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

Alexander Potapenko
2022-07-04T08:21:38Z

On Mon, Jul 4, 2022 at 4:53 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> 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).

I can confirm that your patch fixes KMSAN reports on inode, yet the
following reports still persist:

=====================================================
BUG: KMSAN: uninit-value in walk_component+0x5e7/0x6c0 fs/namei.c:1996
walk_component+0x5e7/0x6c0 fs/namei.c:1996
lookup_last fs/namei.c:2445
path_lookupat+0x27d/0x6f0 fs/namei.c:2468
filename_lookup+0x24c/0x800 fs/namei.c:2497
kern_path+0x79/0x3a0 fs/namei.c:2587
init_stat+0x72/0x13f fs/init.c:132
clean_path+0x74/0x24c init/initramfs.c:339
do_name+0x12d/0xc17 init/initramfs.c:371
write_buffer init/initramfs.c:457
unpack_to_rootfs+0x49a/0xd9e init/initramfs.c:510
do_populate_rootfs+0x57/0x40f init/initramfs.c:699
async_run_entry_fn+0x8f/0x400 kernel/async.c:127
process_one_work+0xb27/0x13e0 kernel/workqueue.c:2289
worker_thread+0x1076/0x1d60 kernel/workqueue.c:2436
kthread+0x31b/0x430 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 ??:?

Local variable seq created at:
walk_component+0x46/0x6c0 fs/namei.c:1981
lookup_last fs/namei.c:2445
path_lookupat+0x27d/0x6f0 fs/namei.c:2468

CPU: 0 PID: 10 Comm: kworker/u9:0 Tainted: G B
5.19.0-rc4-00059-gcf2d25715943-dirty #103
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
Workqueue: events_unbound async_run_entry_fn
=====================================================

What makes you think they are false positives? Is the scenario I
described above:

"""
In particular, if the call to lookup_fast() in walk_component()
returns NULL, and lookup_slow() returns a valid dentry, then the
`seq` and `inode` will remain uninitialized until the call to
step_into()
"""

impossible?

> 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;

--
Alexander Potapenko
Software Engineer

Google Germany GmbH
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