Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into()

Al Viro
2022-07-04T02:54:02Z

On Sat, Jul 02, 2022 at 10:23:16AM -0700, Linus Torvalds wrote:

> Al - can you please take a quick look?

FWIW, trying to write a coherent documentation had its usual effect...
The thing is, we don't really need to fetch the inode that early.
All we really care about is that in RCU mode ->d_seq gets sampled
before we fetch ->d_inode *and* we don't treat "it looks negative"
as hard -ENOENT in case of ->d_seq mismatch.

Which can be bloody well left to step_into(). So we don't need
to pass it inode argument at all - just dentry and seq. Makes
a bunch of functions simpler as well...

It does *not* deal with the "uninitialized" seq argument in
!RCU case; I'll handle that in the followup, but that's a separate
story, IMO (and very clearly a false positive).

Cumulative diff follows; splitup is in #work.namei. Comments?

diff --git a/fs/namei.c b/fs/namei.c
index 1f28d3f463c3..7f4f61ade9e3 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1467,7 +1467,7 @@ EXPORT_SYMBOL(follow_down);
* we meet a managed dentry that would need blocking.
*/
static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
- struct inode **inode, unsigned *seqp)
+ unsigned *seqp)
{
struct dentry *dentry = path->dentry;
unsigned int flags = dentry->d_flags;
@@ -1497,13 +1497,6 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
dentry = path->dentry = mounted->mnt.mnt_root;
nd->state |= ND_JUMPED;
*seqp = read_seqcount_begin(&dentry->d_seq);
- *inode = dentry->d_inode;
- /*
- * We don't need to re-check ->d_seq after this
- * ->d_inode read - there will be an RCU delay
- * between mount hash removal and ->mnt_root
- * becoming unpinned.
- */
flags = dentry->d_flags;
continue;
}
@@ -1515,8 +1508,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
}

static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
- struct path *path, struct inode **inode,
- unsigned int *seqp)
+ struct path *path, unsigned int *seqp)
{
bool jumped;
int ret;
@@ -1525,9 +1517,7 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
path->dentry = dentry;
if (nd->flags & LOOKUP_RCU) {
unsigned int seq = *seqp;
- if (unlikely(!*inode))
- return -ENOENT;
- if (likely(__follow_mount_rcu(nd, path, inode, seqp)))
+ if (likely(__follow_mount_rcu(nd, path, seqp)))
return 0;
if (!try_to_unlazy_next(nd, dentry, seq))
return -ECHILD;
@@ -1547,7 +1537,6 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
if (path->mnt != nd->path.mnt)
mntput(path->mnt);
} else {
- *inode = d_backing_inode(path->dentry);
*seqp = 0; /* out of RCU mode, so the value doesn't matter */
}
return ret;
@@ -1607,9 +1596,7 @@ static struct dentry *__lookup_hash(const struct qstr *name,
return dentry;
}

-static struct dentry *lookup_fast(struct nameidata *nd,
- struct inode **inode,
- unsigned *seqp)
+static struct dentry *lookup_fast(struct nameidata *nd, unsigned *seqp)
{
struct dentry *dentry, *parent = nd->path.dentry;
int status = 1;
@@ -1628,22 +1615,11 @@ static struct dentry *lookup_fast(struct nameidata *nd,
return NULL;
}

- /*
- * This sequence count validates that the inode matches
- * the dentry name information from lookup.
- */
- *inode = d_backing_inode(dentry);
- if (unlikely(read_seqcount_retry(&dentry->d_seq, seq)))
- return ERR_PTR(-ECHILD);
-
- /*
+ /*
* This sequence count validates that the parent had no
* changes while we did the lookup of the dentry above.
- *
- * The memory barrier in read_seqcount_begin of child is
- * enough, we can use __read_seqcount_retry here.
*/
- if (unlikely(__read_seqcount_retry(&parent->d_seq, nd->seq)))
+ if (unlikely(read_seqcount_retry(&parent->d_seq, nd->seq)))
return ERR_PTR(-ECHILD);

*seqp = seq;
@@ -1838,13 +1814,21 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
* for the common case.
*/
static const char *step_into(struct nameidata *nd, int flags,
- struct dentry *dentry, struct inode *inode, unsigned seq)
+ struct dentry *dentry, unsigned seq)
{
struct path path;
- int err = handle_mounts(nd, dentry, &path, &inode, &seq);
+ struct inode *inode;
+ int err = handle_mounts(nd, dentry, &path, &seq);

if (err < 0)
return ERR_PTR(err);
+ inode = path.dentry->d_inode;
+ if (unlikely(!inode)) {
+ if ((nd->flags & LOOKUP_RCU) &&
+ read_seqcount_retry(&path.dentry->d_seq, seq))
+ return ERR_PTR(-ECHILD);
+ return ERR_PTR(-ENOENT);
+ }
if (likely(!d_is_symlink(path.dentry)) ||
((flags & WALK_TRAILING) && !(nd->flags & LOOKUP_FOLLOW)) ||
(flags & WALK_NOFOLLOW)) {
@@ -1870,9 +1854,7 @@ static const char *step_into(struct nameidata *nd, int flags,
return pick_link(nd, &path, inode, seq, flags);
}

