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

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:

quoted

On Thu, Aug 22, 2024 at 12:02 AM Tze-nan Wu (吳澤南)
[off-list ref] wrote:

quoted

BTW, If this should be handled in kernel, modification shown

below

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could fix the issue without breaking the "static_branch" usage in

both

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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,

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     {
        ...
        ...
    +     /* The meaning of `compat` variable could be changed

here

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    +      * to indicate if cgroup_bpf_enabled(CGROUP_SOCK_OPS)

is

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false.
    +      */
        if (!compat)
    -       max_optlen =

BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);

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    +       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