Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags

[Dan Williams]

On Tue, Aug 15, 2017 at 5:27 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Mon 14-08-17 23:12:16, Dan Williams wrote:
>> The mmap syscall suffers from the ABI anti-pattern of not validating
>> unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT need a
>> mechanism to define new behavior that is known to fail on older kernels
>> without the feature. Use the fact that specifying MAP_SHARED and
>> MAP_PRIVATE at the same time is invalid as a cute hack to allow a new
>> set of validated flags to be introduced.
>>
>> This also introduces the ->fmmap() file operation that is ->mmap() plus
>> flags. Each ->fmmap() implementation must fail requests when a locally
>> unsupported flag is specified.
> ...
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 1104e5df39ef..bbe755d0caee 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -1674,6 +1674,7 @@ struct file_operations {
>>       long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
>>       long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
>>       int (*mmap) (struct file *, struct vm_area_struct *);
>> +     int (*fmmap) (struct file *, struct vm_area_struct *, unsigned long);
>>       int (*open) (struct inode *, struct file *);
>>       int (*flush) (struct file *, fl_owner_t id);
>>       int (*release) (struct inode *, struct file *);
>> @@ -1748,6 +1749,12 @@ static inline int call_mmap(struct file *file, struct vm_area_struct *vma)
>>       return file->f_op->mmap(file, vma);
>>  }
>>
>> +static inline int call_fmmap(struct file *file, struct vm_area_struct *vma,
>> +             unsigned long flags)
>> +{
>> +     return file->f_op->fmmap(file, vma, flags);
>> +}
>> +
>
> Hum, I dislike a new file op for this when the only problem with ->mmap is
> that it misses 'flags' argument. I understand there are lots of ->mmap
> implementations out there and modifying prototype of them all is painful
> but is it so bad? Coccinelle patch for this should be rather easy...

So it wasn't all that easy, and Linus declined to take it. I think we
should add a new ->mmap_validate() file operation and save the
tree-wide cleanup until later.