[RFC][PATCH v4 26/69] sanitize handling of nd->last_type, kill LAST_BIND

Al Viro
2020-03-13T23:57:34Z

->last_type values are set in 3 places: path_init() (sets to LAST_ROOT),
link_path_walk (LAST_NORM/DOT/DOTDOT) and pick_link (LAST_BIND).

The are checked in walk_component(), lookup_last() and do_last().
They also get copied to the caller by filename_parentat(). In the last
3 cases the value is what we had at the return from link_path_walk().
In case of walk_component() it's either directly downstream from
assignment in link_path_walk() or, when called by lookup_last(), the
value we have at the return from link_path_walk().

The value at the entry into link_path_walk() can survive to return only
if the pathname contains nothing but slashes. Note that pick_link()
never returns such - pure jumps are handled directly. So for the calls
of link_path_walk() for trailing symlinks it does not matter what value
had been there at the entry; the value at the return won't depend upon it.

There are 3 call chains that might have pick_link() storing LAST_BIND:

1) pick_link() from step_into() from walk_component() from
link_path_walk(). In that case we will either be parsing the next
component immediately after return into link_path_walk(), which will
overwrite the ->last_type before anyone has a chance to look at it,
or we'll fail, in which case nobody will be looking at ->last_type at all.

2) pick_link() from step_into() from walk_component() from lookup_last().
The value is never looked at due to the above; it won't affect the value
seen at return from any link_path_walk().

3) pick_link() from step_into() from do_last(). Ditto.

In other words, assignemnt in pick_link() is pointless, and so is
LAST_BIND itself; nothing ever looks at that value. Kill it off.
And make link_path_walk() _always_ assign ->last_type - in the only
case when the value at the entry might survive to the return that value
is always LAST_ROOT, inherited from path_init(). Move that assignment
from path_init() into the beginning of link_path_walk(), to consolidate
the things.

Historical note: LAST_BIND used to be used for the kludge with trailing
pure jump symlinks (extra iteration through the top-level loop).
No point keeping it anymore...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
Documentation/filesystems/path-lookup.rst | 7 ++-----
fs/namei.c | 3 +--
include/linux/namei.h | 2 +-
3 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index a3216979298b..f46b05e9b96c 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -404,11 +404,8 @@ that is the "next" component in the pathname.
``int last_type``
~~~~~~~~~~~~~~~~~

-This is one of ``LAST_NORM``, ``LAST_ROOT``, ``LAST_DOT``, ``LAST_DOTDOT``, or
-``LAST_BIND``. The ``last`` field is only valid if the type is
-``LAST_NORM``. ``LAST_BIND`` is used when following a symlink and no
-components of the symlink have been processed yet. Others should be
-fairly self-explanatory.
+This is one of ``LAST_NORM``, ``LAST_ROOT``, ``LAST_DOT`` or ``LAST_DOTDOT``.
+The ``last`` field is only valid if the type is ``LAST_NORM``.

``struct path root``
~~~~~~~~~~~~~~~~~~~~
diff --git a/fs/namei.c b/fs/namei.c
index 9b08c64397c8..438717b462fb 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1785,7 +1785,6 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
if (unlikely(error))
return ERR_PTR(error);

- nd->last_type = LAST_BIND;
res = READ_ONCE(inode->i_link);
if (!res) {
const char * (*get)(struct dentry *, struct inode *,
@@ -2123,6 +2122,7 @@ static int link_path_walk(const char *name, struct nameidata *nd)
{
int err;

+ nd->last_type = LAST_ROOT;
if (IS_ERR(name))
return PTR_ERR(name);
while (*name=='/')
@@ -2224,7 +2224,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
if (flags & LOOKUP_RCU)
rcu_read_lock();

- nd->last_type = LAST_ROOT; /* if there are only slashes... */
nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT;
nd->depth = 0;

diff --git a/include/linux/namei.h b/include/linux/namei.h
index d9576a051808..a4bb992623c4 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -15,7 +15,7 @@ enum { MAX_NESTED_LINKS = 8 };
/*
* Type of the last component on LOOKUP_PARENT
*/
-enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
+enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};

/* pathwalk mode */
#define LOOKUP_FOLLOW 0x0001 /* follow links at the end */
--
2.11.0