bpf/arena: add bpf_arena_reserve_pages kfunc

Add a new BPF arena kfunc for reserving a range of arena virtual
addresses without backing them with pages. This prevents the range from
being populated using bpf_arena_alloc_pages().

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250709191312.29840-2-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 0d56cea71602..5b37753799d2 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -550,6 +550,34 @@ static void arena_free_pages(struct bpf_arena *arena, long uaddr, long page_cnt)
 	}
 }
 
+/*
+ * Reserve an arena virtual address range without populating it. This call stops
+ * bpf_arena_alloc_pages from adding pages to this range.
+ */
+static int arena_reserve_pages(struct bpf_arena *arena, long uaddr, u32 page_cnt)
+{
+	long page_cnt_max = (arena->user_vm_end - arena->user_vm_start) >> PAGE_SHIFT;
+	long pgoff;
+	int ret;
+
+	if (uaddr & ~PAGE_MASK)
+		return 0;
+
+	pgoff = compute_pgoff(arena, uaddr);
+	if (pgoff + page_cnt > page_cnt_max)
+		return -EINVAL;
+
+	guard(mutex)(&arena->lock);
+
+	/* Cannot guard already allocated pages. */
+	ret = is_range_tree_set(&arena->rt, pgoff, page_cnt);
+	if (ret)
+		return -EBUSY;
+
+	/* "Allocate" the region to prevent it from being allocated. */
+	return range_tree_clear(&arena->rt, pgoff, page_cnt);
+}
+
 __bpf_kfunc_start_defs();
 
 __bpf_kfunc void *bpf_arena_alloc_pages(void *p__map, void *addr__ign, u32 page_cnt,
@@ -573,11 +601,26 @@ __bpf_kfunc void bpf_arena_free_pages(void *p__map, void *ptr__ign, u32 page_cnt
 		return;
 	arena_free_pages(arena, (long)ptr__ign, page_cnt);
 }
+
+__bpf_kfunc int bpf_arena_reserve_pages(void *p__map, void *ptr__ign, u32 page_cnt)
+{
+	struct bpf_map *map = p__map;
+	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
+
+	if (map->map_type != BPF_MAP_TYPE_ARENA)
+		return -EINVAL;
+
+	if (!page_cnt)
+		return 0;
+
+	return arena_reserve_pages(arena, (long)ptr__ign, page_cnt);
+}
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(arena_kfuncs)
 BTF_ID_FLAGS(func, bpf_arena_alloc_pages, KF_TRUSTED_ARGS | KF_SLEEPABLE | KF_ARENA_RET | KF_ARENA_ARG2)
 BTF_ID_FLAGS(func, bpf_arena_free_pages, KF_TRUSTED_ARGS | KF_SLEEPABLE | KF_ARENA_ARG2)
+BTF_ID_FLAGS(func, bpf_arena_reserve_pages, KF_TRUSTED_ARGS | KF_SLEEPABLE | KF_ARENA_ARG2)
 BTF_KFUNCS_END(arena_kfuncs)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {