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SubjectRe: [PATCH v3 3/6] sysctl: Add flags to support min/max range clamping
On Thu, Mar 01, 2018 at 01:31:17PM -0800, Andrew Morton wrote:
> On Thu, 1 Mar 2018 12:43:37 -0500 Waiman Long <longman@redhat.com> wrote:
>
> > When minimum/maximum values are specified for a sysctl parameter in
> > the ctl_table structure with proc_dointvec_minmax() handler, update
> > to that parameter will fail with error if the given value is outside
> > of the required range.
> >
> > There are use cases where it may be better to clamp the value of
> > the sysctl parameter to the given range without failing the update,
> > especially if the users are not aware of the actual range limits.
> > Reading the value back after the update will now be a good practice
> > to see if the provided value exceeds the range limits.
> >
> > To provide this less restrictive form of range checking, a new flags
> > field is added to the ctl_table structure. The new field is a 16-bit
> > value that just fits into the hole left by the 16-bit umode_t field
> > without increasing the size of the structure.
> >
> > When the CTL_FLAGS_CLAMP_RANGE flag is set in the ctl_table entry,
> > any update from the userspace will be clamped to the given range
> > without error.
> >
> > ...
> >
> > --- a/include/linux/sysctl.h
> > +++ b/include/linux/sysctl.h
> > @@ -116,6 +116,7 @@ struct ctl_table
> > void *data;
> > int maxlen;
> > umode_t mode;
> > + uint16_t flags;
>
> It would be nice to make this have type `enum ctl_table_flags', but I
> guess there's then no reliable way of forcing it to be 16-bit.
>
> I guess this is the best we can do...
>

We can add this to the enum:

enum ctl_table_flags {
CTL_FLAGS_CLAMP_RANGE = BIT(0),
+ __CTL_FLAGS_CLAMP_MAX = BIT(16),
};


Then also:

#define CTL_TABLE_FLAGS_ALL ((BIT(__CTL_FLAGS_CLAMP_MAX + 1))-1)

at the end of the definition, then a helper which can be used during
parsing:

static int check_ctl_table_flags(u16 flags)
{
if (flags & ~(CTL_TABLE_FLAGS_ALL))
return -ERANGE;
return 0;
}

Waiman please evaluate and add.

Luis

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