On 10/13/2016 01:26 PM, Al Viro wrote:
On Thu, Oct 13, 2016 at 01:09:04PM -0700, Christoph Hellwig wrote:
On Thu, Oct 13, 2016 at 07:58:51PM +0000, Vineeth Remanan Pillai wrote:
filename_lookup used to return success for non-existing file when called
with LOOKUP_PARENT flag. This behaviour was changed with
commit 8bcb77fabd7c ("namei: split off filename_lookupat()

The above patch split parent lookup functionality to a different function
filename_parentat and changed all calls to filename_lookup(LOOKUP_PARENT)
to the new function filename_parentat. But functions like kern_path which
passed the flags directly to filename_lookup regressed due to this.

This patch aims to fix the regressed behaviour by calling
filename_parentat from filename_lookup if the flags contain LOOKUP_PARENT.
What callers shows te problems?  That's probaby were the fix need to got
in, and even if not that's still part of a good bug report.
Out-of-tree, at a guess...  Incidentally, since filename_lookup() is not
exported, it's probably kern_path() that gets used wherever it is.
Depending on the details of what's being attempted,  kern_path_locked()
might or might not be the right primitive to use, but I would probably
start with checking if that's what the code in question really wants.

        // kernel_string is an kernel pointer to NUL-terminated array of char

        struct path path;
        struct dentry *dentry;

        dentry = kern_path_locked(kernel_string, &path);
        if (IS_ERR(dentry))
                // failed while getting the parent, or not a regular last
                // component (/, ., .., <something>/., <something>/..)
                sod off // no cleanup needed

        // now path contains the resolved parent and dentry points to the
        // dentry of child, possibly negative; the last component of the
        // name can be determined from dentry->d_name.  Parent directory
        // is locked, making sure that directory entry won't be changed
        // until you are done.

        if (d_is_really_negative(dentry)) {
                // parent exists, but child doesn't
        } else {
                // child exists

        // clean up: drop dentry, unlock parent, drop dentry/vfsmount of parent

Yes, the use case is out-of-tree and the code snippet above depicts the use .
Since kern_path_locked is also not exported, out-of-tree code used kern_path
for the existence check for directories.

One reference about this issue can be seen here.


We also have a customer who complained about this functionality change.

I understand that there has been no API promises been made to this API. But since this is an exported function, the change in function could cause break in out-of-tree kernel code. I will rephrase the commit message to say "change in functionality" instead of regression

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