Re: [PATCH mm/bpf v2] mm: bpf: add find_vma_no_check() without lockdep_assert on mm->mmap_lock

[Yonghong Song <yhs@fb.com>]

On 9/8/21 11:30 AM, Liam Howlett wrote:
> * Alexei Starovoitov <alexei.starovoitov@gmail.com> [210908 14:21]:
>> On Wed, Sep 8, 2021 at 11:15 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>>>
>>> On Wed, 8 Sep 2021 11:02:58 -0700 Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>>>
>>>>> Please describe the expected userspace-visible change from Peter's
>>>>> patch in full detail?
>>>>
>>>> User space expects build_id to be available. Peter patch simply removes
>>>> that feature.
>>>
>>> Are you sure?  He ends up with
>>
>> More than sure :)
>> Just look at below.
>>
>>> static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>>>                                            u64 *ips, u32 trace_nr, bool user)
>>> {
>>>          int i;
>>>
>>>          /* cannot access current->mm, fall back to ips */
>>>          for (i = 0; i < trace_nr; i++) {
>>>                  id_offs[i].status = BPF_STACK_BUILD_ID_IP;
>>>                  id_offs[i].ip = ips[i];
>>>                  memset(id_offs[i].build_id, 0, BUILD_ID_SIZE_MAX);
>>>          }
>>>          return;
>>> }
>>>
>>> and you're saying that userspace won't like this because we didn't set
>>> BPF_STACK_BUILD_ID_VALID?
>>
>> The patch forces the "fallback path" that in production is seen 0.001%
>> Meaning that user space doesn't see build_id any more. It sees IPs only.
>> The user space cannot correlate IPs to binaries. That's what build_id enabled.
> 
> I was thinking of decomposing the checks in my first response to two
> functions.
> 
> Something like this:
> --------------
> diff --git a/mm/mmap.c b/mm/mmap.c
> index dce46105e3df..8afc1d22aa61 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2293,12 +2293,13 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>   EXPORT_SYMBOL(get_unmapped_area);
>   
>   /* Look up the first VMA which satisfies  addr < vm_end,  NULL if none. */
> -struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
> +struct vm_area_struct *find_vma_non_owner(struct mm_struct *mm,
> +					 unsigned long addr)
>   {
>   	struct rb_node *rb_node;
>   	struct vm_area_struct *vma;
>   
> -	mmap_assert_locked(mm);
> +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
>   	/* Check the cache first. */
>   	vma = vmacache_find(mm, addr);
>   	if (likely(vma))
> @@ -2325,6 +2326,11 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>   	return vma;
>   }
>   
> +struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
> +{
> +	lockdep_assert_held(&mm->mmap_lock);
> +	return find_vma_non_owner(mm, addr);
> +}
>   EXPORT_SYMBOL(find_vma);
>   
>   /*
> 
> --------------
> 
> Although this leaks more into the mm API and was referred to as ugly
> previously, it does provide a working solution and still maintains the
> same level of checking.
> 
> Would it push the back actors to just switch to non_owner though?

Thanks, Liam. This should work for bpf side as well as we can just call
find_vma_no_owner(). I will submit v3 soon.

> 
> 
> Thanks,
> Liam
>