Messages in this thread | | | From | Peter Collingbourne <> | Date | Thu, 9 Dec 2021 14:13:06 -0800 | Subject | Re: [PATCH v3 2/6] uaccess-buffer: add core code |
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On Wed, Dec 8, 2021 at 2:21 AM Dmitry Vyukov <dvyukov@google.com> wrote: > > On Wed, 8 Dec 2021 at 05:48, Peter Collingbourne <pcc@google.com> wrote: > > > > Add the core code to support uaccess logging. Subsequent patches will > > hook this up to the arch-specific kernel entry and exit code for > > certain architectures. > > > > Link: https://linux-review.googlesource.com/id/I6581765646501a5631b281d670903945ebadc57d > > Signed-off-by: Peter Collingbourne <pcc@google.com> > > --- > > v3: > > - performance optimizations for entry/exit code > > - don't use kcur == NULL to mean overflow > > - fix potential double free in clone() > > - don't allocate a new kernel-side uaccess buffer for each syscall > > - fix uaccess buffer leak on exit > > - fix some sparse warnings > > > > v2: > > - New interface that avoids multiple syscalls per real syscall and > > is arch-generic > > - Avoid logging uaccesses done by BPF programs > > - Add documentation > > - Split up into multiple patches > > - Various code moves, renames etc as requested by Marco > > > > fs/exec.c | 3 + > > include/linux/instrumented-uaccess.h | 6 +- > > include/linux/sched.h | 5 ++ > > include/linux/uaccess-buffer-info.h | 46 ++++++++++ > > include/linux/uaccess-buffer.h | 112 +++++++++++++++++++++++ > > include/uapi/linux/prctl.h | 3 + > > include/uapi/linux/uaccess-buffer.h | 27 ++++++ > > kernel/Makefile | 1 + > > kernel/bpf/helpers.c | 7 +- > > kernel/fork.c | 4 + > > kernel/signal.c | 4 +- > > kernel/sys.c | 6 ++ > > kernel/uaccess-buffer.c | 129 +++++++++++++++++++++++++++ > > 13 files changed, 350 insertions(+), 3 deletions(-) > > create mode 100644 include/linux/uaccess-buffer-info.h > > create mode 100644 include/linux/uaccess-buffer.h > > create mode 100644 include/uapi/linux/uaccess-buffer.h > > create mode 100644 kernel/uaccess-buffer.c > > > > diff --git a/fs/exec.c b/fs/exec.c > > index 537d92c41105..c9975e790f30 100644 > > --- a/fs/exec.c > > +++ b/fs/exec.c > > @@ -65,6 +65,7 @@ > > #include <linux/vmalloc.h> > > #include <linux/io_uring.h> > > #include <linux/syscall_user_dispatch.h> > > +#include <linux/uaccess-buffer.h> > > > > #include <linux/uaccess.h> > > #include <asm/mmu_context.h> > > @@ -1313,6 +1314,8 @@ int begin_new_exec(struct linux_binprm * bprm) > > me->personality &= ~bprm->per_clear; > > > > clear_syscall_work_syscall_user_dispatch(me); > > + uaccess_buffer_set_descriptor_addr_addr(0); > > + uaccess_buffer_free(current); > > > > /* > > * We have to apply CLOEXEC before we change whether the process is > > diff --git a/include/linux/instrumented-uaccess.h b/include/linux/instrumented-uaccess.h > > index ece549088e50..b967f4436d15 100644 > > --- a/include/linux/instrumented-uaccess.h > > +++ b/include/linux/instrumented-uaccess.h > > @@ -2,7 +2,8 @@ > > > > /* > > * This header provides generic wrappers for memory access instrumentation for > > - * uaccess routines that the compiler cannot emit for: KASAN, KCSAN. > > + * uaccess routines that the compiler cannot emit for: KASAN, KCSAN, > > + * uaccess buffers. > > */ > > #ifndef _LINUX_INSTRUMENTED_UACCESS_H > > #define _LINUX_INSTRUMENTED_UACCESS_H > > @@ -11,6 +12,7 @@ > > #include <linux/kasan-checks.h> > > #include <linux/kcsan-checks.h> > > #include <linux/types.h> > > +#include <linux/uaccess-buffer.h> > > > > /** > > * instrument_copy_to_user - instrument reads of copy_to_user > > @@ -27,6 +29,7 @@ instrument_copy_to_user(void __user *to, const void *from, unsigned long n) > > { > > kasan_check_read(from, n); > > kcsan_check_read(from, n); > > + uaccess_buffer_log_write(to, n); > > } > > > > /** > > @@ -44,6 +47,7 @@ instrument_copy_from_user(const void *to, const void __user *from, unsigned long > > { > > kasan_check_write(to, n); > > kcsan_check_write(to, n); > > + uaccess_buffer_log_read(from, n); > > } > > > > #endif /* _LINUX_INSTRUMENTED_UACCESS_H */ > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index 78c351e35fec..7c5278d7b57d 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -34,6 +34,7 @@ > > #include <linux/rseq.h> > > #include <linux/seqlock.h> > > #include <linux/kcsan.h> > > +#include <linux/uaccess-buffer-info.h> > > #include <asm/kmap_size.h> > > > > /* task_struct member predeclarations (sorted alphabetically): */ > > @@ -1484,6 +1485,10 @@ struct task_struct { > > struct callback_head l1d_flush_kill; > > #endif > > > > +#ifdef CONFIG_HAVE_ARCH_UACCESS_BUFFER > > + struct uaccess_buffer_info uaccess_buffer; > > +#endif > > + > > /* > > * New fields for task_struct should be added above here, so that > > * they are included in the randomized portion of task_struct. > > diff --git a/include/linux/uaccess-buffer-info.h b/include/linux/uaccess-buffer-info.h > > new file mode 100644 > > index 000000000000..15e2d8f7c074 > > --- /dev/null > > +++ b/include/linux/uaccess-buffer-info.h > > @@ -0,0 +1,46 @@ > > +/* SPDX-License-Identifier: GPL-2.0 */ > > +#ifndef _LINUX_UACCESS_BUFFER_INFO_H > > +#define _LINUX_UACCESS_BUFFER_INFO_H > > + > > +#ifdef CONFIG_HAVE_ARCH_UACCESS_BUFFER > > + > > +struct uaccess_buffer_info { > > + /* > > + * The pointer to pointer to struct uaccess_descriptor. This is the > > + * value controlled by prctl(PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR). > > + */ > > + struct uaccess_descriptor __user *__user *desc_ptr_ptr; > > + > > + /* > > + * The pointer to struct uaccess_descriptor read at syscall entry time. > > + */ > > + struct uaccess_descriptor __user *desc_ptr; > > + > > + /* > > + * A pointer to the kernel's temporary copy of the uaccess log for the > > + * current syscall. We log to a kernel buffer in order to avoid leaking > > + * timing information to userspace. > > + */ > > + struct uaccess_buffer_entry *kbegin; > > + > > + /* > > + * The position of the next uaccess buffer entry for the current > > + * syscall, or NULL if we are not logging the current syscall. > > + */ > > + struct uaccess_buffer_entry *kcur; > > + > > + /* > > + * A pointer to the end of the kernel's uaccess log. > > + */ > > + struct uaccess_buffer_entry *kend; > > + > > + /* > > + * The pointer to the userspace uaccess log, as read from the > > + * struct uaccess_descriptor. > > + */ > > + struct uaccess_buffer_entry __user *ubegin; > > +}; > > + > > +#endif > > + > > +#endif /* _LINUX_UACCESS_BUFFER_INFO_H */ > > diff --git a/include/linux/uaccess-buffer.h b/include/linux/uaccess-buffer.h > > new file mode 100644 > > index 000000000000..f2f46db274f3 > > --- /dev/null > > +++ b/include/linux/uaccess-buffer.h > > @@ -0,0 +1,112 @@ > > +/* SPDX-License-Identifier: GPL-2.0 */ > > +#ifndef _LINUX_UACCESS_BUFFER_H > > +#define _LINUX_UACCESS_BUFFER_H > > + > > +#include <linux/sched.h> > > +#include <uapi/linux/uaccess-buffer.h> > > + > > +#include <asm-generic/errno-base.h> > > + > > +#ifdef CONFIG_HAVE_ARCH_UACCESS_BUFFER > > + > > +static inline bool uaccess_buffer_maybe_blocked(struct task_struct *tsk) > > +{ > > + return test_task_syscall_work(tsk, UACCESS_BUFFER_ENTRY); > > +} > > + > > +void __uaccess_buffer_syscall_entry(void); > > +static inline void uaccess_buffer_syscall_entry(void) > > +{ > > + __uaccess_buffer_syscall_entry(); > > +} > > + > > +void __uaccess_buffer_syscall_exit(void); > > +static inline void uaccess_buffer_syscall_exit(void) > > +{ > > + __uaccess_buffer_syscall_exit(); > > +} > > + > > +bool __uaccess_buffer_pre_exit_loop(void); > > +static inline bool uaccess_buffer_pre_exit_loop(void) > > +{ > > + if (!test_syscall_work(UACCESS_BUFFER_ENTRY)) > > + return false; > > + return __uaccess_buffer_pre_exit_loop(); > > +} > > + > > +void __uaccess_buffer_post_exit_loop(void); > > +static inline void uaccess_buffer_post_exit_loop(bool pending) > > +{ > > + if (pending) > > + __uaccess_buffer_post_exit_loop(); > > +} > > + > > +static inline int uaccess_buffer_set_descriptor_addr_addr(unsigned long addr) > > I would move the implementation to .c file. It's a rare path.
Done.
> > +{ > > + current->uaccess_buffer.desc_ptr_ptr = > > + (struct uaccess_descriptor __user * __user *)addr; > > + if (addr) > > + set_syscall_work(UACCESS_BUFFER_ENTRY); > > + else > > + clear_syscall_work(UACCESS_BUFFER_ENTRY); > > + return 0; > > +} > > + > > +size_t copy_from_user_nolog(void *to, const void __user *from, size_t len); > > + > > +void uaccess_buffer_free(struct task_struct *tsk); > > + > > +void __uaccess_buffer_log_read(const void __user *from, unsigned long n); > > +static inline void uaccess_buffer_log_read(const void __user *from, unsigned long n) > > +{ > > + if (unlikely(test_syscall_work(UACCESS_BUFFER_EXIT))) > > UACCESS_BUFFER_EXIT is only defined in future patches, so this won't compile.
Right, but there's no way for CONFIG_UACCESS_BUFFER to be defined at this point, so this won't compile anyway. We define the constants for this (TIF_UACCESS_BUFFER_EXIT for arm64, SYSCALL_WORK_UACCESS_BUFFER_EXIT for GENERIC_ENTRY) at the same time as we enable the respective architecture support.
> > > + __uaccess_buffer_log_read(from, n); > > +} > > + > > +void __uaccess_buffer_log_write(void __user *to, unsigned long n); > > +static inline void uaccess_buffer_log_write(void __user *to, unsigned long n) > > +{ > > + if (unlikely(test_syscall_work(UACCESS_BUFFER_EXIT))) > > + __uaccess_buffer_log_write(to, n); > > +} > > + > > +#else > > + > > +static inline bool uaccess_buffer_maybe_blocked(struct task_struct *tsk) > > +{ > > + return false; > > +} > > +static inline void uaccess_buffer_syscall_entry(void) > > +{ > > +} > > +static inline void uaccess_buffer_syscall_exit(void) > > +{ > > +} > > +static inline bool uaccess_buffer_pre_exit_loop(void) > > +{ > > + return false; > > +} > > +static inline void uaccess_buffer_post_exit_loop(bool pending) > > +{ > > +} > > +static inline int uaccess_buffer_set_descriptor_addr_addr(unsigned long addr) > > +{ > > + return -EINVAL; > > +} > > +static inline void uaccess_buffer_free(struct task_struct *tsk) > > +{ > > +} > > + > > +#define copy_from_user_nolog(to, from, len) copy_from_user(to, from, len) > > + > > +static inline void uaccess_buffer_log_read(const void __user *from, > > + unsigned long n) > > +{ > > +} > > +static inline void uaccess_buffer_log_write(void __user *to, unsigned long n) > > +{ > > +} > > + > > +#endif > > + > > +#endif /* _LINUX_UACCESS_BUFFER_H */ > > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h > > index bb73e9a0b24f..74b37469c7b3 100644 > > --- a/include/uapi/linux/prctl.h > > +++ b/include/uapi/linux/prctl.h > > @@ -272,4 +272,7 @@ struct prctl_mm_map { > > # define PR_SCHED_CORE_SCOPE_THREAD_GROUP 1 > > # define PR_SCHED_CORE_SCOPE_PROCESS_GROUP 2 > > > > +/* Configure uaccess logging feature */ > > +#define PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR 63 > > + > > #endif /* _LINUX_PRCTL_H */ > > diff --git a/include/uapi/linux/uaccess-buffer.h b/include/uapi/linux/uaccess-buffer.h > > new file mode 100644 > > index 000000000000..bf10f7c78857 > > --- /dev/null > > +++ b/include/uapi/linux/uaccess-buffer.h > > @@ -0,0 +1,27 @@ > > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ > > +#ifndef _UAPI_LINUX_UACCESS_BUFFER_H > > +#define _UAPI_LINUX_UACCESS_BUFFER_H > > + > > +#include <linux/types.h> > > + > > +/* Location of the uaccess log. */ > > +struct uaccess_descriptor { > > + /* Address of the uaccess_buffer_entry array. */ > > + __u64 addr; > > + /* Size of the uaccess_buffer_entry array in number of elements. */ > > + __u64 size; > > +}; > > + > > +/* Format of the entries in the uaccess log. */ > > +struct uaccess_buffer_entry { > > + /* Address being accessed. */ > > + __u64 addr; > > + /* Number of bytes that were accessed. */ > > + __u64 size; > > + /* UACCESS_BUFFER_* flags. */ > > + __u64 flags; > > +}; > > + > > +#define UACCESS_BUFFER_FLAG_WRITE 1 /* access was a write */ > > + > > +#endif /* _UAPI_LINUX_UACCESS_BUFFER_H */ > > diff --git a/kernel/Makefile b/kernel/Makefile > > index 186c49582f45..d4d9be5146c3 100644 > > --- a/kernel/Makefile > > +++ b/kernel/Makefile > > @@ -114,6 +114,7 @@ obj-$(CONFIG_KCSAN) += kcsan/ > > obj-$(CONFIG_SHADOW_CALL_STACK) += scs.o > > obj-$(CONFIG_HAVE_STATIC_CALL_INLINE) += static_call.o > > obj-$(CONFIG_CFI_CLANG) += cfi.o > > +obj-$(CONFIG_HAVE_ARCH_UACCESS_BUFFER) += uaccess-buffer.o > > > > obj-$(CONFIG_PERF_EVENTS) += events/ > > > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c > > index 649f07623df6..ab6520a633ef 100644 > > --- a/kernel/bpf/helpers.c > > +++ b/kernel/bpf/helpers.c > > @@ -15,6 +15,7 @@ > > #include <linux/pid_namespace.h> > > #include <linux/proc_ns.h> > > #include <linux/security.h> > > +#include <linux/uaccess-buffer.h> > > > > #include "../../lib/kstrtox.h" > > > > @@ -637,7 +638,11 @@ const struct bpf_func_proto bpf_event_output_data_proto = { > > BPF_CALL_3(bpf_copy_from_user, void *, dst, u32, size, > > const void __user *, user_ptr) > > { > > - int ret = copy_from_user(dst, user_ptr, size); > > + /* > > + * Avoid logging uaccesses here as the BPF program may not