tail_call fix (#670)

Co-authored-by: Dave Thaler <dthaler@microsoft.com>

libs/execution_context/ebpf_maps.c

@@ -400,9 +400,6 @@ _update_array_map_entry_with_handle(
     if (index >= map->ebpf_map_definition.max_entries)
         return EBPF_INVALID_ARGUMENT;
 
-    // The following addition is safe since it was checked during map creation.
-    size_t actual_value_size = ((size_t)map->ebpf_map_definition.value_size) + sizeof(struct _ebpf_object*);
-
     ebpf_result_t result = EBPF_SUCCESS;
 
     ebpf_lock_state_t lock_state = ebpf_lock_lock(&map->lock);
@@ -414,7 +411,7 @@ _update_array_map_entry_with_handle(
     }
 
     // Release the reference on the old ID stored here, if any.
-    uint8_t* entry = &map->data[*key * actual_value_size];
+    uint8_t* entry = &map->data[*key * map->ebpf_map_definition.value_size];
     ebpf_id_t old_id = *(ebpf_id_t*)entry;
     if (old_id) {
         ebpf_object_dereference_by_id(old_id, value_type);

tests/sample/tail_call.c

@@ -4,7 +4,7 @@
 #include "bpf_helpers.h"
 
 SEC("maps")
-struct bpf_map map = {sizeof(struct bpf_map), BPF_MAP_TYPE_PROG_ARRAY, sizeof(uint32_t), sizeof(uint32_t), 1};
+struct bpf_map map = {sizeof(struct bpf_map), BPF_MAP_TYPE_PROG_ARRAY, sizeof(uint32_t), sizeof(uint32_t), 10};
 
 SEC("maps") struct bpf_map canary = {sizeof(struct bpf_map), BPF_MAP_TYPE_ARRAY, sizeof(uint32_t), sizeof(uint32_t), 1};
 
@@ -13,7 +13,7 @@ SEC("xdp_prog") int caller(struct xdp_md* ctx)
     uint32_t key = 0;
     uint32_t* value;
 
-    bpf_tail_call(ctx, &map, 0);
+    bpf_tail_call(ctx, &map, 9);
 
     // If we get to here it means bpf_tail_call failed.
     value = bpf_map_lookup_elem(&canary, &key);

tests/sample/tail_call_bad.c

@@ -4,7 +4,7 @@
 #include "bpf_helpers.h"
 
 SEC("maps")
-struct bpf_map map = {sizeof(struct bpf_map), BPF_MAP_TYPE_PROG_ARRAY, sizeof(uint32_t), sizeof(uint32_t), 1};
+struct bpf_map map = {sizeof(struct bpf_map), BPF_MAP_TYPE_PROG_ARRAY, sizeof(uint32_t), sizeof(uint32_t), 10};
 
 SEC("maps") struct bpf_map canary = {sizeof(struct bpf_map), BPF_MAP_TYPE_ARRAY, sizeof(uint32_t), sizeof(uint32_t), 1};
 
@@ -14,7 +14,7 @@ SEC("xdp_prog") int caller(struct xdp_md* ctx)
     uint32_t* value;
 
     // This should fail since the index is past the end of the array.
-    long error = bpf_tail_call(ctx, &map, 1);
+    long error = bpf_tail_call(ctx, &map, 10);
 
     value = bpf_map_lookup_elem(&canary, &key);
     if (value) {

tests/unit/libbpf_test.cpp

@@ -410,7 +410,7 @@ _ebpf_test_tail_call(_In_z_ const char* filename, int expected_result)
     REQUIRE(canary_map_fd >= 0);
 
     // First do some negative tests.
-    int index = 1;
+    int index = 10;
     error = bpf_map_update_elem(map_fd, (uint8_t*)&index, (uint8_t*)&callee_fd, 0);
     REQUIRE(error < 0);
     REQUIRE(errno == -error);
@@ -421,6 +421,7 @@ _ebpf_test_tail_call(_In_z_ const char* filename, int expected_result)
     REQUIRE(errno == -error);
 
     // Finally store the correct program fd.
+    index = 9;
     error = bpf_map_update_elem(map_fd, (uint8_t*)&index, (uint8_t*)&callee_fd, 0);
     REQUIRE(error == 0);