Re: [RFC PATCH] kmod: add ability to swap root in usermode helper

From: Oleg Nesterov
Date: Mon May 20 2013 - 11:13:42 EST

On 05/20, Stanislav Kinsbursky wrote:
> 20.05.2013 17:57, Oleg Nesterov ÐÐÑÐÑ:
>> Why do we need the new member/arguments?
>>> @@ -215,6 +216,9 @@ static int ____call_usermodehelper(void *data)
>>> */
>>> set_user_nice(current, 0);
>>> + if (sub_info->root)
>>> + set_fs_root(current->fs, sub_info->root);
>> Can't subprocess_info->init() do this? You can pass root as ->data.
>> IOW, unless I missed something, nfs can do this without any changes
>> in kmod.c.
>> Oleg.
> Thanks for the comment.
> Yes, it definitely can. But, NFS server in the the only place. Usermode helper in
> called from NFS client code and thus the same functionality is required there as well.

Not sure I understand... OK, and why NFS client can't use the same

> Moreover, set_fs_root() is not exported.

Then it should be exported, I think ;)

Or you can export the new helper.

> And adding an ability of a root swap to usermode helper looks quite logical. At least from the
> "containers" point of view, which usually have it's own root.

But it is not logical to uglify the code, imho.

OK, why nfs can't simply use this code

static int umh_set_fs_root(struct subprocess_info *info, struct cred *new)
set_fs_root(current->fs, sub_info->data);
return 0;

int call_usermodehelper_root(char *path, char **argv, char **envp, int wait,
struct path *root)

struct subprocess_info *info;

info = call_usermodehelper_setup(path, argv, envp, gfp_mask,
umh_set_fs_root, NULL, root);
if (info == NULL)
return -ENOMEM;
return call_usermodehelper_exec(info, wait);

? Why do you want to add the new member, the new arguments, the new helpers?


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