[PATCH bpf-next v2 1/2] bpf: Fix updating attached freplace prog to prog_array map

[Leon Hwang <leon.hwang@linux.dev>]

The commit f7866c3587337731 ("bpf: Fix null pointer dereference in
resolve_prog_type() for BPF_PROG_TYPE_EXT") fixed a NULL pointer
dereference panic, but didn't fix the issue that fails to update attached
freplace prog to prog_array map.

Since commit 1c123c567fb138eb ("bpf: Resolve fext program type when
checking map compatibility"), freplace prog and its target prog are able
to tail call each other.

And the commit 3aac1ead5eb6b76f ("bpf: Move prog->aux->linked_prog and
trampoline into bpf_link on attach") sets prog->aux->dst_prog as NULL
after attaching freplace prog to its target prog.

Then, as for following example:

tailcall_freplace.c:

// SPDX-License-Identifier: GPL-2.0

\#include <linux/bpf.h>
\#include <bpf/bpf_helpers.h>

struct {
	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
	__uint(max_entries, 1);
	__uint(key_size, sizeof(__u32));
	__uint(value_size, sizeof(__u32));
} jmp_table SEC(".maps");

int count = 0;

SEC("freplace")
int entry_freplace(struct __sk_buff *skb)
{
	count++;

	bpf_tail_call_static(skb, &jmp_table, 0);

	return count;
}

char __license[] SEC("license") = "GPL";

tc_bpf2bpf.c:

// SPDX-License-Identifier: GPL-2.0

\#include <linux/bpf.h>
\#include <bpf/bpf_helpers.h>

__noinline
int subprog(struct __sk_buff *skb)
{
	volatile int ret = 1;

	asm volatile (""::"r+"(ret));
	return ret;
}

SEC("tc")
int entry_tc(struct __sk_buff *skb)
{
	return subprog(skb);
}

char __license[] SEC("license") = "GPL";

And entry_freplace's target is the entry_tc's subprog.

After loading entry_freplace, the jmp_table's owner type is
BPF_PROG_TYPE_SCHED_CLS.

Next, after attaching entry_freplace to entry_tc's subprog, its prog->aux->
dst_prog is NULL.

Next, while updating entry_freplace to jmp_table, bpf_prog_map_compatible()
returns false because resolve_prog_type() returns BPF_PROG_TYPE_EXT instead
of BPF_PROG_TYPE_SCHED_CLS.

With this patch, resolve_prog_type() returns BPF_PROG_TYPE_SCHED_CLS to
support updating the attached entry_freplace to jmp_table.

Fixes: f7866c358733 ("bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT")
Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 include/linux/bpf_verifier.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 5cea15c81b8a8..bfd093ac333f2 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -874,8 +874,8 @@ static inline u32 type_flag(u32 type)
 /* only use after check_attach_btf_id() */
 static inline enum bpf_prog_type resolve_prog_type(const struct bpf_prog *prog)
 {
-	return (prog->type == BPF_PROG_TYPE_EXT && prog->aux->dst_prog) ?
-		prog->aux->dst_prog->type : prog->type;
+	return (prog->type == BPF_PROG_TYPE_EXT && prog->aux->saved_dst_prog_type) ?
+		prog->aux->saved_dst_prog_type : prog->type;
 }
 
 static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
-- 
2.44.0