[PATCH bpf] bpf: Proper R0 zero-extension for BPF_CALL instructions

["Björn Töpel" <bjorn@kernel.org>]

From: Björn Töpel <bjorn@rivosinc.com>

A BPF call instruction can be, correctly, marked with zext_dst set to
true. An example of this can be found in the BPF selftests
progs/bpf_cubic.c:

  ...
  extern __u32 tcp_reno_undo_cwnd(struct sock *sk) __ksym;

  __u32 BPF_STRUCT_OPS(bpf_cubic_undo_cwnd, struct sock *sk)
  {
          return tcp_reno_undo_cwnd(sk);
  }
  ...

which compiles to:
  0:  r1 = *(u64 *)(r1 + 0x0)
  1:  call -0x1
  2:  exit

The call will be marked as zext_dst set to true, and for some backends
(bpf_jit_needs_zext() returns true) expanded to:
  0:  r1 = *(u64 *)(r1 + 0x0)
  1:  call -0x1
  2:  w0 = w0
  3:  exit

The opt_subreg_zext_lo32_rnd_hi32() function which is responsible for
the zext patching, relies on insn_def_regno() to fetch the register to
zero-extend. However, this function does not handle call instructions
correctly, and opt_subreg_zext_lo32_rnd_hi32() fails the verification.

Make sure that R0 is correctly resolved for (BPF_JMP | BPF_CALL)
instructions.

Fixes: 83a2881903f3 ("bpf: Account for BPF_FETCH in insn_has_def32()")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
---
I'm not super happy about the additional special case -- first
cmpxchg, and now call. :-( A more elegant/generic solution is welcome!
---
 kernel/bpf/verifier.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 264b3dc714cc..4f9660eafc72 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -13386,6 +13386,9 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
 		if (!bpf_jit_needs_zext() && !is_cmpxchg_insn(&insn))
 			continue;
 
+		if (insn.code == (BPF_JMP | BPF_CALL))
+			load_reg = BPF_REG_0;
+
 		if (WARN_ON(load_reg == -1)) {
 			verbose(env, "verifier bug. zext_dst is set, but no reg is defined\n");
 			return -EFAULT;

base-commit: 01f856ae6d0ca5ad0505b79bf2d22d7ca439b2a1
-- 
2.37.2