arm64: bpf: implement bpf_tail_call() helper
Add support for JMP_CALL_X (tail call) introduced by commit 04fd61ab ("bpf: allow bpf programs to tail-call other bpf programs"). bpf_tail_call() arguments: ctx - context pointer passed to next program array - pointer to map which type is BPF_MAP_TYPE_PROG_ARRAY index - index inside array that selects specific program to run In this implementation arm64 JIT jumps into callee program after prologue, so callee program reuses the same stack. For tail_call_cnt, we use the callee-saved R26 (which was already saved/restored but previously unused by JIT). With this patch a tail call generates the following code on arm64: if (index >= array->map.max_entries) goto out; 34: mov x10, #0x10 // #16 38: ldr w10, [x1,x10] 3c: cmp w2, w10 40: b.ge 0x0000000000000074 if (tail_call_cnt > MAX_TAIL_CALL_CNT) goto out; tail_call_cnt++; 44: mov x10, #0x20 // #32 48: cmp x26, x10 4c: b.gt 0x0000000000000074 50: add x26, x26, #0x1 prog = array->ptrs[index]; if (prog == NULL) goto out; 54: mov x10, #0x68 // #104 58: ldr x10, [x1,x10] 5c: ldr x11, [x10,x2] 60: cbz x11, 0x0000000000000074 goto *(prog->bpf_func + prologue_size); 64: mov x10, #0x20 // #32 68: ldr x10, [x11,x10] 6c: add x10, x10, #0x20 70: br x10 74: Signed-off-by: NZi Shen Lim <zlim.lnx@gmail.com> Signed-off-by: NDavid S. Miller <davem@davemloft.net>
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