be following > > + * the uaccess log rules. > > + */ > > + int ret = copy_from_user_nolog(dst, user_ptr, size); > > > > if (unlikely(ret)) { > > memset(dst, 0, size); > > diff --git a/kernel/fork.c b/kernel/fork.c > > index 3244cc56b697..8be2ca528a65 100644 > > --- a/kernel/fork.c > > +++ b/kernel/fork.c > > @@ -96,6 +96,7 @@ > > #include <linux/scs.h> > > #include <linux/io_uring.h> > > #include <linux/bpf.h> > > +#include <linux/uaccess-buffer.h> > > > > #include <asm/pgalloc.h> > > #include <linux/uaccess.h> > > @@ -754,6 +755,7 @@ void __put_task_struct(struct task_struct *tsk) > > delayacct_tsk_free(tsk); > > put_signal_struct(tsk->signal); > > sched_core_free(tsk); > > + uaccess_buffer_free(tsk); > > > > if (!profile_handoff_task(tsk)) > > free_task(tsk); > > @@ -890,6 +892,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) > > if (memcg_charge_kernel_stack(tsk)) > > goto free_stack; > > > > + uaccess_buffer_free(orig); > > + > > stack_vm_area = task_stack_vm_area(tsk); > > > > err = arch_dup_task_struct(tsk, orig); > > diff --git a/kernel/signal.c b/kernel/signal.c > > index a629b11bf3e0..69bf21518bd0 100644 > > --- a/kernel/signal.c > > +++ b/kernel/signal.c > > @@ -45,6 +45,7 @@ > > #include <linux/posix-timers.h> > > #include <linux/cgroup.h> > > #include <linux/audit.h> > > +#include <linux/uaccess-buffer.h> > > > > #define CREATE_TRACE_POINTS > > #include <trace/events/signal.h> > > @@ -1031,7 +1032,8 @@ static void complete_signal(int sig, struct task_struct *p, enum pid_type type) > > if (sig_fatal(p, sig) && > > !(signal->flags & SIGNAL_GROUP_EXIT) && > > !sigismember(&t->real_blocked, sig) && > > - (sig == SIGKILL || !p->ptrace)) { > > + (sig == SIGKILL || > > + !(p->ptrace || uaccess_buffer_maybe_blocked(p)))) { > > /* > > * This signal will be fatal to the whole group. > > */ > > diff --git a/kernel/sys.c b/kernel/sys.c > > index 8fdac0d90504..c71a9a9c0f68 100644 > > --- a/kernel/sys.c > > +++ b/kernel/sys.c > > @@ -42,6 +42,7 @@ > > #include <linux/version.h> > > #include <linux/ctype.h> > > #include <linux/syscall_user_dispatch.h> > > +#include <linux/uaccess-buffer.h> > > > > #include <linux/compat.h> > > #include <linux/syscalls.h> > > @@ -2530,6 +2531,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > > error = sched_core_share_pid(arg2, arg3, arg4, arg5); > > break; > > #endif > > + case PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR: > > + if (arg3 || arg4 || arg5) > > + return -EINVAL; > > + error = uaccess_buffer_set_descriptor_addr_addr(arg2); > > Does this miss uaccess_buffer_free() for the 0 case? > It seems that when we set it to 0 we always want to free as well (e.g. > in exec). I wonder if freeing should be done by > uaccess_buffer_set_descriptor_addr_addr() itself. > Both uaccess_buffer_set_descriptor_addr_addr() and > uaccess_buffer_free() reset task work, which is fine but is somewhat > suboptimal logically. > Then task exit could do uaccess_buffer_set_descriptor_addr_addr(0).
We don't need to free in that case because we can just log to the original location. I originally had uaccess_buffer_set_descriptor_addr_addr() do the freeing, but you asked me to change it to avoid the free [1]. I guess I don't have a strong opinion but slightly prefer having the two functions do orthogonal things.
