[RFC][PATCHSET][CFT] pathwalk cleanups and fixes

[Al Viro]

	OK, vfs.git #work.namei seems to survive xfstests.  I think
it cleans the things quite a bit, but it obviously needs more
review and testing.

	Review and testing would be _very_ welcome; it does a lot
of massage, so there had been a plenty of opportunities to fuck up
and fail to spot that.  The same goes for profiling - it doesn't
seem to slow the things down, but that needs to be verified.

	It does include #work.openat2.  Topology: 17 commits, followed
by clean merge with #work.openat2, followed by 9 followups.  The
part is #work.openat2 is as posted by Aleksa; I can repost it, but
I don't see much point.  Description of the rest follows; patches
themselves will be in followups.

part 1: follow_automount() cleanups and fixes.

	Quite a bit of that function had been about working around the
wrong calling conventions of finish_automount().  The problem is that
finish_automount() misuses the primitive intended for mount(2) and
friends, where we want to mount on top of the pile, even if something
has managed to add to that while we'd been trying to lock the namespace.
For automount that's not the right thing to do - there we want to discard
whatever it was going to attach and just cross into what got mounted
there in the meanwhile (most likely - the results of the same automount
triggered by somebody else).  Current mainline kinda-sorta manages to do
that, but it's unreliable and very convoluted.  Much simpler approach
is to stop using lock_mount() in finish_automount() and have it bail
out if something turns out to have been mounted on top where we wanted
to attach.  That allows to get rid of a lot of PITA in the caller.
Another simplification comes from not trying to cross into the results
of automount - simply ride through the next iteration of the loop and
let it move into overmount.

	Another thing in the same series is divorcing follow_automount()
from nameidata; that'll play later when we get to unifying follow_down()
with the guts of follow_managed().

	4 commits, the second one fixes a hard-to-hit race.  The first
is a prereq for it.

1/17	do_add_mount(): lift lock_mount/unlock_mount into callers
2/17	fix automount/automount race properly
3/17	follow_automount(): get rid of dead^Wstillborn code
4/17	follow_automount() doesn't need the entire nameidata

part 2: unifying mount traversals in pathwalk.

	Handling of mount traversal (follow_managed()) is currently called
in a bunch of places.  Each of them is shortly followed by a call of
step_into() or an open-coded equivalent thereof.  However, the locations
of those step_into() calls are far from preceding follow_managed();
moreover, that preceding call might happen on different paths that
converge to given step_into() call.  It's harder to analyse that it should
be (especially when it comes to liveness analysis) and it forces rather
ugly calling conventions on lookup_fast()/atomic_open()/lookup_open().
The series below massages the code to the point when the calls of
follow_managed() (and __follow_mount_rcu()) move into the beginning of
step_into().

