Merge branch 'add-sock_kmemdup-helper'

Geliang Tang says:

====================
add sock_kmemdup helper

While developing MPTCP BPF path manager [1], I found it's useful to
add a new sock_kmemdup() helper.

My use case is this:

In mptcp_userspace_pm_append_new_local_addr() function (see patch 3
in this patchset), it uses sock_kmalloc() to allocate an address
entry "e", then immediately duplicate the input "entry" to it:

'''
e = sock_kmalloc(sk, sizeof(*e), GFP_ATOMIC);
if (!e) {
ret = -ENOMEM;
goto append_err;
}

*e = *entry;
'''

When I implemented MPTCP BPF path manager, I needed to implement a
code similar to this in BPF.

The kfunc sock_kmalloc() can be easily invoked in BPF to allocate
an entry "e", but the code "*e = *entry;" that assigns "entry" to
"e" is not easy to implemented.

I had to implement such a "copy entry" helper in BPF:

'''
static void mptcp_pm_copy_addr(struct mptcp_addr_info *dst,
struct mptcp_addr_info *src)
{
dst->id = src->id;
dst->family = src->family;
dst->port = src->port;

if (src->family == AF_INET) {
dst->addr.s_addr = src->addr.s_addr;
} else if (src->family == AF_INET6) {
dst->addr6.s6_addr32[0] = src->addr6.s6_addr32[0];
dst->addr6.s6_addr32[1] = src->addr6.s6_addr32[1];
dst->addr6.s6_addr32[2] = src->addr6.s6_addr32[2];
dst->addr6.s6_addr32[3] = src->addr6.s6_addr32[3];
}
}

static void mptcp_pm_copy_entry(struct mptcp_pm_addr_entry *dst,
struct mptcp_pm_addr_entry *src)
{
mptcp_pm_copy_addr(&dst->addr, &src->addr);

dst->flags = src->flags;
dst->ifindex = src->ifindex;
}
'''

And add "write permission" for BPF to each field of mptcp_pm_addr_entry:

'''
@@ static int bpf_mptcp_pm_btf_struct_access(struct bpf_verifier_log *log,
case offsetof(struct mptcp_pm_addr_entry, addr.port):
end = offsetofend(struct mptcp_pm_addr_entry, addr.port);
break;
#if IS_ENABLED(CONFIG_MPTCP_IPV6)
case offsetof(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[0]):
end = offsetofend(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[0]);
break;
case offsetof(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[1]):
end = offsetofend(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[1]);
break;
case offsetof(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[2]):
end = offsetofend(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[2]);
break;
case offsetof(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[3]):
end = offsetofend(struct mptcp_pm_addr_entry, addr.addr6.s6_addr32[3]);
break;
#else
case offsetof(struct mptcp_pm_addr_entry, addr.addr.s_addr):
end = offsetofend(struct mptcp_pm_addr_entry, addr.addr.s_addr);
break;
#endif
'''

But if there's a sock_kmemdup() helper, it will become much simpler,
only need to call kfunc sock_kmemdup() instead in BPF.

So this patchset adds this new helper and uses it in several places.

[1]
https://lore.kernel.org/mptcp/cover.1738924875.git.tanggeliang@kylinos.cn/
====================

Link: https://patch.msgid.link/cover.1740735165.git.tanggeliang@kylinos.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>