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reimplement path_mountpoint() with less magic

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... and get rid of a bunch of bugs in it.  Background:
the reason for path_mountpoint() is that umount() really doesn't
want attempts to revalidate the root of what it's trying to umount.
The thing we want to avoid actually happen from complete_walk();
solution was to do something parallel to normal path_lookupat()
and it both went overboard and got the boilerplate subtly
(and not so subtly) wrong.

A better solution is to do pretty much what the normal path_lookupat()
does, but instead of complete_walk() do unlazy_walk().  All it takes
to avoid that ->d_weak_revalidate() call...  mountpoint_last() goes
away, along with everything it got wrong, and so does the magic around
LOOKUP_NO_REVAL.

Another source of bugs is that when we traverse mounts at the final
location (and we need to do that - umount . expects to get whatever's
overmounting ., if any, out of the lookup) we really ought to take
care of ->d_manage() - as it is, manual umount of autofs automount
in progress can lead to unpleasant surprises for the daemon.  Easily
solved by using handle_lookup_down() instead of follow_mount().

Tested-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2020-01-10 17:17:19 -05:00
parent 1edc8eb2e9
commit c64cd6e34e
3 changed files with 12 additions and 80 deletions
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89
fs/namei.c
View File

@ -1649,17 +1649,15 @@ again:
if (IS_ERR(dentry))
return dentry;
if (unlikely(!d_in_lookup(dentry))) {
if (!(flags & LOOKUP_NO_REVAL)) {
int error = d_revalidate(dentry, flags);
if (unlikely(error <= 0)) {
if (!error) {
d_invalidate(dentry);
dput(dentry);
goto again;
}
int error = d_revalidate(dentry, flags);
if (unlikely(error <= 0)) {
if (!error) {
d_invalidate(dentry);
dput(dentry);
dentry = ERR_PTR(error);
goto again;
}
dput(dentry);
dentry = ERR_PTR(error);
}
} else {
old = inode->i_op->lookup(inode, dentry, flags);
@ -2617,72 +2615,6 @@ int user_path_at_empty(int dfd, const char __user *name, unsigned flags,
}
EXPORT_SYMBOL(user_path_at_empty);
/**
* mountpoint_last - look up last component for umount
* @nd: pathwalk nameidata - currently pointing at parent directory of "last"
*
* This is a special lookup_last function just for umount. In this case, we
* need to resolve the path without doing any revalidation.
*
* The nameidata should be the result of doing a LOOKUP_PARENT pathwalk. Since
* mountpoints are always pinned in the dcache, their ancestors are too. Thus,
* in almost all cases, this lookup will be served out of the dcache. The only
* cases where it won't are if nd->last refers to a symlink or the path is
* bogus and it doesn't exist.
*
* Returns:
* -error: if there was an error during lookup. This includes -ENOENT if the
* lookup found a negative dentry.
*
* 0: if we successfully resolved nd->last and found it to not to be a
* symlink that needs to be followed.
*
* 1: if we successfully resolved nd->last and found it to be a symlink
* that needs to be followed.
*/
static int
mountpoint_last(struct nameidata *nd)
{
int error = 0;
struct dentry *dir = nd->path.dentry;
struct path path;
/* If we're in rcuwalk, drop out of it to handle last component */
if (nd->flags & LOOKUP_RCU) {
if (unlazy_walk(nd))
return -ECHILD;
}
nd->flags &= ~LOOKUP_PARENT;
if (unlikely(nd->last_type != LAST_NORM)) {
error = handle_dots(nd, nd->last_type);
if (error)
return error;
path.dentry = dget(nd->path.dentry);
} else {
path.dentry = d_lookup(dir, &nd->last);
if (!path.dentry) {
/*
* No cached dentry. Mounted dentries are pinned in the
* cache, so that means that this dentry is probably
* a symlink or the path doesn't actually point
* to a mounted dentry.
*/
path.dentry = lookup_slow(&nd->last, dir,
nd->flags | LOOKUP_NO_REVAL);
if (IS_ERR(path.dentry))
return PTR_ERR(path.dentry);
}
}
if (d_flags_negative(smp_load_acquire(&path.dentry->d_flags))) {
dput(path.dentry);
return -ENOENT;
}
path.mnt = nd->path.mnt;
return step_into(nd, &path, 0, d_backing_inode(path.dentry), 0);
}
/**
* path_mountpoint - look up a path to be umounted
* @nd: lookup context
@ -2699,14 +2631,17 @@ path_mountpoint(struct nameidata *nd, unsigned flags, struct path *path)
int err;
while (!(err = link_path_walk(s, nd)) &&
(err = mountpoint_last(nd)) > 0) {
(err = lookup_last(nd)) > 0) {
s = trailing_symlink(nd);
}
if (!err && (nd->flags & LOOKUP_RCU))
err = unlazy_walk(nd);
if (!err)
err = handle_lookup_down(nd);
if (!err) {
*path = nd->path;
nd->path.mnt = NULL;
nd->path.dentry = NULL;
follow_mount(path);
}
terminate_walk(nd);
return err;

2
fs/nfs/nfstrace.h
View File

@ -206,7 +206,6 @@ TRACE_DEFINE_ENUM(LOOKUP_AUTOMOUNT);
TRACE_DEFINE_ENUM(LOOKUP_PARENT);
TRACE_DEFINE_ENUM(LOOKUP_REVAL);
TRACE_DEFINE_ENUM(LOOKUP_RCU);
TRACE_DEFINE_ENUM(LOOKUP_NO_REVAL);
TRACE_DEFINE_ENUM(LOOKUP_OPEN);
TRACE_DEFINE_ENUM(LOOKUP_CREATE);
TRACE_DEFINE_ENUM(LOOKUP_EXCL);
@ -224,7 +223,6 @@ TRACE_DEFINE_ENUM(LOOKUP_DOWN);
{ LOOKUP_PARENT, "PARENT" }, \
{ LOOKUP_REVAL, "REVAL" }, \
{ LOOKUP_RCU, "RCU" }, \
{ LOOKUP_NO_REVAL, "NO_REVAL" }, \
{ LOOKUP_OPEN, "OPEN" }, \
{ LOOKUP_CREATE, "CREATE" }, \
{ LOOKUP_EXCL, "EXCL" }, \

1
include/linux/namei.h
View File

@ -34,7 +34,6 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
/* internal use only */
#define LOOKUP_PARENT 0x0010
#define LOOKUP_NO_REVAL 0x0080
#define LOOKUP_JUMPED 0x1000
#define LOOKUP_ROOT 0x2000
#define LOOKUP_ROOT_GRABBED 0x0008