-static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
- struct inode **inodep,
- unsigned *seqp)
+static struct dentry *follow_dotdot_rcu(struct nameidata *nd, unsigned *seqp)
{
struct dentry *parent, *old;

@@ -1895,7 +1877,6 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
}
old = nd->path.dentry;
parent = old->d_parent;
- *inodep = parent->d_inode;
*seqp = read_seqcount_begin(&parent->d_seq);
if (unlikely(read_seqcount_retry(&old->d_seq, nd->seq)))
return ERR_PTR(-ECHILD);
@@ -1910,9 +1891,7 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
return NULL;
}

-static struct dentry *follow_dotdot(struct nameidata *nd,
- struct inode **inodep,
- unsigned *seqp)
+static struct dentry *follow_dotdot(struct nameidata *nd, unsigned *seqp)
{
struct dentry *parent;

@@ -1937,7 +1916,6 @@ static struct dentry *follow_dotdot(struct nameidata *nd,
return ERR_PTR(-ENOENT);
}
*seqp = 0;
- *inodep = parent->d_inode;
return parent;

in_root:
@@ -1952,7 +1930,6 @@ static const char *handle_dots(struct nameidata *nd, int type)
if (type == LAST_DOTDOT) {
const char *error = NULL;
struct dentry *parent;
- struct inode *inode;
unsigned seq;

if (!nd->root.mnt) {
@@ -1961,17 +1938,17 @@ static const char *handle_dots(struct nameidata *nd, int type)
return error;
}
if (nd->flags & LOOKUP_RCU)
- parent = follow_dotdot_rcu(nd, &inode, &seq);
+ parent = follow_dotdot_rcu(nd, &seq);
else
- parent = follow_dotdot(nd, &inode, &seq);
+ parent = follow_dotdot(nd, &seq);
if (IS_ERR(parent))
return ERR_CAST(parent);
if (unlikely(!parent))
error = step_into(nd, WALK_NOFOLLOW,
- nd->path.dentry, nd->inode, nd->seq);
+ nd->path.dentry, nd->seq);
else
error = step_into(nd, WALK_NOFOLLOW,
- parent, inode, seq);
+ parent, seq);
if (unlikely(error))
return error;

@@ -1995,7 +1972,6 @@ static const char *handle_dots(struct nameidata *nd, int type)
static const char *walk_component(struct nameidata *nd, int flags)
{
struct dentry *dentry;
- struct inode *inode;
unsigned seq;
/*
* "." and ".." are special - ".." especially so because it has
@@ -2007,7 +1983,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
put_link(nd);
return handle_dots(nd, nd->last_type);
}
- dentry = lookup_fast(nd, &inode, &seq);
+ dentry = lookup_fast(nd, &seq);
if (IS_ERR(dentry))
return ERR_CAST(dentry);
if (unlikely(!dentry)) {
@@ -2017,7 +1993,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
}
if (!(flags & WALK_MORE) && nd->depth)
put_link(nd);
- return step_into(nd, flags, dentry, inode, seq);
+ return step_into(nd, flags, dentry, seq);
}

/*
@@ -2473,8 +2449,7 @@ static int handle_lookup_down(struct nameidata *nd)
{
if (!(nd->flags & LOOKUP_RCU))
dget(nd->path.dentry);
- return PTR_ERR(step_into(nd, WALK_NOFOLLOW,
- nd->path.dentry, nd->inode, nd->seq));
+ return PTR_ERR(step_into(nd, WALK_NOFOLLOW, nd->path.dentry, nd->seq));
}

/* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
@@ -3394,7 +3369,6 @@ static const char *open_last_lookups(struct nameidata *nd,
int open_flag = op->open_flag;
bool got_write = false;
unsigned seq;
- struct inode *inode;
struct dentry *dentry;
const char *res;

@@ -3410,7 +3384,7 @@ static const char *open_last_lookups(struct nameidata *nd,
if (nd->last.name[nd->last.len])
nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
/* we _can_ be in RCU mode here */
- dentry = lookup_fast(nd, &inode, &seq);
+ dentry = lookup_fast(nd, &seq);
if (IS_ERR(dentry))
return ERR_CAST(dentry);
if (likely(dentry))
@@ -3464,7 +3438,7 @@ static const char *open_last_lookups(struct nameidata *nd,
finish_lookup:
if (nd->depth)
put_link(nd);
- res = step_into(nd, WALK_TRAILING, dentry, inode, seq);
+ res = step_into(nd, WALK_TRAILING, dentry, seq);
if (unlikely(res))
nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
return res;