Re: [PATCH] sysctl: add proper unsigned int support

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Wed, Feb 01, 2017 at 08:56:46PM +0100, Luis R. Rodriguez wrote:
> On Mon, Jan 30, 2017 at 03:56:20PM +0300, Alexey Dobriyan wrote:
> > On Sun, Jan 29, 2017 at 10:29 PM, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
> > > Commit e7d316a02f6838 ("sysctl: handle error writing UINT_MAX to u32 fields")
> > > added proc_douintvec() to start help adding support for unsigned int,
> > > this however was only half the work needed, all these issues are present
> > > with the current implementation:
> > >
> > >   o Printing the values shows a negative value, this happens
> > >     since do_proc_dointvec() and this uses proc_put_long()
> > >   o We can easily wrap around the int values: UINT_MAX is
> > >     4294967295, if we echo in 4294967295 + 1 we end up with 0,
> > >     using 4294967295 + 2 we end up with 1.
> > >  o We echo negative values in and they are accepted
> > >
> > > Fix all these issues by adding our own do_proc_douintvec().
> > >
> > > Cc: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
> > > Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: "David S. Miller" <davem@davemloft.net>
> > > Cc: Ingo Molnar <mingo@redhat.com>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Fixes: e7d316a02f68 ("sysctl: handle error writing UINT_MAX to u32 fields")
> > > Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
> > > ---
> > >
> > > I split this off as its own atomic fix from a larger RFC series [0].
> > > I've only provided the fix here, and split off further functionality
> > > into a separate patch for the future. Although this is a fix I don't think
> > > its super critical, and specially due to its size do not think it can
> > > be stable material.
> > >
> > > I do have proc_douintvec_minmax() but since we have no users for it
> > > it can wait until I add something that makes use of it. If someone
> > > needs it now though please let me know.
> > >
> > > Likewise adding proc_douintvec_minmax_sysadmin() is very trivial but I have no
> > > immediate users for it so it can wait even longer.
> > >
> > > [0] https://lkml.kernel.org/r/20161208184801.1689-1-mcgrof@kernel.org
> > >
> > >  kernel/sysctl.c | 121 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
> > >  1 file changed, 115 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > > index 8dbaec0e4f7f..118341d3a139 100644
> > > --- a/kernel/sysctl.c
> > > +++ b/kernel/sysctl.c
> > > @@ -2125,12 +2125,12 @@ static int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp,
> > >         return 0;
> > >  }
> > >
> > > -static int do_proc_douintvec_conv(bool *negp, unsigned long *lvalp,
> > > -                                int *valp,
> > > -                                int write, void *data)
> > > +static int do_proc_douintvec_conv(unsigned long *lvalp,
> > > +                                 unsigned int *valp,
> > > +                                 int write, void *data)
> > >  {
> > >         if (write) {
> > > -               if (*negp)
> > > +               if (*lvalp > (unsigned long) UINT_MAX)
> > 
> > Cast is unnecessary here.
> 
> Fixed, thanks!
> 
> > > +static int __do_proc_douintvec(void *tbl_data, struct ctl_table *table,
> > 
> > > +       for (; left && vleft--; i++, first=false) {
> > 
> > I'd suggest to not implement "array of unsigned int" unless
> > such sysctl already exists. Much of the complexity arises from this case.
> 
> Uh, yeah that is such crap.

The more I studied this the more I supported the idea of ripping
the array crap out from unsigned int support.

> As much as I'd like to I'm afraid the problem with this is there
> may be array int setups which should / could be ported to unsigned
> int. I tried to do a grammatical search with Coccinelle but ran into
> issues, I'll follow up with Julia about that. 

I've now completed a preliminary evaluation using Coccinelle to perform a
grammatical search to ask ourselves:                                                        
                                                                                
  o How many sysctl proc_dointvec() (int) users exist which likely              
    should be moved over to proc_douintvec() (unsigned int) ?                   
        Answer: about 8                                                         
        - Of these how many are array users ?                                   
                Answer: Probably only 1                                         
  o How many sysctl array users exist ?                                         
        Answer: about 12                                                        
                                                                                
This last question gives us an idea just how popular arrays: they               
are not. Array support should probably just be kept for strings.

So unless anyone finds evidence to the contrary I will be ripping
out array support from unsigned int. I'll also go ahead and extend
the test cases and add a test_sysctl driver for selftests.

  Luis