bpf: Improve ctx access verifier error message

JIRA: https://issues.redhat.com/browse/RHEL-78204

commit f914876
Author: Paul Chaignon <[email protected]>
Date:   Fri Aug 1 11:49:15 2025 +0200

    bpf: Improve ctx access verifier error message

    We've already had two "error during ctx access conversion" warnings
    triggered by syzkaller. Let's improve the error message by dumping the
    cnt variable so that we can more easily differentiate between the
    different error cases.

    Signed-off-by: Paul Chaignon <[email protected]>
    Acked-by: Eduard Zingerman <[email protected]>
    Link: https://lore.kernel.org/r/cc94316c30dd76fae4a75a664b61a2dbfe68e205.1754039605.git.paul.chaignon@gmail.com
    Signed-off-by: Alexei Starovoitov <[email protected]>

Signed-off-by: Jerome Marchand <[email protected]>

Author: jeromemarchand

kernel/bpf/verifier.c

@@ -21449,7 +21449,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 					 &target_size);
 		if (cnt == 0 || cnt >= INSN_BUF_SIZE ||
 		    (ctx_field_size && !target_size)) {
-			verifier_bug(env, "error during ctx access conversion");
+			verifier_bug(env, "error during ctx access conversion (%d)", cnt);
 			return -EFAULT;
 		}