Quoting Oleg Nesterov (o...@redhat.com):
> 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 <o...@redhat.com>
> Acked-by: Roland McGrath <rol...@redhat.com>
Yes, please.
Acked-by: Serge Hallyn <serge.hal...@canonical.com>
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(¤t->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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