bpf: Move PTR_TO_STACK alignment check to process_dynptr_func

After previous commit, we are minimizing helper specific assumptions
from check_func_arg_reg_off, making it generic, and offloading checks
for a specific argument type to their respective functions called after
check_func_arg_reg_off has been called.

This allows relying on a consistent set of guarantees after that call
and then relying on them in code that deals with registers for each
argument type later. This is in line with how process_spin_lock,
process_timer_func, process_kptr_func check reg->var_off to be constant.
The same reasoning is used here to move the alignment check into
process_dynptr_func. Note that it also needs to check for constant
var_off, and accumulate the constant var_off when computing the spi in
get_spi, but that fix will come in later changes.

Acked-by: Joanne Koong <[e-mail]>
Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
 		verbose(env, "verifier internal error: misconfigured dynptr helper type flags\n");
 		return -EFAULT;
 	}
+	/* CONST_PTR_TO_DYNPTR already has fixed and var_off as 0 due to
+	 * check_func_arg_reg_off's logic. We only need to check offset
+	 * alignment for PTR_TO_STACK.
+	 */
+	if (reg->type == PTR_TO_STACK && (reg->off % BPF_REG_SIZE)) {
+		verbose(env, "cannot pass in dynptr at an offset=%d\n", reg->off);
+		return -EINVAL;
+	}
 	/*  MEM_UNINIT - Points to memory that is an appropriate candidate for
 	 *		 constructing a mutable bpf_dynptr object.
 	 *
···
 	switch (type) {
 	/* Pointer types where both fixed and variable offset is explicitly allowed: */
 	case PTR_TO_STACK:
-		if (arg_type_is_dynptr(arg_type) && reg->off % BPF_REG_SIZE) {
-			verbose(env, "cannot pass in dynptr at an offset\n");
-			return -EINVAL;
-		}
-		fallthrough;
 	case PTR_TO_PACKET:
 	case PTR_TO_PACKET_META:
 	case PTR_TO_MAP_KEY: