Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:
> On Wed, Jun 22, 2022 at 03:45:59PM -0400, Gabriel Krisman Bertazi wrote:
>
>> +static inline int generic_ci_d_revalidate(struct dentry *dentry,
>> + const struct qstr *name,
>> + unsigned int flags)
>> +{
>> + int is_creation = flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET);
>> +
>> + if (d_is_negative(dentry)) {
>> + const struct dentry *parent = READ_ONCE(dentry->d_parent);
>> + const struct inode *dir = READ_ONCE(parent->d_inode);
>> +
>> + if (dir && needs_casefold(dir)) {
>> + if (!d_is_casefold_lookup(dentry))
>> + return 0;
>
> In which conditions does that happen?
Hi Al,
This can happen right after a case-sensitive directory is converted to
case-insensitive. A previous case-sensitive lookup could have left a
negative dentry in the dcache that we need to reject, because it doesn't
have the same assurance of absence of all-variation of names as a
negative dentry created during a case-insensitive lookup.
>> + if (is_creation &&
>> + (dentry->d_name.len != name->len ||
>> + memcmp(dentry->d_name.name, name->name, name->len)))
>> + return 0;
>> + }
>> + }
>> + return 1;
>> +}
>
> Analysis of stability of ->d_name, please. It's *probably* safe, but
> the details are subtle and IMO should be accompanied by several asserts.
> E.g. "we never get LOOKUP_CREATE in op->intent without O_CREAT in op->open_flag
> for such and such reasons, and we verify that in such and such place"...
>
> A part of that would be "the call in lookup_dcache() can only get there
> with non-zero flags when coming from __lookup_hash(), and that has parent locked,
> stabilizing the name; the same goes for the call in __lookup_slow(), with the
> only call chain with possibly non-zero flags is through lookup_slow(), where we
> have the parent locked". However, lookup_fast() and lookup_open() have the
> flags come from nd->flags, and LOOKUP_CREATE can be found there in several areas.
> I _think_ we are guaranteed the parent locked in all such call chains, but that
> is definitely worth at least a comment.
Thanks for the example of the analysis what you are looking for here.
That will help me quite a bit. I wrote this code a while ago and I
don't recall the exact details. I will go through the code again and
send a new version with the detailed analysis.
--
Gabriel Krisman Bertazi
