Re: [PATCH 11/19] bpf: Add support to load multi func tracing program

Jiri Olsa
2021-06-07T18:19:20Z

On Sun, Jun 06, 2021 at 08:56:47PM -0700, Yonghong Song wrote:
>
>
> On 6/5/21 4:10 AM, Jiri Olsa wrote:
> > Adding support to load tracing program with new BPF_F_MULTI_FUNC flag,
> > that allows the program to be loaded without specific function to be
> > attached to.
> >
> > The verifier assumes the program is using all (6) available arguments
>
> Is this a verifier failure or it is due to the check in the
> beginning of function arch_prepare_bpf_trampoline()?
>
> /* x86-64 supports up to 6 arguments. 7+ can be added in the future
> */
> if (nr_args > 6)
> return -ENOTSUPP;

yes, that's the limit.. it allows the traced program to
touch 6 arguments, because it's the maximum for JIT

>
> If it is indeed due to arch_prepare_bpf_trampoline() maybe we
> can improve it instead of specially processing the first argument
> "ip" in quite some places?

do you mean to teach JIT to process more than 6 arguments?

>
> > as unsigned long values. We can't add extra ip argument at this time,
> > because JIT on x86 would fail to process this function. Instead we
> > allow to access extra first 'ip' argument in btf_ctx_access.
> >
> > Such program will be allowed to be attached to multiple functions
> > in following patches.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > include/linux/bpf.h | 1 +
> > include/uapi/linux/bpf.h | 7 +++++++
> > kernel/bpf/btf.c | 5 +++++
> > kernel/bpf/syscall.c | 35 +++++++++++++++++++++++++++++-----
> > kernel/bpf/verifier.c | 3 ++-
> > tools/include/uapi/linux/bpf.h | 7 +++++++
> > 6 files changed, 52 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 6cbf3c81c650..23221e0e8d3c 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -845,6 +845,7 @@ struct bpf_prog_aux {
> > bool sleepable;
> > bool tail_call_reachable;
> > struct hlist_node tramp_hlist;
> > + bool multi_func;
>
> Move this field right after "tail_call_reachable"?
>
> > /* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
> > const struct btf_type *attach_func_proto;
> > /* function name for valid attach_btf_id */
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 2c1ba70abbf1..ad9340fb14d4 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -1109,6 +1109,13 @@ enum bpf_link_type {
> > */
> > #define BPF_F_SLEEPABLE (1U << 4)
> > +/* If BPF_F_MULTI_FUNC is used in BPF_PROG_LOAD command, the verifier does
> > + * not expect BTF ID for the program, instead it assumes it's function
> > + * with 6 u64 arguments. No trampoline is created for the program. Such
> > + * program can be attached to multiple functions.
> > + */
> > +#define BPF_F_MULTI_FUNC (1U << 5)
> > +
> > /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
> > * the following extensions:
> > *
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index a6e39c5ea0bf..c233aaa6a709 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -4679,6 +4679,11 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> > args++;
> > nr_args--;
> > }
> > + if (prog->aux->multi_func) {
> > + if (arg == 0)
> > + return true;
> > + arg--;
>
> Some comments in the above to mention like "the first 'ip' argument
> is omitted" will be good.

will do, thanks

jirka

>
> > + }
> > if (arg > nr_args) {
> > bpf_log(log, "func '%s' doesn't have %d-th argument\n",
> [...]
>