Re: [PATCH 5/7] uprobes: introduce MMF_HAS_UPROBES

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Oleg Nesterov <oleg@redhat.com> [2012-08-08 19:37:47]:

> Add the new MMF_HAS_UPROBES flag. It is set by install_breakpoint()
> and it is copied by dup_mmap(), uprobe_pre_sstep_notifier() checks
> it to avoid the slow path if the task was never probed. Perhaps it
> makes sense to check it in valid_vma(is_register => false) as well.
> 
> This needs the new dup_mmap()->uprobe_dup_mmap() hook. We can't use
> uprobe_reset_state() or put MMF_HAS_UPROBES into MMF_INIT_MASK, we
> need oldmm->mmap_sem to avoid the race with uprobe_register() or
> mmap() from another thread.
> 
> Currently we never clear this bit, it can be false-positive after
> uprobe_unregister() or uprobe_munmap() or if dup_mmap() hits the
> probed VM_DONTCOPY vma. But this is fine correctness-wise and has
> no effect unless the task hits the non-uprobe breakpoint.
> 

In which case, cant we just delete uprobe_munmap() altogether.

> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  include/linux/sched.h   |    2 ++
>  include/linux/uprobes.h |    5 +++++
>  kernel/events/uprobes.c |   22 +++++++++++++++++++++-
>  kernel/fork.c           |    1 +
>  4 files changed, 29 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index cbf3a71..c0fcfb7 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -441,6 +441,8 @@ extern int get_dumpable(struct mm_struct *mm);
>  #define MMF_VM_HUGEPAGE		17	/* set when VM_HUGEPAGE is set on vma */
>  #define MMF_EXE_FILE_CHANGED	18	/* see prctl_set_mm_exe_file() */
> 
> +#define MMF_HAS_UPROBES		19	/* might have uprobes */
> +
>  #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK)
> 
>  struct sighand_struct {
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index 03ae547..4a37ab1 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -108,6 +108,7 @@ extern int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_con
>  extern void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc);
>  extern int uprobe_mmap(struct vm_area_struct *vma);
>  extern void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned long end);
> +extern void uprobe_dup_mmap(struct mm_struct *oldmm, struct mm_struct *newmm);
>  extern void uprobe_free_utask(struct task_struct *t);
>  extern void uprobe_copy_process(struct task_struct *t);
>  extern unsigned long __weak uprobe_get_swbp_addr(struct pt_regs *regs);
> @@ -138,6 +139,10 @@ static inline void
>  uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned long end)
>  {
>  }
> +static inline void
> +uprobe_dup_mmap(struct mm_struct *oldmm, struct mm_struct *newmm)
> +{
> +}
>  static inline void uprobe_notify_resume(struct pt_regs *regs)
>  {
>  }
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 309309e..2a42319 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -649,6 +649,7 @@ static int
>  install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
>  			struct vm_area_struct *vma, unsigned long vaddr)
>  {
> +	bool first_uprobe;
>  	int ret;
> 
>  	/*
> @@ -680,7 +681,17 @@ install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
>  		uprobe->flags |= UPROBE_COPY_INSN;
>  	}
> 
> +	/*
> +	 * set MMF_HAS_UPROBES in advance for uprobe_pre_sstep_notifier(),
> +	 * the task can hit this breakpoint right after __replace_page().
> +	 */
> +	first_uprobe = !test_bit(MMF_HAS_UPROBES, &mm->flags);
> +	if (first_uprobe)
> +		set_bit(MMF_HAS_UPROBES, &mm->flags);
> +
>  	ret = set_swbp(&uprobe->arch, mm, vaddr);
> +	if (ret && first_uprobe)
> +		clear_bit(MMF_HAS_UPROBES, &mm->flags);
> 
>  	return ret;
>  }
> @@ -1034,6 +1045,9 @@ void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned lon
>  	if (!atomic_read(&vma->vm_mm->mm_users)) /* called by mmput() ? */
>  		return;
> 
> +	if (!test_bit(MMF_HAS_UPROBES, &vma->vm_mm->flags))
> +		return;
> +

I am not sure whats the purpose of the above test 


>  	/* TODO: unmapping uprobe(s) will need more work */

and I am unable to think what more we would want to do here.

>  }
> 
> @@ -1144,6 +1158,12 @@ void uprobe_reset_state(struct mm_struct *mm)
>  	mm->uprobes_state.xol_area = NULL;
>  }
> 
> +void uprobe_dup_mmap(struct mm_struct *oldmm, struct mm_struct *newmm)
> +{
> +	if (test_bit(MMF_HAS_UPROBES, &oldmm->flags))
> +		set_bit(MMF_HAS_UPROBES, &newmm->flags);
> +}
> +
>  /*
>   *  - search for a free slot.
>   */
> @@ -1511,7 +1531,7 @@ int uprobe_pre_sstep_notifier(struct pt_regs *regs)
>  {
>  	struct uprobe_task *utask;
> 
> -	if (!current->mm)
> +	if (!current->mm || !test_bit(MMF_HAS_UPROBES, &current->mm->flags))
>  		return 0;
> 
>  	utask = current->utask;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 0e419ab..ab970c3 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -350,6 +350,7 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
> 
>  	down_write(&oldmm->mmap_sem);
>  	flush_cache_dup_mm(oldmm);
> +	uprobe_dup_mmap(oldmm, mm);
>  	/*
>  	 * Not linked in yet - no deadlock potential:
>  	 */
> -- 
> 1.5.5.1
>