Re: [PATCH 1/1][3rd resend] sys_unshare: remove the deadCLONE_THREAD/SIGHAND/VM code

[Serge E. Hallyn]

Quoting Oleg Nesterov (oleg@xxxxxxxxxx):
> Cleanup: kill the dead code which does nothing but complicates the code
> and confuses the reader.
> 
> sys_unshare(CLONE_THREAD/SIGHAND/VM) is not really implemented, and I doubt
> very much it will ever work. At least, nobody even tried since the original
> "unshare system call -v5: system call handler function" commit
> 99d1419d96d7df9cfa56bc977810be831bd5ef64 was applied more than 4 years ago.
> 
> And the code is not consistent. unshare_thread() always fails unconditionally,
> while unshare_sighand() and unshare_vm() pretend to work if there is nothing
> to unshare.
> 
> Remove unshare_thread(), unshare_sighand(), unshare_vm() helpers and related
> variables and add a simple CLONE_THREAD | CLONE_SIGHAND| CLONE_VM check into
> check_unshare_flags().
> 
> Also, move the "CLONE_NEWNS needs CLONE_FS" check from check_unshare_flags()
> to sys_unshare(). This looks more consistent and matches the similar
> do_sysvsem check in sys_unshare().
> 
> Note: with or without this patch "atomic_read(mm->mm_users) > 1" can give
> a false positive due to get_task_mm().
> 
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Acked-by: Roland McGrath <roland@xxxxxxxxxx>

Yes, please.

Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>

thanks,
-serge


> ---
> 
>  kernel/fork.c |  123 +++++++++++-----------------------------------------------
>  1 file changed, 25 insertions(+), 98 deletions(-)
> 
> --- 2.6.37/kernel/fork.c~unshare-killcrap	2010-11-05 18:03:28.000000000 +0100
> +++ 2.6.37/kernel/fork.c	2010-11-05 18:09:52.000000000 +0100
> @@ -1522,38 +1522,24 @@ void __init proc_caches_init(void)
>  }
>  
>  /*
> - * Check constraints on flags passed to the unshare system call and
> - * force unsharing of additional process context as appropriate.
> + * Check constraints on flags passed to the unshare system call.
>   */
> -static void check_unshare_flags(unsigned long *flags_ptr)
> +static int check_unshare_flags(unsigned long unshare_flags)
>  {
> +	if (unshare_flags & ~(CLONE_THREAD|CLONE_FS|CLONE_NEWNS|CLONE_SIGHAND|
> +				CLONE_VM|CLONE_FILES|CLONE_SYSVSEM|
> +				CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWNET))
> +		return -EINVAL;
>  	/*
> -	 * If unsharing a thread from a thread group, must also
> -	 * unshare vm.
> -	 */
> -	if (*flags_ptr & CLONE_THREAD)
> -		*flags_ptr |= CLONE_VM;
> -
> -	/*
> -	 * If unsharing vm, must also unshare signal handlers.
> -	 */
> -	if (*flags_ptr & CLONE_VM)
> -		*flags_ptr |= CLONE_SIGHAND;
> -
> -	/*
> -	 * If unsharing namespace, must also unshare filesystem information.
> +	 * Not implemented, but pretend it works if there is nothing to
> +	 * unshare. Note that unsharing CLONE_THREAD or CLONE_SIGHAND
> +	 * needs to unshare vm.
>  	 */
> -	if (*flags_ptr & CLONE_NEWNS)
> -		*flags_ptr |= CLONE_FS;
> -}
> -
> -/*
> - * Unsharing of tasks created with CLONE_THREAD is not supported yet
> - */
> -static int unshare_thread(unsigned long unshare_flags)
> -{
> -	if (unshare_flags & CLONE_THREAD)
> -		return -EINVAL;
> +	if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
> +		/* FIXME: get_task_mm() increments ->mm_users */
> +		if (atomic_read(&current->mm->mm_users) > 1)
> +			return -EINVAL;
> +	}
>  
>  	return 0;
>  }
> @@ -1580,34 +1566,6 @@ static int unshare_fs(unsigned long unsh
>  }
>  
>  /*
> - * Unsharing of sighand is not supported yet
> - */
> -static int unshare_sighand(unsigned long unshare_flags, struct sighand_struct **new_sighp)
> -{
> -	struct sighand_struct *sigh = current->sighand;
> -
> -	if ((unshare_flags & CLONE_SIGHAND) && atomic_read(&sigh->count) > 1)
> -		return -EINVAL;
> -	else
> -		return 0;
> -}
> -
> -/*
> - * Unshare vm if it is being shared
> - */
> -static int unshare_vm(unsigned long unshare_flags, struct mm_struct **new_mmp)
> -{
> -	struct mm_struct *mm = current->mm;
> -
> -	if ((unshare_flags & CLONE_VM) &&
> -	    (mm && atomic_read(&mm->mm_users) > 1)) {
> -		return -EINVAL;
> -	}
> -
> -	return 0;
> -}
> -
> -/*
>   * Unshare file descriptor table if it is being shared
>   */
>  static int unshare_fd(unsigned long unshare_flags, struct files_struct **new_fdp)
> @@ -1635,45 +1593,37 @@ static int unshare_fd(unsigned long unsh
>   */
>  SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
>  {
> -	int err = 0;
>  	struct fs_struct *fs, *new_fs = NULL;
> -	struct sighand_struct *new_sigh = NULL;
> -	struct mm_struct *mm, *new_mm = NULL, *active_mm = NULL;
>  	struct files_struct *fd, *new_fd = NULL;
>  	struct nsproxy *new_nsproxy = NULL;
>  	int do_sysvsem = 0;
> +	int err;
>  
> -	check_unshare_flags(&unshare_flags);
> -
> -	/* Return -EINVAL for all unsupported flags */
> -	err = -EINVAL;
> -	if (unshare_flags & ~(CLONE_THREAD|CLONE_FS|CLONE_NEWNS|CLONE_SIGHAND|
> -				CLONE_VM|CLONE_FILES|CLONE_SYSVSEM|
> -				CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWNET))
> +	err = check_unshare_flags(unshare_flags);
> +	if (err)
>  		goto bad_unshare_out;
>  
>  	/*
> +	 * If unsharing namespace, must also unshare filesystem information.
> +	 */
> +	if (unshare_flags & CLONE_NEWNS)
> +		unshare_flags |= CLONE_FS;
> +	/*
>  	 * CLONE_NEWIPC must also detach from the undolist: after switching
>  	 * to a new ipc namespace, the semaphore arrays from the old
>  	 * namespace are unreachable.
>  	 */
>  	if (unshare_flags & (CLONE_NEWIPC|CLONE_SYSVSEM))
>  		do_sysvsem = 1;
> -	if ((err = unshare_thread(unshare_flags)))
> -		goto bad_unshare_out;
>  	if ((err = unshare_fs(unshare_flags, &new_fs)))
> -		goto bad_unshare_cleanup_thread;
> -	if ((err = unshare_sighand(unshare_flags, &new_sigh)))
> -		goto bad_unshare_cleanup_fs;
> -	if ((err = unshare_vm(unshare_flags, &new_mm)))
> -		goto bad_unshare_cleanup_sigh;
> +		goto bad_unshare_out;
>  	if ((err = unshare_fd(unshare_flags, &new_fd)))
> -		goto bad_unshare_cleanup_vm;
> +		goto bad_unshare_cleanup_fs;
>  	if ((err = unshare_nsproxy_namespaces(unshare_flags, &new_nsproxy,
>  			new_fs)))
>  		goto bad_unshare_cleanup_fd;
>  
> -	if (new_fs ||  new_mm || new_fd || do_sysvsem || new_nsproxy) {
> +	if (new_fs || new_fd || do_sysvsem || new_nsproxy) {
>  		if (do_sysvsem) {
>  			/*
>  			 * CLONE_SYSVSEM is equivalent to sys_exit().
> @@ -1699,19 +1649,6 @@ SYSCALL_DEFINE1(unshare, unsigned long, 
>  			spin_unlock(&fs->lock);
>  		}
>  
> -		if (new_mm) {
> -			mm = current->mm;
> -			active_mm = current->active_mm;
> -			current->mm = new_mm;
> -			current->active_mm = new_mm;
> -			if (current->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
> -				atomic_dec(&mm->oom_disable_count);
> -				atomic_inc(&new_mm->oom_disable_count);
> -			}
> -			activate_mm(active_mm, new_mm);
> -			new_mm = mm;
> -		}
> -
>  		if (new_fd) {
>  			fd = current->files;
>  			current->files = new_fd;
> @@ -1728,20 +1665,10 @@ bad_unshare_cleanup_fd:
>  	if (new_fd)
>  		put_files_struct(new_fd);
>  
> -bad_unshare_cleanup_vm:
> -	if (new_mm)
> -		mmput(new_mm);
> -
> -bad_unshare_cleanup_sigh:
> -	if (new_sigh)
> -		if (atomic_dec_and_test(&new_sigh->count))
> -			kmem_cache_free(sighand_cachep, new_sigh);
> -
>  bad_unshare_cleanup_fs:
>  	if (new_fs)
>  		free_fs_struct(new_fs);
>  
> -bad_unshare_cleanup_thread:
>  bad_unshare_out:
>  	return err;
>  }
> 
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