[PATCH 3/3] proc: Remove the now unnecessary internal mount of proc

[Eric W. Biederman]

Christian Brauner <christian.brauner at ubuntu.com> writes:

> On Fri, Feb 28, 2020 at 02:19:22PM -0600, Eric W. Biederman wrote:
>> 
>> There remains no more code in the kernel using pids_ns->proc_mnt,
>> therefore remove it from the kernel.
>> 
>> The big benefit of this change is that one of the most error prone and
>> tricky parts of the pid namespace implementation, maintaining kernel
>> mounts of proc is removed.
>> 
>> In addition removing the unnecessary complexity of the kernel mount
>> fixes a regression that caused the proc mount options to be ignored.
>> Now that the initial mount of proc comes from userspace, those mount
>> options are again honored.  This fixes Android's usage of the proc
>> hidepid option.
>> 
>> Reported-by: Alistair Strachan <astrachan at google.com>
>> Fixes: e94591d0d90c ("proc: Convert proc_mount to use mount_ns.")
>> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
>> ---
>>  fs/proc/root.c                | 36 -----------------------------------
>
> What about proc_flush_task()? Or is this on top of your other series?

On top of the other series.  Which is why it is a reply to it.
That I have pushed into linux-next earlier today.

No one seems worried enough about that patchset to comment on v2 so I am
just going with it.

>>  include/linux/pid_namespace.h |  2 --
>>  include/linux/proc_ns.h       |  5 -----
>>  kernel/pid.c                  |  8 --------
>>  kernel/pid_namespace.c        |  7 -------
>>  5 files changed, 58 deletions(-)
>> 
>> diff --git a/fs/proc/root.c b/fs/proc/root.c
>> index 608233dfd29c..2633f10446c3 100644
>> --- a/fs/proc/root.c
>> +++ b/fs/proc/root.c
>> @@ -292,39 +292,3 @@ struct proc_dir_entry proc_root = {
>>  	.subdir		= RB_ROOT,
>>  	.name		= "/proc",
>>  };
>> -
>> -int pid_ns_prepare_proc(struct pid_namespace *ns)
>> -{
>> -	struct proc_fs_context *ctx;
>> -	struct fs_context *fc;
>> -	struct vfsmount *mnt;
>> -
>> -	fc = fs_context_for_mount(&proc_fs_type, SB_KERNMOUNT);
>> -	if (IS_ERR(fc))
>> -		return PTR_ERR(fc);
>> -
>> -	if (fc->user_ns != ns->user_ns) {
>> -		put_user_ns(fc->user_ns);
>> -		fc->user_ns = get_user_ns(ns->user_ns);
>> -	}
>> -
>> -	ctx = fc->fs_private;
>> -	if (ctx->pid_ns != ns) {
>> -		put_pid_ns(ctx->pid_ns);
>> -		get_pid_ns(ns);
>> -		ctx->pid_ns = ns;
>> -	}
>> -
>> -	mnt = fc_mount(fc);
>> -	put_fs_context(fc);
>> -	if (IS_ERR(mnt))
>> -		return PTR_ERR(mnt);
>> -
>> -	ns->proc_mnt = mnt;
>> -	return 0;
>> -}
>> -
>> -void pid_ns_release_proc(struct pid_namespace *ns)
>> -{
>> -	kern_unmount(ns->proc_mnt);
>> -}
>> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
>> index 2ed6af88794b..4956e362e55e 100644
>> --- a/include/linux/pid_namespace.h
>> +++ b/include/linux/pid_namespace.h
>> @@ -33,7 +33,6 @@ struct pid_namespace {
>>  	unsigned int level;
>>  	struct pid_namespace *parent;
>>  #ifdef CONFIG_PROC_FS
>> -	struct vfsmount *proc_mnt;
>>  	struct dentry *proc_self;
>>  	struct dentry *proc_thread_self;
>>  #endif
>> @@ -42,7 +41,6 @@ struct pid_namespace {
>>  #endif
>>  	struct user_namespace *user_ns;
>>  	struct ucounts *ucounts;
>> -	struct work_struct proc_work;
>>  	kgid_t pid_gid;
>>  	int hide_pid;
>>  	int reboot;	/* group exit code if this pidns was rebooted */
>> diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
>> index 4626b1ac3b6c..e1106a077c1a 100644
>> --- a/include/linux/proc_ns.h
>> +++ b/include/linux/proc_ns.h
>> @@ -50,16 +50,11 @@ enum {
>>  
>>  #ifdef CONFIG_PROC_FS
>>  
>> -extern int pid_ns_prepare_proc(struct pid_namespace *ns);
>> -extern void pid_ns_release_proc(struct pid_namespace *ns);
>>  extern int proc_alloc_inum(unsigned int *pino);
>>  extern void proc_free_inum(unsigned int inum);
>>  
>>  #else /* CONFIG_PROC_FS */
>>  
>> -static inline int pid_ns_prepare_proc(struct pid_namespace *ns) { return 0; }
>> -static inline void pid_ns_release_proc(struct pid_namespace *ns) {}
>> -
>>  static inline int proc_alloc_inum(unsigned int *inum)
>>  {
>>  	*inum = 1;
>> diff --git a/kernel/pid.c b/kernel/pid.c
>> index ca08d6a3aa77..60820e72634c 100644
>> --- a/kernel/pid.c
>> +++ b/kernel/pid.c
>> @@ -144,9 +144,6 @@ void free_pid(struct pid *pid)
>>  			/* Handle a fork failure of the first process */
>>  			WARN_ON(ns->child_reaper);
>>  			ns->pid_allocated = 0;
>> -			/* fall through */
>> -		case 0:
>> -			schedule_work(&ns->proc_work);
>>  			break;
>>  		}
>>  
>> @@ -247,11 +244,6 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>>  		tmp = tmp->parent;
>>  	}
>>  
>> -	if (unlikely(is_child_reaper(pid))) {
>> -		if (pid_ns_prepare_proc(ns))
>> -			goto out_free;
>> -	}
>> -
>>  	get_pid_ns(ns);
>>  	refcount_set(&pid->count, 1);
>>  	for (type = 0; type < PIDTYPE_MAX; ++type)
>> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
>> index d40017e79ebe..318fcc6ba301 100644
>> --- a/kernel/pid_namespace.c
>> +++ b/kernel/pid_namespace.c
>> @@ -57,12 +57,6 @@ static struct kmem_cache *create_pid_cachep(unsigned int level)
>>  	return READ_ONCE(*pkc);
>>  }
>>  
>> -static void proc_cleanup_work(struct work_struct *work)
>
> There's a comment in kernel/pid_namespace.c that references
> proc_cleanup_work(). Can you please remove that as well?

Good catch.  It isn't immediately obvious to me how that sentence
needs to be updated but it should be done.

Eric


>> -{
>> -	struct pid_namespace *ns = container_of(work, struct pid_namespace, proc_work);
>> -	pid_ns_release_proc(ns);
>> -}
>> -
>>  static struct ucounts *inc_pid_namespaces(struct user_namespace *ns)
>>  {
>>  	return inc_ucount(ns, current_euid(), UCOUNT_PID_NAMESPACES);
>> @@ -114,7 +108,6 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
>>  	ns->user_ns = get_user_ns(user_ns);
>>  	ns->ucounts = ucounts;
>>  	ns->pid_allocated = PIDNS_ADDING;
>> -	INIT_WORK(&ns->proc_work, proc_cleanup_work);
>>  
>>  	return ns;
>>  
>> -- 
>> 2.25.0
>>