On Sat, May 30, 2020 at 02:17:02AM -0400, Gabriel Krisman Bertazi wrote:
- /*
* If the dentry name is stored in-line, then it may be concurrently* modified by a rename. If this happens, the VFS will eventually retry* the lookup, so it doesn't matter what ->d_compare() returns.* However, it's unsafe to call utf8_strncasecmp() with an unstable* string. Therefore, we have to copy the name into a temporary buffer.*/- if (len <= DNAME_INLINE_LEN - 1) {
unsigned int i;for (i = 0; i < len; i++)strbuf[i] = READ_ONCE(str[i]);strbuf[len] = 0;qstr.name = strbuf;- }
Could we avoid this if the casefolded version were cached in the dentry? Then we could use utf8_strncasecmp_folded which would be safe. Would this be acceptable for vfs?
The VFS assumes that each dentry has one name, the one in d_name. That's what it passes to ->d_compare(), and that's what it updates in __d_move().
So while ext4 and f2fs could put the casefolded name in ->d_fsdata, ->d_compare() wouldn't actually have access to it (unless we added d_fsdata as a parameter to ->d_compare()). Also, the casefolded name would get outdated when __d_move() changes d_name.
We could instead make d_name always be the casefolded name. I'm not sure that would be possible, though. For one, I don't think ->lookup() is allowed to just change the dentry name. It would also make getcwd(), /proc/*/fd/, etc. always show casefolded names, which could be problematic. And probably other issues I can't think of off the top of my head.
- Eric