5/17	make build_open_flags() treat O_CREAT | O_EXCL as implying O_NOFOLLOW
	gets EEXIST handling in do_last() past the step_into() call there.
6/17	handle_mounts(): start building a sane wrapper for follow_managed()
	rather than mangling follow_managed() itself (and creating conflicts
	with openat2 series), add a wrapper that will absorb the required
	interface changes.
7/17	atomic_open(): saner calling conventions (return dentry on success)
	struct path passed to it is pure out parameter; only dentry part
	ever varies, though - mnt is always nd->path.mnt.  Just return
	the dentry on success, and ERR_PTR(-E...) on failure.
8/17	lookup_open(): saner calling conventions (return dentry on success)
	propagate the same change one level up the call chain.
9/17	do_last(): collapse the call of path_to_nameidata()
	struct path filled in lookup_open() call is eventually given to
	handle_mounts(); the only use it has before that is path_to_nameidata()
	call in "->atomic_open() has actually opened it" case, and there
	path_to_nameidata() is an overkill - we are guaranteed to replace
	only nd->path.dentry.  So have the struct path filled only immediately
	prior to handle_mounts().
10/17	handle_mounts(): pass dentry in, turn path into a pure out argument
	now all callers of handle_mount() are directly preceded by filling
	struct path it gets.  path->mnt is nd->path.mnt in all cases, so we can
	pass just the dentry instead and fill path in handle_mount() itself.
	Some boilerplate gone, path is pure out argument of handle_mount()
	now.
11/17	lookup_fast(): consolidate the RCU success case
	massage to gather what will become an RCU case equivalent of
	handle_mounts(); basically, that's what we do if revalidate succeeds
	in RCU case of lookup_fast(), including unlazy and fallback to
	handle_mounts() if __follow_mount_rcu() says "it's too tricky".
12/17	teach handle_mounts() to handle RCU mode
	... and take that into handle_mount() itself.  The other caller of
	__follow_mount_rcu() is fine with the same fallback (it just didn't
	bother since it's in the very beginning of pathwalk), switched to
	handle_mount() as well.
13/17	lookup_fast(): take mount traversal into callers
	Now we are getting somewhere - both RCU and non-RCU success cases of
	lookup_fast() are ended with the same return handle_mounts(...);
	move that to the callers - there it will merge with the identical calls
	that had been on the paths where we had to do slow lookups.
	lookup_fast() returns dentry now.
14/17	new step_into() flag: WALK_NOFOLLOW
	use step_into() instead of open-coding it in handle_lookup_down().
	Add a flag for "don't follow symlinks regardless of LOOKUP_FOLLOW" for
	that (and eventually, I hope, for .. handling).
	Now *all* calls of handle_mounts() and step_into() are right next to
	each other.
15/17	fold handle_mounts() into step_into()
	... and we can move the call of handle_mounts() into step_into(),
	getting a slightly saner calling conventions out of that.
16/17	LOOKUP_MOUNTPOINT: fold path_mountpointat() into path_lookupat()
	another payoff from 14/17 - we can teach path_lookupat() to do
	what path_mountpointat() used to.  And kill the latter, along with
	its wrappers.
17/17	expand the only remaining call of path_lookup_conditional()
	minor cleanup - RIP path_lookup_conditional().  Only one caller left.

At that point we run out of things that can be done without textual conflicts
with openat2 series.  Changes so far:
	* mount traversal is taken into step_into().
	* lookup_fast(), atomic_open() and lookup_open() calling conventions
are slightly changed.  All of them return dentry now, instead of returning
an int and filling struct path on success.  For lookup_fast() the old
"0 for cache miss, 1 for cache hit" is replaced with "NULL stands for cache
miss, dentry - for hit".
	* step_into() can be called in RCU mode as well.  Takes nameidata,
WALK_... flags, dentry and, in RCU case, corresponding inode and seq value.
Handles mount traversals, decides whether it's a symlink to be followed.
Error => returns -E...; symlink to follow => returns 1, puts symlink on stack;
non-symlink or symlink not to follow => returns 0, moves nd->path to new location.
	* LOOKUP_MOUNTPOINT introduced; user_path_mountpoint_at() and friends
became calls of user_path_at() et.al. with LOOKUP_MOUNTPOINT in flags.

Next comes the merge with Aleksa's openat2 patchset; everything up to that point
had been non-conflicting with it.  That patchset has been posted earlier;
it's in #work.openat2.  The next series comes on top of the merge.

part 3: untangling the symlink handling.

	Right now when we decide to follow a symlink it happens this way:
	* step_into() decides that it has been given a symlink that needs to
be followed.
	* it calls pick_link(), which pushes the symlink on stack and
returns 1 on success / -E... on error.  Symlink's mount/dentry/seq is
stored on stack and the inode is stashed in nd->link_inode.
	* step_into() passes that 1 to its callers, which proceed to pass it
up the call chain for several layers.  In all cases we get to get_link()
call shortly afterwards.
	* get_link() is called, picks the inode stashed in nd->link_inode
by the pick_link(), does some checks, touches the atime, etc.
	* get_link() either picks the link body out of inode or calls
->get_link().  If it's an absolute symlink, we move to the root and return
the relative portion of the body; if it's a relative one - just return the
body.  If it's a procfs-style one, the call of nd_jump_link() has been
made and we'd moved to whatever location is desired.  And return NULL,
same as we do for symlink to "/".
	* the caller proceeds to deal with the string returned to it.

