whether follow it when it is a symlink and call ``may_follow_link()`` to
check if we have privilege to follow it.
Symlinks with no final component
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A pair of special-case symlinks deserve a little further explanation.
Both result in a new ``struct path`` (with mount and dentry) being set
up in the ``nameidata``, and result in ``get_link()`` returning ``NULL``.
up in the ``nameidata``, and result in pick_link() returning ``NULL``.
The more obvious case is a symlink to "``/``". All symlinks starting
with "``/``" are detected in ``get_link()`` which resets the ``nameidata``
with "``/``" are detected in pick_link() which resets the ``nameidata``
to point to the effective filesystem root. If the symlink only
contains "``/``" then there is nothing more to do, no components at all,
so ``NULL`` is returned to indicate that the symlink can be released and
@ -1175,12 +1156,11 @@ something that looks like a symlink. It is really a reference to the
target file, not just the name of it. When you ``readlink`` these
objects you get a name that might refer to the same file - unless it
has been unlinked or mounted over. When ``walk_component()`` follows
one of these, the ``->follow_link()`` method in "procfs" doesn't return
a string name, but instead calls ``nd_jump_link()`` which updates the
``nameidata`` in place to point to that target. ``->follow_link()`` then
returns ``NULL``. Again there is no final component and ``get_link()``
reports this by leaving the ``last_type`` field of ``nameidata`` as
``LAST_BIND``.
one of these, the ``->get_link()`` method in "procfs" doesn't return
a string name, but instead calls nd_jump_link() which updates the
``nameidata`` in place to point to that target. ``->get_link()`` then
returns ``NULL``. Again there is no final component and pick_link()
returns ``NULL``.
Following the symlink in the final component
--------------------------------------------
@ -1197,42 +1177,38 @@ potentially need to call ``link_path_walk()`` again and again on
successive symlinks until one is found that doesn't point to another
symlink.
This case is handled by the relevant caller of ``link_path_walk()``, such as
``path_lookupat()`` using a loop that calls ``link_path_walk()``, and then
handles the final component. If the final component is a symlink
that needs to be followed, then ``trailing_symlink()`` is called to set
things up properly and the loop repeats, calling ``link_path_walk()``
again. This could loop as many as 40 times if the last component of
each symlink is another symlink.
This case is handled by relevant callers of link_path_walk(), such as
path_lookupat(), path_openat() using a loop that calls link_path_walk(),
and then handles the final component by calling open_last_lookups() or
lookup_last(). If it is a symlink that needs to be followed,
open_last_lookups() or lookup_last() will set things up properly and
return the path so that the loop repeats, calling
link_path_walk() again. This could loop as many as 40 times if the last
component of each symlink is another symlink.
The various functions that examine the final component and possibly
report that it is a symlink are ``lookup_last()``, ``mountpoint_last()``
and ``do_last()``, each of which use the same convention as
``walk_component()`` of returning ``1`` if a symlink was found that needs
to be followed.
Of the various functions that examine the final component,
open_last_lookups() is the most interesting as it works in tandem
with do_open() for opening a file. Part of open_last_lookups() runs
with ``i_rwsem`` held and this part is in a separate function: lookup_open().
Of these, ``do_last()`` is the most interesting as it is used for
opening a file. Part of ``do_last()`` runs with ``i_rwsem`` held and this
part is in a separate function: ``lookup_open()``.
Explaining open_last_lookups() and do_open() completely is beyond the scope
of this article, but a few highlights should help those interested in exploring
the code.
Explaining ``do_last()`` completely is beyond the scope of this article,
but a few highlights should help those interested in exploring the
code.
1. Rather than just finding the target file, ``do_last()`` needs to open
1. Rather than just finding the target file, do_open() is used after
open_last_lookup() to open
it. If the file was found in the dcache, then ``vfs_open()`` is used for
this. If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
the filesystem provides it) to combine the final lookup with the open, or
will perform the separate ``lookup_real()`` and ``vfs_create()`` steps
will perform the separate ``i_op->lookup()`` and ``i_op->create()`` steps
directly. In the later case the actual "open" of this newly found or
created file will be performed by ``vfs_open()``, just as if the name
created file will be performed by vfs_open(), just as if the name
were found in the dcache.
2. ``vfs_open()`` can fail with ``-EOPENSTALE`` if the cached information
wasn't quite current enough. Rather than restarting the lookup from
the top with ``LOOKUP_REVAL`` set, ``lookup_open()`` is called instead,
giving the filesystem a chance to resolve small inconsistencies.
If that doesn't work, only then is the lookup restarted from the top.
2. vfs_open() can fail with ``-EOPENSTALE`` if the cached information
wasn't quite current enough. If it's in RCU-walk ``-ECHILD`` will be returned
otherwise ``-ESTALE`` is returned. When ``-ESTALE`` is returned, the caller may
retry with ``LOOKUP_REVAL`` flag set.
3. An open with O_CREAT **does** follow a symlink in the final component,
unlike other creation system calls (like ``mkdir``). So the sequence::
@ -1242,8 +1218,8 @@ code.
will create a file called ``/tmp/bar``. This is not permitted if
``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
like for a non-creating open: ``should_follow_link()`` returns ``1``, and
so does ``do_last()`` so that ``trailing_symlink()`` gets called and the
like for a non-creating open: lookup_last() or open_last_lookup()
returns a non ``NULL`` value, and link_path_walk() gets called and the
open process continues on the symlink that was found.
Updating the access time