Re: [PATCH bpf-next v6 2/3] bpf: try to avoid kzalloc in cgroup/{s,g}etsockopt

[Stanislav Fomichev <sdf@google.com>]

On Fri, Jan 8, 2021 at 5:56 PM Martin KaFai Lau <kafai@fb.com> wrote:
>
> On Fri, Jan 08, 2021 at 01:02:22PM -0800, Stanislav Fomichev wrote:
> > When we attach a bpf program to cgroup/getsockopt any other getsockopt()
> > syscall starts incurring kzalloc/kfree cost.
> >
> > Let add a small buffer on the stack and use it for small (majority)
> > {s,g}etsockopt values. The buffer is small enough to fit into
> > the cache line and cover the majority of simple options (most
> > of them are 4 byte ints).
> >
> > It seems natural to do the same for setsockopt, but it's a bit more
> > involved when the BPF program modifies the data (where we have to
> > kmalloc). The assumption is that for the majority of setsockopt
> > calls (which are doing pure BPF options or apply policy) this
> > will bring some benefit as well.
> >
> > Without this patch (we remove about 1% __kmalloc):
> >      3.38%     0.07%  tcp_mmap  [kernel.kallsyms]  [k] __cgroup_bpf_run_filter_getsockopt
> >             |
> >              --3.30%--__cgroup_bpf_run_filter_getsockopt
> >                        |
> >                         --0.81%--__kmalloc
> >
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > Cc: Martin KaFai Lau <kafai@fb.com>
> > Cc: Song Liu <songliubraving@fb.com>
> > ---
> >  include/linux/filter.h |  5 ++++
> >  kernel/bpf/cgroup.c    | 52 ++++++++++++++++++++++++++++++++++++------
> >  2 files changed, 50 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > index 29c27656165b..8739f1d4cac4 100644
> > --- a/include/linux/filter.h
> > +++ b/include/linux/filter.h
> > @@ -1281,6 +1281,11 @@ struct bpf_sysctl_kern {
> >       u64 tmp_reg;
> >  };
> >
> > +#define BPF_SOCKOPT_KERN_BUF_SIZE    32
> > +struct bpf_sockopt_buf {
> > +     u8              data[BPF_SOCKOPT_KERN_BUF_SIZE];
> > +};
> > +
> >  struct bpf_sockopt_kern {
> >       struct sock     *sk;
> >       u8              *optval;
> > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > index c41bb2f34013..a9aad9c419e1 100644
> > --- a/kernel/bpf/cgroup.c
> > +++ b/kernel/bpf/cgroup.c
> > @@ -1298,7 +1298,8 @@ static bool __cgroup_bpf_prog_array_is_empty(struct cgroup *cgrp,
> >       return empty;
> >  }
> >
> > -static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
> > +static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen,
> > +                          struct bpf_sockopt_buf *buf)
> >  {
> >       if (unlikely(max_optlen < 0))
> >               return -EINVAL;
> > @@ -1310,6 +1311,15 @@ static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
> >               max_optlen = PAGE_SIZE;
> >       }
> >
> > +     if (max_optlen <= sizeof(buf->data)) {
> > +             /* When the optval fits into BPF_SOCKOPT_KERN_BUF_SIZE
> > +              * bytes avoid the cost of kzalloc.
> > +              */
> > +             ctx->optval = buf->data;
> > +             ctx->optval_end = ctx->optval + max_optlen;
> > +             return max_optlen;
> > +     }
> > +
> >       ctx->optval = kzalloc(max_optlen, GFP_USER);
> >       if (!ctx->optval)
> >               return -ENOMEM;
> > @@ -1319,16 +1329,26 @@ static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
> >       return max_optlen;
> >  }
> >
> > -static void sockopt_free_buf(struct bpf_sockopt_kern *ctx)
> > +static void sockopt_free_buf(struct bpf_sockopt_kern *ctx,
> > +                          struct bpf_sockopt_buf *buf)
> >  {
> > +     if (ctx->optval == buf->data)
> > +             return;
> >       kfree(ctx->optval);
> >  }
> >
> > +static bool sockopt_buf_allocated(struct bpf_sockopt_kern *ctx,
> > +                               struct bpf_sockopt_buf *buf)
> > +{
> > +     return ctx->optval != buf->data;
> > +}
> > +
> >  int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> >                                      int *optname, char __user *optval,
> >                                      int *optlen, char **kernel_optval)
> >  {
> >       struct cgroup *cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
> > +     struct bpf_sockopt_buf buf = {};
> >       struct bpf_sockopt_kern ctx = {
> >               .sk = sk,
> >               .level = *level,
> > @@ -1350,7 +1370,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> >        */
> >       max_optlen = max_t(int, 16, *optlen);
> >
> > -     max_optlen = sockopt_alloc_buf(&ctx, max_optlen);
> > +     max_optlen = sockopt_alloc_buf(&ctx, max_optlen, &buf);
> >       if (max_optlen < 0)
> >               return max_optlen;
> >
> > @@ -1390,13 +1410,30 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> >                */
> >               if (ctx.optlen != 0) {
> When ctx.optlen == 0, is sockopt_free_buf() called?
> Did I miss something?
Ouch, good catch, it looks like we do leak the buf here with optlen == 0.

We should probably change the following below to:
out:
      if (!*kernel_optval)
           sockopt_free_buf(...);

I'll send a bpf patch with a Fixes tag, thanks!