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

[Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>]

Cc'ing Jakub.

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?

Thanks.