	The sequence is the same in all cases (nested symlink, trailing
symlink on lookup, trailing symlink on open), but its pieces are not close
to each other and the bit between the call of pick_link() and (inevitable)
call of get_link() afterwards is not easy to follow.  Moreover, a bunch
of functions (walk_component/lookup_last/do_last) ends up with the same
conventions for return values as step_into().  And those conventions
(see above) are not pretty - 0/1/-E... is asking for mistakes, especially
when returned 1 is used only to direct control flow on a rather twisted
way to matching get_link() call.  And that path can be seriously twisted.
E.g. when we are trying to open /dev/stdin, we get the following sequence:
	* path_init() has put us into root and returned "/dev/stdin"
	* link_path_walk() has eventually reached /dev and left
<LAST_NORM, "stdin"> in nd->last_type/nd->last
	* we call do_last(), which sees that we have LAST_NORM and calls
lookup_fast().  Let's assume that everything is in dcache; we get the
dentry of /dev/stdin and proceed to finish_lookup:, where we call step_into()
	* it's a symlink, we have LOOKUP_FOLLOW, so we decide to pick the
damn thing.  Into the stack it goes and we return 1.
	* do_last() sees 1 and returns it.
	* trailing_symlink() is called (in the top-level loop) and it
calls get_link().  OK, we get "/proc/self/fd/0" for body, move to
root again and return "proc/self/fd/0".
	* link_path_walk() is given that string, eventually leading us into
/proc/self/fd, with <LAST_NORM, "0"> left as the component to handle.
	* do_last() is called, and similar to the previous case we
eventually reach the call of step_into() with dentry of /proc/self/fd/0.
	* _now_ we can discard /dev/stdin from the stack (we'd been
using its body until now).  It's dropped (from step_into()) and we get
to look at what we'd been given.  A symlink to follow, so on the stack
it goes and we return 1.
	* again, do_last() passes 1 to caller
	* trailing_symlink() is called and calls get_link().
	* this time it's a procfs symlink and its ->get_link() method
moves us to the mount/dentry of our stdin.  And returns NULL.  But the
fun doesn't stop yet.
	* trailing_symlink() returns "" to the caller
	* link_path_walk() is called on that and does nothing
whatsoever.
	* do_last() is called and sees LAST_BIND left by the get_link().
It calls handle_dots()
	* handle_dots() drops the symlink from stack and returns
	* do_last() *FINALLY* proceeds to the point after its call of
step_into() (finish_open:) and gets around to opening the damn thing.

	Making sense of the control flow through all of that is not fun,
to put it mildly; debugging anything in that area can be a massive PITA,
and this example has touched only one of 3 cases.  Arguably, the worst
one, but...  Anyway, it turns out that this code can be massaged to
considerably saner shape - both in terms of control flow and wrt calling
conventions.

