RE: [PATCH v9 01/10] btf: Add a new kfunc flag which allows to mark a function to be sleepable

[Roberto Sassu]

> From: Jarkko Sakkinen [mailto:jarkko@xxxxxxxxxx]
> Sent: Tuesday, August 9, 2022 6:55 PM
> On Tue, Aug 09, 2022 at 03:45:54PM +0200, Roberto Sassu wrote:
> > From: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> >
> > From: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> >
> > This allows to declare a kfunc as sleepable and prevents its use in
> > a non sleepable program.
> 
> Nit: "Declare a kfunc as sleepable and prevent its use in a
> non-sleepable program."
> 
> It's missing the part *how* the patch accomplishes its goals.

I will add:

If an eBPF program is going to call a kfunc declared as sleepable,
eBPF will look at the eBPF program flags. If BPF_F_SLEEPABLE is
not set, execution of that program is denied.

Roberto

> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> > Co-developed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > ---
> >  Documentation/bpf/kfuncs.rst | 6 ++++++
> >  include/linux/btf.h          | 1 +
> >  kernel/bpf/btf.c             | 9 +++++++++
> >  3 files changed, 16 insertions(+)
> >
> > diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
> > index c0b7dae6dbf5..c8b21de1c772 100644
> > --- a/Documentation/bpf/kfuncs.rst
> > +++ b/Documentation/bpf/kfuncs.rst
> > @@ -146,6 +146,12 @@ that operate (change some property, perform some
> operation) on an object that
> >  was obtained using an acquire kfunc. Such kfuncs need an unchanged pointer
> to
> >  ensure the integrity of the operation being performed on the expected object.
> >
> > +2.4.6 KF_SLEEPABLE flag
> > +-----------------------
> > +
> > +The KF_SLEEPABLE flag is used for kfuncs that may sleep. Such kfuncs can
> only
> > +be called by sleepable BPF programs (BPF_F_SLEEPABLE).
> > +
> >  2.5 Registering the kfuncs
> >  --------------------------
> >
> > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > index cdb376d53238..976cbdd2981f 100644
> > --- a/include/linux/btf.h
> > +++ b/include/linux/btf.h
> > @@ -49,6 +49,7 @@
> >   * for this case.
> >   */
> >  #define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer
> arguments */
> > +#define KF_SLEEPABLE   (1 << 5) /* kfunc may sleep */
> >
> >  struct btf;
> >  struct btf_member;
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 7e64447659f3..d3e4c86b8fcd 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -6175,6 +6175,7 @@ static int btf_check_func_arg_match(struct
> bpf_verifier_env *env,
> >  {
> >  	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
> >  	bool rel = false, kptr_get = false, trusted_arg = false;
> > +	bool sleepable = false;
> >  	struct bpf_verifier_log *log = &env->log;
> >  	u32 i, nargs, ref_id, ref_obj_id = 0;
> >  	bool is_kfunc = btf_is_kernel(btf);
> > @@ -6212,6 +6213,7 @@ static int btf_check_func_arg_match(struct
> bpf_verifier_env *env,
> >  		rel = kfunc_flags & KF_RELEASE;
> >  		kptr_get = kfunc_flags & KF_KPTR_GET;
> >  		trusted_arg = kfunc_flags & KF_TRUSTED_ARGS;
> > +		sleepable = kfunc_flags & KF_SLEEPABLE;
> >  	}
> >
> >  	/* check that BTF function arguments match actual types that the
> > @@ -6419,6 +6421,13 @@ static int btf_check_func_arg_match(struct
> bpf_verifier_env *env,
> >  			func_name);
> >  		return -EINVAL;
> >  	}
> > +
> > +	if (sleepable && !env->prog->aux->sleepable) {
> > +		bpf_log(log, "kernel function %s is sleepable but the program is
> not\n",
> > +			func_name);
> > +		return -EINVAL;
> > +	}
> > +
> >  	/* returns argument register number > 0 in case of reference release
> kfunc */
> >  	return rel ? ref_regno : 0;
> >  }
> > --
> > 2.25.1
> >
> 
> BR, Jarkko