by Gabriel Krisman Bertazi

[permalink] [raw]

Subject: Re: [PATCH v5 04/12] fscrypt: Drop d_revalidate for valid dentries during lookup

Christian Brauner <[email protected]> writes:

> On Fri, Feb 02, 2024 at 11:50:07AM -0300, Gabriel Krisman Bertazi wrote:
>> Eric Biggers <[email protected]> writes:
>>
>> > On Wed, Jan 31, 2024 at 03:35:40PM -0300, Gabriel Krisman Bertazi wrote:
>> >> Eric Biggers <[email protected]> writes:
>> >>
>> >> > On Mon, Jan 29, 2024 at 05:43:22PM -0300, Gabriel Krisman Bertazi wrote:
>> >> >> Unencrypted and encrypted-dentries where the key is available don't need
>> >> >> to be revalidated with regards to fscrypt, since they don't go stale
>> >> >> from under VFS and the key cannot be removed for the encrypted case
>> >> >> without evicting the dentry. Mark them with d_set_always_valid, to
>> >> >
>> >> > "d_set_always_valid" doesn't appear in the diff itself.
>> >> >
>> >> >> diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
>> >> >> index 4aaf847955c0..a22997b9f35c 100644
>> >> >> --- a/include/linux/fscrypt.h
>> >> >> +++ b/include/linux/fscrypt.h
>> >> >> @@ -942,11 +942,22 @@ static inline int fscrypt_prepare_rename(struct inode *old_dir,
>> >> >> static inline void fscrypt_prepare_lookup_dentry(struct dentry *dentry,
>> >> >> bool is_nokey_name)
>> >> >> {
>> >> >> - if (is_nokey_name) {
>> >> >> - spin_lock(&dentry->d_lock);
>> >> >> + spin_lock(&dentry->d_lock);
>> >> >> +
>> >> >> + if (is_nokey_name)
>> >> >> dentry->d_flags |= DCACHE_NOKEY_NAME;
>> >> >> - spin_unlock(&dentry->d_lock);
>> >> >> + else if (dentry->d_flags & DCACHE_OP_REVALIDATE &&
>> >> >> + dentry->d_op->d_revalidate == fscrypt_d_revalidate) {
>> >> >> + /*
>> >> >> + * Unencrypted dentries and encrypted dentries where the
>> >> >> + * key is available are always valid from fscrypt
>> >> >> + * perspective. Avoid the cost of calling
>> >> >> + * fscrypt_d_revalidate unnecessarily.
>> >> >> + */
>> >> >> + dentry->d_flags &= ~DCACHE_OP_REVALIDATE;
>> >> >> }
>> >> >> +
>> >> >> + spin_unlock(&dentry->d_lock);
>> >> >
>> >> > This makes lookups in unencrypted directories start doing the
>> >> > spin_lock/spin_unlock pair. Is that really necessary?
>> >> >
>> >> > These changes also make the inline function fscrypt_prepare_lookup() very long
>> >> > (when including the fscrypt_prepare_lookup_dentry() that's inlined into it).
>> >> > The rule that I'm trying to follow is that to the extent that the fscrypt helper
>> >> > functions are inlined, the inline part should be a fast path for unencrypted
>> >> > directories. Encrypted directories should be handled out-of-line.
>> >> >
>> >> > So looking at the original fscrypt_prepare_lookup():
>> >> >
>> >> > static inline int fscrypt_prepare_lookup(struct inode *dir,
>> >> > struct dentry *dentry,
>> >> > struct fscrypt_name *fname)
>> >> > {
>> >> > if (IS_ENCRYPTED(dir))
>> >> > return __fscrypt_prepare_lookup(dir, dentry, fname);
>> >> >
>> >> > memset(fname, 0, sizeof(*fname));
>> >> > fname->usr_fname = &dentry->d_name;
>> >> > fname->disk_name.name = (unsigned char *)dentry->d_name.name;
>> >> > fname->disk_name.len = dentry->d_name.len;
>> >> > return 0;
>> >> > }
>> >> >
>> >> > If you could just add the DCACHE_OP_REVALIDATE clearing for dentries in
>> >> > unencrypted directories just before the "return 0;", hopefully without the
>> >> > spinlock, that would be good. Yes, that does mean that
>> >> > __fscrypt_prepare_lookup() will have to handle it too, for the case of dentries
>> >> > in encrypted directories, but that seems okay.
>> >>
>> >> ok, will do. IIUC, we might be able to do without the d_lock
>> >> provided there is no store tearing.
>> >>
>> >> But what was the reason you need the d_lock to set DCACHE_NOKEY_NAME
>> >> during lookup? Is there a race with parallel lookup setting d_flag that
>> >> I couldn't find? Or is it another reason?
>> >
>> > d_flags is documented to be protected by d_lock. So for setting
>> > DCACHE_NOKEY_NAME, fs/crypto/ just does the safe thing of taking d_lock. I
>> > never really looked into whether the lock can be skipped there (i.e., whether
>> > anything else can change d_flags while ->lookup is running), since this code
>> > only ran for no-key names, for which performance isn't really important.
>>
>> Yes, I was looking for the actual race that could happen here, and
>> couldn't find one. As far as I understand it, the only thing that could
>> see the dentry during a lookup would be a parallel lookup, but those
>> will be held waiting for completion in d_alloc_parallel, and won't touch
>> d_flags. Currently, right after this code, we call d_set_d_op() in
>> generic_set_encrypted_ci_d_ops(), which will happily write d_flags without
>> the d_lock. If this is a problem here, we have a problem there.
>>
>> What I really don't want to do is keep the lock for DCACHE_NOKEY_NAME,
>> but drop it for unsetting DCACHE_OP_REVALIDATE right in the same field,
>> without a good reason. I get the argument that unencrypted
>> dentries are a much hotter path and we care more. But the locking rules
>> of ->d_lookup don't change for both cases.
>
> Even if it were to work in this case I don't think it is generally safe
> to do. But also, for DCACHE_OP_REVALIDATE afaict this is an
> optimization. Why don't you simply accept the raciness, just like fuse
> does in fuse_dentry_settime(), check for DCACHE_OP_REVALIDATE locklessly
> and only take the lock if that thing is set?

That sounds extremely reasonable. I will follow that approach!

Thanks,

--
Gabriel Krisman Bertazi