| Message ID | 20240301012845.2951053-2-kuba@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | netlink: handle EMSGSIZE errors in the core | expand |
On Fri, Mar 1, 2024 at 2:31 AM Jakub Kicinski <kuba@kernel.org> wrote: > > Eric points out that our current suggested way of handling > EMSGSIZE errors ((err == -EMSGSIZE) ? skb->len : err) will > break if we didn't fit even a single object into the buffer > provided by the user. This should not happen for well behaved > applications, but we can fix that, and free netlink families > from dealing with that completely by moving error handling > into the core. > > Let's assume from now on that all EMSGSIZE errors in dumps are > because we run out of skb space. Families can now propagate > the error nla_put_*() etc generated and not worry about any > return value magic. If some family really wants to send EMSGSIZE > to user space, assuming it generates the same error on the next > dump iteration the skb->len should be 0, and user space should > still see the EMSGSIZE. > > This should simplify families and prevent mistakes in return > values which lead to DONE being forced into a separate recv() > call as discovered by Ido some time ago. > > Signed-off-by: Jakub Kicinski <kuba@kernel.org> Reviewed-by: Eric Dumazet <edumazet@google.com>
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c index ad7b645e3ae7..da846212fb9b 100644 --- a/net/netlink/af_netlink.c +++ b/net/netlink/af_netlink.c @@ -2267,6 +2267,15 @@ static int netlink_dump(struct sock *sk, bool lock_taken) if (extra_mutex) mutex_unlock(extra_mutex); + /* EMSGSIZE plus something already in the skb means + * that there's more to dump but current skb has filled up. + * If the callback really wants to return EMSGSIZE to user space + * it needs to do so again, on the next cb->dump() call, + * without putting data in the skb. + */ + if (nlk->dump_done_errno == -EMSGSIZE && skb->len) + nlk->dump_done_errno = skb->len; + cb->extack = NULL; }
Eric points out that our current suggested way of handling EMSGSIZE errors ((err == -EMSGSIZE) ? skb->len : err) will break if we didn't fit even a single object into the buffer provided by the user. This should not happen for well behaved applications, but we can fix that, and free netlink families from dealing with that completely by moving error handling into the core. Let's assume from now on that all EMSGSIZE errors in dumps are because we run out of skb space. Families can now propagate the error nla_put_*() etc generated and not worry about any return value magic. If some family really wants to send EMSGSIZE to user space, assuming it generates the same error on the next dump iteration the skb->len should be 0, and user space should still see the EMSGSIZE. This should simplify families and prevent mistakes in return values which lead to DONE being forced into a separate recv() call as discovered by Ido some time ago. Signed-off-by: Jakub Kicinski <kuba@kernel.org> --- CC: kuniyu@amazon.com --- net/netlink/af_netlink.c | 9 +++++++++ 1 file changed, 9 insertions(+)