Re: [PATCH bpf-next v2] libbpf: don't allocate 16M for log buffer by default

[Stanislav Fomichev <sdf@fomichev.me>]

On 03/25, Andrii Nakryiko wrote:
> On Wed, Mar 25, 2020 at 9:20 AM Stanislav Fomichev <sdf@google.com> wrote:
> >
> > For each prog/btf load we allocate and free 16 megs of verifier buffer.
> > On production systems it doesn't really make sense because the
> > programs/btf have gone through extensive testing and (mostly) guaranteed
> > to successfully load.
> >
> > Let's assume successful case by default and skip buffer allocation
> > on the first try. If there is an error, start with BPF_LOG_BUF_SIZE
> > and double it on each ENOSPC iteration.
> >
> > v2:
> > * Don't allocate the buffer at all on the first try (Andrii Nakryiko)
> >
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> >  tools/lib/bpf/btf.c    | 20 +++++++++++++++-----
> >  tools/lib/bpf/libbpf.c | 22 ++++++++++++++--------
> >  2 files changed, 29 insertions(+), 13 deletions(-)
> >
> > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > index 3d1c25fc97ae..bfef3d606b54 100644
> > --- a/tools/lib/bpf/btf.c
> > +++ b/tools/lib/bpf/btf.c
> > @@ -657,22 +657,32 @@ int btf__finalize_data(struct bpf_object *obj, struct btf *btf)
> >
> >  int btf__load(struct btf *btf)
> >  {
> > -       __u32 log_buf_size = BPF_LOG_BUF_SIZE;
> > +       __u32 log_buf_size = 0;
> >         char *log_buf = NULL;
> >         int err = 0;
> >
> >         if (btf->fd >= 0)
> >                 return -EEXIST;
> >
> > -       log_buf = malloc(log_buf_size);
> > -       if (!log_buf)
> > -               return -ENOMEM;
> > +retry_load:
> > +       if (log_buf_size) {
> > +               log_buf = malloc(log_buf_size);
> > +               if (!log_buf)
> > +                       return -ENOMEM;
> >
> > -       *log_buf = 0;
> > +               *log_buf = 0;
> > +       }
> >
> >         btf->fd = bpf_load_btf(btf->data, btf->data_size,
> >                                log_buf, log_buf_size, false);
> >         if (btf->fd < 0) {
> > +               if (!log_buf || errno == ENOSPC) {
> > +                       log_buf_size = max((__u32)BPF_LOG_BUF_SIZE,
> > +                                          log_buf_size << 1);
> > +                       free(log_buf);
> > +                       goto retry_load;
> > +               }
> > +
> >                 err = -errno;
> >                 pr_warn("Error loading BTF: %s(%d)\n", strerror(errno), errno);
> >                 if (*log_buf)
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 085e41f9b68e..b2fd43a03708 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -4855,8 +4855,8 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
> >  {
> >         struct bpf_load_program_attr load_attr;
> >         char *cp, errmsg[STRERR_BUFSIZE];
> > -       int log_buf_size = BPF_LOG_BUF_SIZE;
> > -       char *log_buf;
> > +       size_t log_buf_size = 0;
> > +       char *log_buf = NULL;
> >         int btf_fd, ret;
> >
> >         if (!insns || !insns_cnt)
> > @@ -4896,22 +4896,28 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
> >         load_attr.prog_flags = prog->prog_flags;
> >
> >  retry_load:
> > -       log_buf = malloc(log_buf_size);
> > -       if (!log_buf)
> > -               pr_warn("Alloc log buffer for bpf loader error, continue without log\n");
> > +       if (log_buf_size) {
> > +               log_buf = malloc(log_buf_size);
> > +               if (!log_buf)
> > +                       pr_warn("Alloc log buffer for bpf loader error, continue without log\n");
> > +
> 
> considering that log_buf is not allocated first time, if it fails to
> allocate on retry then it should be an error, no?
> 
> Otherwise looks good, please add my ack after fixing this:
> 
> Acked-by: Andrii Nakryiko <andriin@fb.com>
Thx, will respin a v3. Returning -ENOMEM also makes it more consistent with
btf retry logic.

> > +               *log_buf = 0;
> > +       }
> >
> >         ret = bpf_load_program_xattr(&load_attr, log_buf, log_buf_size);
> >
> >         if (ret >= 0) {
> > -               if (load_attr.log_level)
> > +               if (log_buf && load_attr.log_level)
> >                         pr_debug("verifier log:\n%s", log_buf);
> >                 *pfd = ret;
> >                 ret = 0;
> >                 goto out;
> >         }
> >
> > -       if (errno == ENOSPC) {
> > -               log_buf_size <<= 1;
> > +       if (!log_buf || errno == ENOSPC) {
> > +               log_buf_size = max((size_t)BPF_LOG_BUF_SIZE,
> > +                                  log_buf_size << 1);
> > +
> >                 free(log_buf);
> >                 goto retry_load;
> >         }
> > --
> > 2.25.1.696.g5e7596f4ac-goog
> >