bpf: properly verify tail call behavior

A successful ebpf tail call does not return to the caller, but to the
caller-of-the-caller, often just finishing the ebpf program altogether.

Any restrictions that the verifier needs to take into account - notably
the fact that the tail call might have modified packet pointers - are to
be checked on the caller-of-the-caller. Checking it on the caller made
the verifier refuse perfectly fine programs that would use the packet
pointers after a tail call, which is no problem as this code is only
executed if the tail call was unsuccessful, i.e. nothing happened.

This patch simulates the behavior of a tail call in the verifier. A
conditional jump to the code after the tail call is added for the case
of an unsucessful tail call, and a return to the caller is simulated for
a successful tail call.

For the successful case we assume that the tail call returns an int,
as tail calls are currently only allowed in functions that return and
int. We always assume that the tail call modified the packet pointers,
as we do not know what the tail call did.

For the unsuccessful case we know nothing happened, so we do not need to
add new constraints.

This approach also allows to check other problems that may occur with
tail calls, namely we are now able to check that precision is properly
propagated into subprograms using tail calls, as well as checking the
live slots in such a subprogram.

Fixes: 1a4607ffba35 ("bpf: consider that tail calls invalidate packet pointers")
Link: https://lore.kernel.org/bpf/20251029105828.1488347-1-martin.teichmann@xfel.eu/
Signed-off-by: Martin Teichmann <martin.teichmann@xfel.eu>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251119160355.1160932-2-martin.teichmann@xfel.eu
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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