Re: [net-next PATCH v4 1/3] sysctl: refactor integer handling proc code

[Octavian Purdila <opurdila@ixiacom.com>]

On Wednesday 17 February 2010 15:33:03 you wrote:
> Cong Wang <amwang@redhat.com> writes:
> > Octavian Purdila wrote:
> >> On Tuesday 16 February 2010 15:09:51 you wrote:
> >>> Octavian Purdila wrote:
> >>>> On Tuesday 16 February 2010 10:41:07 you wrote:
> >>>>>> +static int proc_skip_wspace(char __user **buf, size_t *size)
> >>>>>> +{
> >>>>>> +     char c;
> >>>>>> +
> >>>>>> +     while (*size) {
> >>>>>> +             if (get_user(c, *buf))
> >>>>>> +                     return -EFAULT;
> >>>>>> +             if (!isspace(c))
> >>>>>> +                     break;
> >>>>>> +             (*size)--; (*buf)++;
> >>>>>> +     }
> >>>>>> +
> >>>>>> +     return 0;
> >>>>>> +}
> >>>>>
> >>>>> In lib/string.c we have skip_spaces(), I think we can use it
> >>>>> here instead of inventing another one.
> >>>>
> >>>> I'm afraid we can't, skip_spaces does not accept userspace buffers.
> >>>
> >>> Well, you need to use copy_from_user() before call it.
> >>
> >> And how much would you copy? You need to either use a stack buffer and
> >> do a loop copy or you would need to copy the whole userspace buffer
> >> which means we need to allocate a kernel buffer. I think its much
> >> cleaner the way is currently done.
> >
> > Yeah, maybe just a personal preference. :-/
> 
> There can be valid security reasons for copying all of the data before
> processing it.
> 

How so? I only know about security issues when you copy & process the same 
data more than one time.

And all existing code I've looked over (proc_dointvec, proc_dostring, 
bitmap_parse_user) does not copy all of the data. In fact the code in question 
here is just existing code moved to its own function.

There must be a reason for doing that, as copying whole buffer seems like a 
much simple implementation? (my guess is to avoid an extra allocation)

> Semantically if we an guarantee that we either have processed the
> entire buffer or failed the entire buffer and no changes have occurred
> in the kernel that seems like a much easier semantic to work with in
> user space.
> 

OK, but this is not how various proc routines currently handles it. For 
example proc_dointvec won't undo the changes for previous items when we get an 
error and I think for good reasons as I don't see a clean and generic way to 
do the undo stuff.