Re: [PATCH net v4] bpf, net: Check cgroup_bpf_enabled() only once in do_sock_getsockopt()

["Tze-nan Wu (吳澤南)" <Tze-nan.Wu@mediatek.com>]

On Fri, 2024-08-23 at 19:04 -0700, Stanislav Fomichev wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  On 08/22, Alexei Starovoitov wrote:
> > On Thu, Aug 22, 2024 at 12:02 AM Tze-nan Wu (吳澤南)
> > <Tze-nan.Wu@mediatek.com> wrote:
> > >
> > >
> > > BTW, If this should be handled in kernel, modification shown
> below
> > > could fix the issue without breaking the "static_branch" usage in
> both
> > > macros:
> > >
> > >
> > > +++ /include/linux/bpf-cgroup.h:
> > >     -#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen)
> > >     +#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, compat)
> > >      ({
> > >             int __ret = 0;
> > >             if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT))
> > >                 copy_from_sockptr(&__ret, optlen, sizeof(int));
> > >      +      else
> > >      +          *compat = true;
> > >             __ret;
> > >      })
> > >
> > >     #define BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock, level, optname,
> > > optval, optlen, max_optlen, retval)
> > >      ({
> > >          int __ret = retval;
> > >     -    if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT) &&
> > >     -        cgroup_bpf_sock_enabled(sock, CGROUP_GETSOCKOPT))
> > >     +    if (cgroup_bpf_sock_enabled(sock, CGROUP_GETSOCKOPT))
> > >              if (!(sock)->sk_prot->bpf_bypass_getsockopt ||
> > >                ...
> > >
> > >   +++ /net/socket.c:
> > >     int do_sock_getsockopt(struct socket *sock, bool compat, int
> level,
> > >      {
> > >         ...
> > >         ...
> > >     +     /* The meaning of `compat` variable could be changed
> here
> > >     +      * to indicate if cgroup_bpf_enabled(CGROUP_SOCK_OPS)
> is
> > > false.
> > >     +      */
> > >         if (!compat)
> > >     -       max_optlen =
> BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);
> > >     +       max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen,
> > > &compat);
> > 
> > This is better, but it's still quite a hack. Let's not override it.
> > We can have another bool, but the question:
> > do we really need BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN  ?
> > copy_from_sockptr(&__ret, optlen, sizeof(int));
> > should be fast enough to do it unconditionally.
> > What are we saving here?
> > 
> > Stan ?
> 
> Agreed, most likely nobody would notice :-)

Sorry for my late reply, just have the mailer fixed.

If it is feasible to make the `copy_from_sockptr` unconditionally,
should I submit a new patch that resolve the issue by removing
`BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN`? Patch A shown as below.

  +++ /net/socket.c:
   int do_sock_getsockopt(...)
   {
  -     int max_optlen __maybe_unused;
  +     int max_optlen __maybe_unused = 0;
        const struct proto_ops *ops;
        int err;
  ...
  ...
        if (!compat) <== wonder if we should keep the condition here?
  -         max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);
  +         copy_from_sockptr(&max_optlen, optlen, sizeof(int));

        ops = READ_ONCE(sock->ops);
        if (level == SOL_SOCKET) {
-----------------------------------------

Or perhaps adding another variable "enabled" is the preferable way?
As it keeps the static_branch behavior.
Patch B shown as below:

  +++ /include/linux/bpf-cgroup.h:
  -#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen)
  +#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, enabled)
   ({
        int __ret = 0;
        if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT))
            copy_from_sockptr(&__ret, optlen, sizeof(int));
  +     else
  +         *enabled = false;
        __ret;
   })

  +++ /net/socket.c:
   int do_sock_getsockopt(...)
   {
  +   bool enabled __maybe_unused = !compat;
      int max_optlen __maybe_unused;
      const struct proto_ops *ops;
      int err;
      if (!compat)
  -       max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);
  +       max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen,
&enabled);

      ops = READ_ONCE(sock->ops);
      ...
      ...
  -   if (!compat)
  +   if (enabled)
          err = BPF_CGROUP_RUN_PROG_GETSOCKOPT(...);
-----------------------------------------

Any comments would be appreciated.
--Tze-nan