[1] https://lore.kernel.org/all/CACT4Y+aoiT+z+3CMBNmO0SwXBXpfDCsHY7pPLf54S8V=c-a8ag@mail.gmail.com/#:~:text=Is%20this%20necessary
> > > + break; > > default: > > error = -EINVAL; > > break; > > diff --git a/kernel/uaccess-buffer.c b/kernel/uaccess-buffer.c > > new file mode 100644 > > index 000000000000..088e43f7611c > > --- /dev/null > > +++ b/kernel/uaccess-buffer.c > > @@ -0,0 +1,129 @@ > > +// SPDX-License-Identifier: GPL-2.0 > > +/* > > + * Support for uaccess logging via uaccess buffers. > > + * > > + * Copyright (C) 2021, Google LLC. > > + */ > > + > > +#include <linux/compat.h> > > +#include <linux/mm.h> > > +#include <linux/prctl.h> > > +#include <linux/ptrace.h> > > +#include <linux/sched.h> > > +#include <linux/signal.h> > > +#include <linux/slab.h> > > +#include <linux/uaccess.h> > > +#include <linux/uaccess-buffer.h> > > + > > +static void uaccess_buffer_log(unsigned long addr, unsigned long size, > > + unsigned long flags) > > +{ > > + struct uaccess_buffer_info *buf = ¤t->uaccess_buffer; > > + struct uaccess_buffer_entry *entry = buf->kcur; > > + > > + if (entry == buf->kend || unlikely(uaccess_kernel())) > > + return; > > + entry->addr = addr; > > + entry->size = size; > > + entry->flags = flags; > > + > > + ++buf->kcur; > > +} > > + > > +void __uaccess_buffer_log_read(const void __user *from, unsigned long n) > > +{ > > + uaccess_buffer_log((unsigned long)from, n, 0); > > +} > > +EXPORT_SYMBOL(__uaccess_buffer_log_read); > > + > > +void __uaccess_buffer_log_write(void __user *to, unsigned long n) > > +{ > > + uaccess_buffer_log((unsigned long)to, n, UACCESS_BUFFER_FLAG_WRITE); > > +} > > +EXPORT_SYMBOL(__uaccess_buffer_log_write); > > + > > +bool __uaccess_buffer_pre_exit_loop(void) > > +{ > > + struct uaccess_buffer_info *buf = ¤t->uaccess_buffer; > > + struct uaccess_descriptor __user *desc_ptr; > > + sigset_t tmp_mask; > > + > > + if (get_user(desc_ptr, buf->desc_ptr_ptr) || !desc_ptr) > > + return false; > > + > > + current->real_blocked = current->blocked; > > + sigfillset(&tmp_mask); > > This and __uaccess_buffer_post_exit_loop() runs only when we have a > signal/timer interrupt between setting the descriptor address in > userspace and entering the next syscall, right? > Just want to make sure this code is not executed for normal uaccess > tracing for performance reasons.
They only need to run if something in _TIF_WORK_MASK (arm64) or EXIT_TO_USER_MODE_WORK (GENERIC_ENTRY) is set, i.e. signals pending or some kind of tracing enabled. That's how it works on the arm64 side (check is in the caller of do_notify_resume) but I had neglected to move the calls into the if statement on the GENERIC_ENTRY side; done in v4.
> > + set_current_blocked(&tmp_mask); > > + return true; > > +} > > + > > +void __uaccess_buffer_post_exit_loop(void) > > +{ > > + spin_lock_irq(¤t->sighand->siglock); > > + current->blocked = current->real_blocked; > > + recalc_sigpending(); > > + spin_unlock_irq(¤t->sighand->siglock); > > +} > > + > > +void uaccess_buffer_free(struct task_struct *tsk) > > +{ > > + struct uaccess_buffer_info *buf = &tsk->uaccess_buffer; > > + > > + kfree(buf->kbegin); > > + clear_syscall_work(UACCESS_BUFFER_EXIT); > > + buf->kbegin = buf->kcur = buf->kend = NULL; > > +} > > + > > +void __uaccess_buffer_syscall_entry(void) > > +{ > > + struct uaccess_buffer_info *buf = ¤t->uaccess_buffer; > > + struct uaccess_descriptor desc; > > + > > + if (get_user(buf->desc_ptr, buf->desc_ptr_ptr) || !buf->desc_ptr || > > + put_user(0, buf->desc_ptr_ptr) || > > + copy_from_user(&desc, buf->desc_ptr, sizeof(desc))) > > + return; > > + > > + if (desc.size > 1024) > > + desc.size = 1024; > > + > > + if (buf->kend - buf->kbegin != desc.size) > > + buf->kbegin = > > + krealloc_array(buf->kbegin, desc.size, > > + sizeof(struct uaccess_buffer_entry), > > + GFP_KERNEL); > > + if (!buf->kbegin) > > I think we also need to set at least buf->kend to NULL here. > I am not sure what can go wrong now, but it's a strange state. On next > iteration we will do "buf->kend - buf->kbegin", where kend is a > dangling pointer and kbegin is NULL.
Done.
Peter
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