bpf: Introduce may_goto instruction

bpf: Introduce may_goto instruction

Introduce may_goto instruction that from the verifier pov is similar to
open coded iterators bpf_for()/bpf_repeat() and bpf_loop() helper, but it
doesn't iterate any objects.
In assembly 'may_goto' is a nop most of the time until bpf runtime has to
terminate the program for whatever reason. In the current implementation
may_goto has a hidden counter, but other mechanisms can be used.
For programs written in C the later patch introduces 'cond_break' macro
that combines 'may_goto' with 'break' statement and has similar semantics:
cond_break is a nop until bpf runtime has to break out of this loop.
It can be used in any normal "for" or "while" loop, like

  for (i = zero; i < cnt; cond_break, i++) {

The verifier recognizes that may_goto is used in the program, reserves
additional 8 bytes of stack, initializes them in subprog prologue, and
replaces may_goto instruction with:
aux_reg = *(u64 *)(fp - 40)
if aux_reg == 0 goto pc+off
aux_reg -= 1
*(u64 *)(fp - 40) = aux_reg

may_goto instruction can be used by LLVM to implement __builtin_memcpy,
__builtin_strcmp.

may_goto is not a full substitute for bpf_for() macro.
bpf_for() doesn't have induction variable that verifiers sees,
so 'i' in bpf_for(i, 0, 100) is seen as imprecise and bounded.

But when the code is written as:
for (i = 0; i < 100; cond_break, i++)
the verifier see 'i' as precise constant zero,
hence cond_break (aka may_goto) doesn't help to converge the loop.
A static or global variable can be used as a workaround:
static int zero = 0;
for (i = zero; i < 100; cond_break, i++) // works!

may_goto works well with arena pointers that don't need to be bounds
checked on access. Load/store from arena returns imprecise unbounded
scalar and loops with may_goto pass the verifier.

Reserve new opcode BPF_JMP | BPF_JCOND for may_goto insn.
JCOND stands for conditional pseudo jump.
Since goto_or_nop insn was proposed, it may use the same opcode.
may_goto vs goto_or_nop can be distinguished by src_reg:
code = BPF_JMP | BPF_JCOND
src_reg = 0 - may_goto
src_reg = 1 - goto_or_nop

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Tested-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20240306031929.42666-2-alexei.starovoitov@gmail.com