in native eBPF programs userspace is using pseudo BPF_CALL instructions
which encode one of 'enum bpf_func_id' inside insn->imm field.
Verifier checks that program using correct function arguments to given func_id.
If all checks passed, kernel needs to fixup BPF_CALL->imm fields by
replacing func_id with in-kernel function pointer.
eBPF interpreter just calls the function.
In-kernel eBPF users continue to use generic BPF_CALL.
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxxxx>
---
kernel/bpf/syscall.c | 37 +++++++++++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4c5f5169f6fc..5a336af61858 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -338,6 +338,40 @@ void bpf_register_prog_type(struct bpf_prog_type_list *tl)
list_add(&tl->list_node, &bpf_prog_types);
}
+/* fixup insn->imm field of bpf_call instructions:
+ * if (insn->imm == BPF_FUNC_map_lookup_elem)
+ * insn->imm = bpf_map_lookup_elem - __bpf_call_base;
+ * else if (insn->imm == BPF_FUNC_map_update_elem)
+ * insn->imm = bpf_map_update_elem - __bpf_call_base;
+ * else ...
+ *
+ * this function is called after eBPF program passed verification
+ */
+static void fixup_bpf_calls(struct bpf_prog *prog)
+{
+ const struct bpf_func_proto *fn;
+ int i;
+
+ for (i = 0; i < prog->len; i++) {
+ struct bpf_insn *insn = &prog->insnsi[i];
+
+ if (insn->code == (BPF_JMP | BPF_CALL)) {
+ /* we reach here when program has bpf_call instructions
+ * and it passed bpf_check(), means that
+ * ops->get_func_proto must have been supplied, check it
+ */
+ BUG_ON(!prog->info->ops->get_func_proto);
+
+ fn = prog->info->ops->get_func_proto(insn->imm);
+ /* all functions that have prototype and verifier allowed
+ * programs to call them, must be real in-kernel functions
+ */
+ BUG_ON(!fn->func);
+ insn->imm = fn->func - __bpf_call_base;
+ }
+ }
+}
+
/* drop refcnt on maps used by eBPF program and free auxilary data */
static void free_bpf_prog_info(struct bpf_prog_info *info)
{
@@ -485,6 +519,9 @@ static int bpf_prog_load(enum bpf_prog_type type, struct nlattr __user *uattr,
if (err < 0)
goto free_prog_info;
+ /* fixup BPF_CALL->imm field */
+ fixup_bpf_calls(prog);
+
/* eBPF program is ready to be JITed */
bpf_prog_select_runtime(prog);
--
1.7.9.5
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