ebpf: remove self-assignment in interpreter's tail call

ARG1 = BPF_R1 as it stands, evaluates to regs[BPF_REG_1] = regs[BPF_REG_1]
and thus has no effect. Add a comment instead, explaining what happens and
why it's okay to just remove it. Since from user space side, a tail call is
invoked as a pseudo helper function via bpf_tail_call_proto, the verifier
checks the arguments just like with any other helper function and makes
sure that the first argument (regs[BPF_REG_1])'s type is ARG_PTR_TO_CTX.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 1 deletions

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index c5bedc82bc1c..bf38f5e8196c 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -453,7 +453,11 @@ select_insn:
                 if (unlikely(!prog))
                         goto out;
 
-                ARG1 = BPF_R1;
+                /* ARG1 at this point is guaranteed to point to CTX from
+                 * the verifier side due to the fact that the tail call is
+                 * handeled like a helper, that is, bpf_tail_call_proto,
+                 * where arg1_type is ARG_PTR_TO_CTX.
+                 */
                 insn = prog->insnsi;
                 goto select_insn;
 out: