Re: [PATCH v5 bpf-next 0/5] bpf path iterator

[Song Liu <songliubraving@meta.com>]

> On Jul 9, 2025, at 3:24 PM, NeilBrown <neil@brown.name> wrote:
[...]
>> 
>> How should the user handle -ECHILD without LOOKUP_RCU flag? Say the
>> following code in landlocked:
>> 
>> /* Try RCU walk first */
>> err = vfs_walk_ancestors(path, ll_cb, data, LOOKUP_RCU);
>> 
>> if (err == -ECHILD) {
>> struct path walk_path = *path;
>> 
>> /* reset any data changed by the walk */
>> reset_data(data);
>> 
>> /* now do ref-walk */
>> err = vfs_walk_ancestors(&walk_path, ll_cb, data, 0);
>> }
>> 
>> Or do you mean vfs_walk_ancestors will never return -ECHILD?
>> Then we need vfs_walk_ancestors to call reset_data logic, right?
> 
> It isn't clear to me that vfs_walk_ancestors() needs to return anything.
> All the communication happens through walk_cb()
> 
> walk_cb() is called with a path, the data, and a "may_sleep" flag.
> If it needs to sleep but may_sleep is not set, it returns "-ECHILD"
> which causes the walk to restart and use refcounts.
> If it wants to stop, it returns 0.
> If it wants to continue, it returns 1.
> If it wants a reference to the path then it can use (new)
> vfs_legitimize_path() which might fail.
> If it wants a reference to the path and may_sleep is true, it can use
> path_get() which won't fail.
> 
> When returning -ECHILD (either because of a need to sleep or because
> vfs_legitimize_path() fails), walk_cb() would reset_data().

This might actually work. 

My only concern is with vfs_legitimize_path. It is probably safer if 
we only allow taking references with may_sleep==true, so that path_get
won’t fail. In this case, we will not need walk_cb() to call 
vfs_legitimize_path. If the user want a reference, the walk_cb will 
first return -ECHILD, and call path_get when may_sleep is true. 

Does this make sense? Did I miss any cases? 

Thanks,
Song