[PATCH bpf-next v3 3/4] libbpf: Move section-defined program flags to prog_flags

["Toke Høiland-Jørgensen" <toke@redhat.com>]

The libbpf section definition modifiers for XDP frags support and
sleepable programs stores the flags bits only in the private section
definition cookie from object open to load time. This has the
unfortunate consequence that API consumers cannot see (or manipulate)
the flag between object open and program load.

In particular, libxdp has special handling of frags-enabled programs to
make them compatible with the dispatcher. This doesn't work on XDP
programs that enable frags through the 'xdp.frags' section definition
because the flag is not visible through bpf_program__flags()[0].

Fix this by changing how libbpf loads the program flags from section
definitions: instead of using the private section definition cookie, add
a setup function to the default section definitions that stores the
flags for sleepable and XDP frags programs in the prog_flags field of
struct bpf_program.

Exposing the flags this way means that any use of
bpf_program__set_flags() will override the flags unless the caller takes
care of updating flags in a non-destructive way. This is unavoidable
with the set-only API, and any user setting flags unconditionally is
already broken in the sense that they will also override any other
current and future flags.

In addition, prog_flags survives program type changes through
bpf_program__set_type(). It is the responsibility of the caller to
ensure the flags are cleared if they are incompatible with the new
program type.

[0] https://github.com/xdp-project/xdp-tools/issues/587

Fixes: 082c4bfba4f7 ("libbpf: Add SEC name for xdp frags programs")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 tools/lib/bpf/libbpf.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index b749c01742ee..27779b4cddd0 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -7879,6 +7879,19 @@ static int tracing_multi_mod_fd(struct bpf_program *prog, int *btf_obj_fd)
 	return 0;
 }
 
+static int libbpf_setup_prog_flags(struct bpf_program *prog, long cookie)
+{
+	enum sec_def_flags def = cookie;
+
+	if (def & SEC_SLEEPABLE)
+		prog->prog_flags |= BPF_F_SLEEPABLE;
+
+	if (def & SEC_XDP_FRAGS)
+		prog->prog_flags |= BPF_F_XDP_HAS_FRAGS;
+
+	return 0;
+}
+
 /* this is called as prog->sec_def->prog_prepare_load_fn for libbpf-supported sec_defs */
 static int libbpf_prepare_prog_load(struct bpf_program *prog,
 				    struct bpf_prog_load_opts *opts, long cookie)
@@ -7889,12 +7902,6 @@ static int libbpf_prepare_prog_load(struct bpf_program *prog,
 	if ((def & SEC_EXP_ATTACH_OPT) && !kernel_supports(prog->obj, FEAT_EXP_ATTACH_TYPE))
 		opts->expected_attach_type = 0;
 
-	if (def & SEC_SLEEPABLE)
-		opts->prog_flags |= BPF_F_SLEEPABLE;
-
-	if (prog->type == BPF_PROG_TYPE_XDP && (def & SEC_XDP_FRAGS))
-		opts->prog_flags |= BPF_F_XDP_HAS_FRAGS;
-
 	/* special check for usdt to use uprobe_multi link */
 	if ((def & SEC_USDT) && kernel_supports(prog->obj, FEAT_UPROBE_MULTI_LINK)) {
 		/* for BPF_TRACE_UPROBE_MULTI, user might want to query expected_attach_type
@@ -10099,6 +10106,7 @@ int bpf_program__clone(struct bpf_program *prog, const struct bpf_prog_load_opts
 	.prog_type = BPF_PROG_TYPE_##ptype,				    \
 	.expected_attach_type = atype,					    \
 	.cookie = (long)(flags),					    \
+	.prog_setup_fn = libbpf_setup_prog_flags,			    \
 	.prog_prepare_load_fn = libbpf_prepare_prog_load,		    \
 	__VA_ARGS__							    \
 }

-- 
2.55.0