1/9	merging pick_link() with get_link(), part 1
	prep work: move the "hardening" crap from trailing_symlink() into
get_link() (conditional on the absense of LOOKUP_PARENT in nd->flags).
We'll be moving the calls of get_link() around quite a bit through that
series, and the next step will be to eliminate trailing_symlink().
2/9	merging pick_link() with get_link(), part 2
	fold trailing_symlink() into lookup_last() and do_last().
Now these are returning strings; it's not the final calling conventions,
but it's almost there.  NULL => old 0, we are done.  ERR_PTR(-E...) =>
old -E..., we'd failed.  string => old 1, and the string is the symlink
body to follow.  Just as for trailing_symlink(), "/" and procfs ones
(where get_link() returns NULL) yield "", so the ugly song and dance
with no-op trip through link_path_walk()/handle_dots() still remains.
3/9	merging pick_link() with get_link(), part 3
	elimination of that round-trip.  In *all* cases having
get_link() return NULL on such symlinks means that we'll proceed to
drop the symlink from stack and get back to the point near that
get_link() call - basically, where we would be if it hadn't been
a symlink at all.  The path by which we are getting there depends
upon the call site; the end result is the same in all cases - such
symlinks (procfs ones and symlink to "/") are fully processed by
the time get_link() returns, so we could as well drop them from the
stack right in get_link().  Makes life simpler in terms of control
flow analysis...
	And now the calling conventions for do_last() and lookup_last()
have reached the final shape - ERR_PTR(-E...) for error, NULL for
"we are done", string for "traverse this".
4/9	merging pick_link() with get_link(), part 4
	now all calls of walk_component() are followed by the same
boilerplate - "if it has returned 1, call get_link() and if that
has returned NULL treat that as if walk_component() has returned 0".
Eliminate by folding that into walk_component() itself.  Now
walk_component() return value conventions have joined those of
do_last()/lookup_last().
5/9	merging pick_link() with get_link(), part 5
	same as for the previous, only this time the boilerplate
migrates one level down, into step_into().  Only one caller of
get_link() left, step_into() has joined the same return value
conventions.
6/9	merging pick_link() with get_link(), part 6
	move that thing into pick_link().  Now all traces of
"return 1 if we are following a symlink" are gone.
7/9	finally fold get_link() into pick_link()
	ta-da - expand get_link() into the only caller.  As a side
benefit, we get rid of stashing the inode in nd->link_inode - it
was done only to carry that piece of information from pick_link()
to eventual get_link().  That's not the main benefit, though - the
control flow became considerably easier to reason about.

For what it's worth, the example above (/dev/stdin) becomes
	* path_init() has put us into root and returned "/dev/stdin"
	* link_path_walk() has eventually reached /dev and left
<LAST_NORM, "stdin"> in nd->last_type/nd->last
	* we call do_last(), which sees that we have LAST_NORM and calls
lookup_fast().  Let's assume that everything is in dcache; we get the
dentry of /dev/stdin and proceed to finish_lookup:, where we call step_into()
	* it's a symlink, we have LOOKUP_FOLLOW, so we decide to pick the
damn thing.  On the stack it goes and we get its body.  Which is
"/proc/self/fd/0", so we move to root and return "proc/self/fd/0".
	* do_last() sees non-NULL and returns it - whether it's an error
or a pathname to traverse, we hadn't reached something we'll be opening.
	* link_path_walk() is given that string, eventually leading us into
/proc/self/fd, with <LAST_NORM, "0"> left as the component to handle.
	* do_last() is called, and similar to the previous case we
eventually reach the call of step_into() with dentry of /proc/self/fd/0.
	* _now_ we can discard /dev/stdin from the stack (we'd been
using its body until now).  It's dropped (from step_into()) and we get
to look at what we'd been given.  A symlink to follow, so on the stack
it goes.   This time it's a procfs symlink and its ->get_link() method
moves us to the mount/dentry of our stdin.  And returns NULL.  So we
drop symlink from stack and return that NULL to caller.
	* that NULL is returned by step_into(), same as if we had just
moved to a non-symlink.
	* do_last() proceeds to open the damn thing.

part 4.  some mount traversal cleanups.

8/9	massage __follow_mount_rcu() a bit
	make it more similar to non-RCU counterpart
9/9	new helper: traverse_mounts()
	the guts of follow_managed() are very similar to
follow_down().  The calling conventions are different (follow_managed()
works with nameidata, follow_down() - with standalone struct path),
but the core loop is pretty much the same in both.  Turned that loop
into a common helper (traverse_mounts()) and since follow_managed()
becomes a very thin wrapper around it, expand follow_managed() at its
only call site (in handle_mounts()),

That's where the series stands right now.  FWIW, at 5.5-rc1 fs/namei.c
had been 4867 lines, at the tip of #work.openat2 - 4998, at the
tip of #work.namei (containing #work.openat2) - 4730...  And IMO
the thing has become considerably easier to follow.

What's more, it might be possible to untangle the control flow in
do_last() now.  Probably a separate series, though - do_last() is
one hell of a tarpit, so I'm not stepping into it for the rest
of this cycle...