[PATCH bpf-next v4 1/2] bpf: allow map helpers access to map values directly

[Paul Chaignon]

Helpers that expect ARG_PTR_TO_MAP_KEY and ARG_PTR_TO_MAP_VALUE can only
access stack and packet memory.  Allow these helpers to directly access
map values by passing registers of type PTR_TO_MAP_VALUE.

This change removes the need for an extra copy to the stack when using a
map value to perform a second map lookup, as in the following:

struct bpf_map_def SEC("maps") infobyreq = {
    .type = BPF_MAP_TYPE_HASHMAP,
    .key_size = sizeof(struct request *),
    .value_size = sizeof(struct info_t),
    .max_entries = 1024,
};
struct bpf_map_def SEC("maps") counts = {
    .type = BPF_MAP_TYPE_HASHMAP,
    .key_size = sizeof(struct info_t),
    .value_size = sizeof(u64),
    .max_entries = 1024,
};
SEC("kprobe/blk_account_io_start")
int bpf_blk_account_io_start(struct pt_regs *ctx)
{
    struct info_t *info = bpf_map_lookup_elem(&infobyreq, &ctx->di);
    u64 *count = bpf_map_lookup_elem(&counts, info);
    (*count)++;
}

Signed-off-by: Paul Chaignon <paul.chaig...@orange.com>
---
 kernel/bpf/verifier.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5dd1dcb902bf..70e00beade03 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1914,7 +1914,7 @@ static int check_func_arg(struct bpf_verifier_env *env, 
u32 regno,
        if (arg_type == ARG_PTR_TO_MAP_KEY ||
            arg_type == ARG_PTR_TO_MAP_VALUE) {
                expected_type = PTR_TO_STACK;
-               if (!type_is_pkt_pointer(type) &&
+               if (!type_is_pkt_pointer(type) && type != PTR_TO_MAP_VALUE &&
                    type != expected_type)
                        goto err_type;
        } else if (arg_type == ARG_CONST_SIZE ||
@@ -1970,6 +1970,9 @@ static int check_func_arg(struct bpf_verifier_env *env, 
u32 regno,
                        err = check_packet_access(env, regno, reg->off,
                                                  meta->map_ptr->key_size,
                                                  false);
+               else if (type == PTR_TO_MAP_VALUE)
+                       err = check_map_access(env, regno, reg->off,
+                                              meta->map_ptr->key_size, false);
                else
                        err = check_stack_boundary(env, regno,
                                                   meta->map_ptr->key_size,
@@ -1987,6 +1990,10 @@ static int check_func_arg(struct bpf_verifier_env *env, 
u32 regno,
                        err = check_packet_access(env, regno, reg->off,
                                                  meta->map_ptr->value_size,
                                                  false);
+               else if (type == PTR_TO_MAP_VALUE)
+                       err = check_map_access(env, regno, reg->off,
+                                              meta->map_ptr->value_size,
+                                              false);
                else
                        err = check_stack_boundary(env, regno,
                                                   meta->map_ptr->value_size,
-- 
2.14.1