Re: [PATCH libmnl] src: attr: Add mnl_attr_get_uint() function

[Petr Machata <petrm@nvidia.com>]

Pablo Neira Ayuso <pablo@netfilter.org> writes:

> On Mon, Sep 30, 2024 at 01:45:09PM +0200, Jakub Kicinski wrote:
>> On Mon, 30 Sep 2024 12:56:20 +0200 Pablo Neira Ayuso wrote:
>> > On Mon, Sep 30, 2024 at 12:28:08PM +0200, Pablo Neira Ayuso wrote:
>> > > On Sun, Sep 29, 2024 at 10:42:44AM +0000, Danielle Ratson wrote:  
>> > > > Hi,
>> > > > 
>> > > > Is there a plan to build a new version soon? 
>> > > > I am asking since I am planning to use this function in ethtool.  
>> > > 
>> > > ASAP  
>> > 
>> > but one question before... Is this related to NLA_UINT in the kernel?
>> > 
>> > /**
>> >  * nla_put_uint - Add a variable-size unsigned int to a socket buffer
>> >  * @skb: socket buffer to add attribute to
>> >  * @attrtype: attribute type
>> >  * @value: numeric value
>> >  */
>> > static inline int nla_put_uint(struct sk_buff *skb, int attrtype, u64 value)
>> > {
>> >         u64 tmp64 = value;
>> >         u32 tmp32 = value;
>> > 
>> >         if (tmp64 == tmp32)
>> >                 return nla_put_u32(skb, attrtype, tmp32);
>> >         return nla_put(skb, attrtype, sizeof(u64), &tmp64);
>> > }
>> > 
>> > if I'm correct, it seems kernel always uses either u32 or u64.
>> > 
>> > Userspace assumes u8 and u16 are possible though:
>> > 
>> > +/**
>> > + * mnl_attr_get_uint - returns 64-bit unsigned integer attribute.
>> > + * \param attr pointer to netlink attribute
>> > + *
>> > + * This function returns the 64-bit value of the attribute payload.
>> > + */
>> > +EXPORT_SYMBOL uint64_t mnl_attr_get_uint(const struct nlattr *attr)
>> > +{
>> > +       switch (mnl_attr_get_payload_len(attr)) {
>> > +       case sizeof(uint8_t):
>> > +               return mnl_attr_get_u8(attr);
>> > +       case sizeof(uint16_t):
>> > +               return mnl_attr_get_u16(attr);
>> > +       case sizeof(uint32_t):
>> > +               return mnl_attr_get_u32(attr);
>> > +       case sizeof(uint64_t):
>> > +               return mnl_attr_get_u64(attr);
>> > +       }
>> > +
>> > +       return -1ULL;
>> > +}
>> > 
>> > Or this is an attempt to provide a helper that allows you fetch for
>> > payload value of 2^3..2^6 bytes?
>> 
>> No preference here, FWIW. Looks like this patch does a different thing
>> than the kernel. But maybe a broader "automatic" helper is useful for
>> user space code.
>
> Not sure. @Danielle: could you clarify your intention?

This follows the iproute2 helper, where I was asked to support >32-bit
fields purely as a service to the users, so that one helper can be used
for any integral field.