Is it possible to tail call eBPF codes that use different modes?

Question

Is it possible to tail call eBPF codes that use different modes?

For example, if I coded a code that printk("hello world") using kprobe,

would I be able to tail call a XDP code afterwards or vice versa?

I programmed something on eBPF that uses a socket buffer and seems like when I try to tail call another code that uses kprobe, it doesn't load the program.

I wanted to tail call a code that uses XDP_PASS after using a BPF.SOCKET_FILTER mode but seems like tail call isn't working.

I've been trying to figure this out but I can't find any documentations regarding tail calling codes that use different modes :P

Thanks in advance!

Answer 1

No, it is not.

Have a look at kernel commit 04fd61ab36ec, which introduced tail calls: the comment in the first piece of code (in internal kernel header bpf.h), defining the struct bpf_array, sets a owner_prog_type member, and explains the following in a comment:

/* 'ownership' of prog_array is claimed by the first program that
 * is going to use this map or by the first program which FD is stored
 * in the map to make sure that all callers and callees have the same
 * prog_type and JITed flag
 */

So once the program type associated with a BPF program array, used for tail calls, has been defined, it is not possible to use it with other program types. Which makes sense, since different program types work with different context (packet data VS traced function context VS ...), can use different helpers, have return functions with different meanings, necessitate different checks from the verifier, ... So it's hard to see how jumping from one type to another would work. How could you start with processing a network packet, and all of a sudden jump to a piece of code that is supposed to trace some internals of the kernel? :)

Note that it is also impossible to mix JIT-ed and non-JIT-ed programs, as indicated by the owner_jited of the struct.