Re: [PATCH 1/3] bpf: allow zero-initializing hash map seed

Add a new flag BPF_F_ZERO_SEED, which forces a hash map
to initialize the seed to zero.
---
 include/uapi/linux/bpf.h | 2 ++
 kernel/bpf/hashtab.c     | 8 ++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index aa5ccd2385ed..9d15c8f179ac 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -252,6 +252,8 @@ enum bpf_attach_type {
 #define BPF_F_NO_COMMON_LRU    (1U << 1)
 /* Specify numa node during map creation */
 #define BPF_F_NUMA_NODE                (1U << 2)
+/* Zero-initialize hash function seed */
+#define BPF_F_ZERO_SEED                (1U << 6)

 /* flags for BPF_PROG_QUERY */
 #define BPF_F_QUERY_EFFECTIVE  (1U << 0)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 2c1790288138..a79e123dae62 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c

@@ -23,7 +23,7 @@

 #define HTAB_CREATE_FLAG_MASK                                          \
        (BPF_F_NO_PREALLOC | BPF_F_NO_COMMON_LRU | BPF_F_NUMA_NODE |    \
-        BPF_F_RDONLY | BPF_F_WRONLY)
+        BPF_F_RDONLY | BPF_F_WRONLY | BPF_F_ZERO_SEED)

 struct bucket {
        struct hlist_nulls_head head;

@@ -373,7 +373,11 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
        if (!htab->buckets)
                goto free_htab;

-       htab->hashrnd = get_random_int();
+       if (htab->map.map_flags & BPF_F_ZERO_SEED)
+               htab->hashrnd = 0;
+       else
+               htab->hashrnd = get_random_int();
+