bpf: Support associating BPF program with struct_ops

Add a new BPF command BPF_PROG_ASSOC_STRUCT_OPS to allow associating
a BPF program with a struct_ops map. This command takes a file
descriptor of a struct_ops map and a BPF program and set
prog->aux->st_ops_assoc to the kdata of the struct_ops map.

The command does not accept a struct_ops program nor a non-struct_ops
map. Programs of a struct_ops map is automatically associated with the
map during map update. If a program is shared between two struct_ops
maps, prog->aux->st_ops_assoc will be poisoned to indicate that the
associated struct_ops is ambiguous. The pointer, once poisoned, cannot
be reset since we have lost track of associated struct_ops. For other
program types, the associated struct_ops map, once set, cannot be
changed later. This restriction may be lifted in the future if there is
a use case.

A kernel helper bpf_prog_get_assoc_struct_ops() can be used to retrieve
the associated struct_ops pointer. The returned pointer, if not NULL, is
guaranteed to be valid and point to a fully updated struct_ops struct.
For struct_ops program reused in multiple struct_ops map, the return
will be NULL.

prog->aux->st_ops_assoc is protected by bumping the refcount for
non-struct_ops programs and RCU for struct_ops programs. Since it would
be inefficient to track programs associated with a struct_ops map, every
non-struct_ops program will bump the refcount of the map to make sure
st_ops_assoc stays valid. For a struct_ops program, it is protected by
RCU as map_free will wait for an RCU grace period before disassociating
the program with the map. The helper must be called in BPF program
context or RCU read-side critical section.

struct_ops implementers should note that the struct_ops returned may not
be initialized nor attached yet. The struct_ops implementer will be
responsible for tracking and checking the state of the associated
struct_ops map if the use case expects an initialized or attached
struct_ops.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20251203233748.668365-3-ameryhung@gmail.com

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index c8ae6ab31651..67226145a4db 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -136,6 +136,7 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
 	mutex_init(&fp->aux->used_maps_mutex);
 	mutex_init(&fp->aux->ext_mutex);
 	mutex_init(&fp->aux->dst_mutex);
+	mutex_init(&fp->aux->st_ops_assoc_mutex);
 
 #ifdef CONFIG_BPF_SYSCALL
 	bpf_prog_stream_init(fp);
@@ -286,6 +287,7 @@ void __bpf_prog_free(struct bpf_prog *fp)
 	if (fp->aux) {
 		mutex_destroy(&fp->aux->used_maps_mutex);
 		mutex_destroy(&fp->aux->dst_mutex);
+		mutex_destroy(&fp->aux->st_ops_assoc_mutex);
 		kfree(fp->aux->poke_tab);
 		kfree(fp->aux);
 	}
@@ -2896,6 +2898,7 @@ static void bpf_prog_free_deferred(struct work_struct *work)
 #endif
 	bpf_free_used_maps(aux);
 	bpf_free_used_btfs(aux);
+	bpf_prog_disassoc_struct_ops(aux->prog);
 	if (bpf_prog_is_dev_bound(aux))
 		bpf_prog_dev_bound_destroy(aux->prog);
 #ifdef CONFIG_PERF_EVENTS