On Thu, Jan 4, 2024 at 10:16 PM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote:
>
>
>
> On 5/1/24 12:15, Alexei Starovoitov wrote:
> > On Thu, Jan 4, 2024 at 6:23 AM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote:
> >>
> >>
> >> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> >> index fe30b9ebb8de4..67fa337fc2e0c 100644
> >> --- a/arch/x86/net/bpf_jit_comp.c
> >> +++ b/arch/x86/net/bpf_jit_comp.c
> >> @@ -259,7 +259,7 @@ struct jit_context {
> >> /* Number of bytes emit_patch() needs to generate instructions */
> >> #define X86_PATCH_SIZE 5
> >> /* Number of bytes that will be skipped on tailcall */
> >> -#define X86_TAIL_CALL_OFFSET (11 + ENDBR_INSN_SIZE)
> >> +#define X86_TAIL_CALL_OFFSET (22 + ENDBR_INSN_SIZE)
> >>
> >> static void push_r12(u8 **pprog)
> >> {
> >> @@ -406,14 +406,21 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
> >> */
> >> emit_nops(&prog, X86_PATCH_SIZE);
> >> if (!ebpf_from_cbpf) {
> >> - if (tail_call_reachable && !is_subprog)
> >> + if (tail_call_reachable && !is_subprog) {
> >> /* When it's the entry of the whole tailcall context,
> >> * zeroing rax means initialising tail_call_cnt.
> >> */
> >> - EMIT2(0x31, 0xC0); /* xor eax, eax */
> >> - else
> >> - /* Keep the same instruction layout. */
> >> - EMIT2(0x66, 0x90); /* nop2 */
> >> + EMIT2(0x31, 0xC0); /* xor eax, eax */
> >> + EMIT1(0x50); /* push rax */
> >> + /* Make rax as ptr that points to tail_call_cnt. */
> >> + EMIT3(0x48, 0x89, 0xE0); /* mov rax, rsp */
> >> + EMIT1_off32(0xE8, 2); /* call main prog */
> >> + EMIT1(0x59); /* pop rcx, get rid of tail_call_cnt */
> >> + EMIT1(0xC3); /* ret */
> >> + } else {
> >> + /* Keep the same instruction size. */
> >> + emit_nops(&prog, 13);
> >> + }
> >
> > I'm afraid the extra call breaks stack unwinding and many other things.
>
> I was worried about it. But I'm not sure how it breaks stack unwinding.
>
> However, without the extra call, I've tried another approach:
>
> * [RFC PATCH bpf-next 1/3] bpf, x64: Fix tailcall hierarchy
> https://lore.kernel.org/bpf/20231005145814.83122-2-hffilwlqm@xxxxxxxxx/
>
> It's to propagate tail_call_cnt_ptr, too. But more complicated:
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 8c10d9abc..001c5e4b7 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -313,24 +332,15 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
> bool tail_call_reachable, bool is_subprog,
> bool is_exception_cb)
> {
> + int tcc_ptr_off = round_up(stack_depth, 8) + 8;
> + int tcc_off = tcc_ptr_off + 8;
> u8 *prog = *pprog;
>
> /* BPF trampoline can be made to work without these nops,
> * but let's waste 5 bytes for now and optimize later
> */
> EMIT_ENDBR();
> - memcpy(prog, x86_nops[5], X86_PATCH_SIZE);
> - prog += X86_PATCH_SIZE;
> - if (!ebpf_from_cbpf) {
> - if (tail_call_reachable && !is_subprog)
> - /* When it's the entry of the whole tailcall context,
> - * zeroing rax means initialising tail_call_cnt.
> - */
> - EMIT2(0x31, 0xC0); /* xor eax, eax */
> - else
> - /* Keep the same instruction layout. */
> - EMIT2(0x66, 0x90); /* nop2 */
> - }
> + emit_nops(&prog, X86_PATCH_SIZE);
> /* Exception callback receives FP as third parameter */
> if (is_exception_cb) {
> EMIT3(0x48, 0x89, 0xF4); /* mov rsp, rsi */
> @@ -347,15 +357,52 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
> EMIT1(0x55); /* push rbp */
> EMIT3(0x48, 0x89, 0xE5); /* mov rbp, rsp */
> }
> + if (!ebpf_from_cbpf) {
> + if (tail_call_reachable && !is_subprog) {
> + /* Make rax as ptr that points to tail_call_cnt. */
> + EMIT3(0x48, 0x89, 0xE8); /* mov rax, rbp */
> + EMIT2_off32(0x48, 0x2D, tcc_off); /* sub rax, tcc_off */
> + /* When it's the entry of the whole tail call context,
> + * storing 0 means initialising tail_call_cnt.
> + */
> + EMIT2_off32(0xC7, 0x00, 0); /* mov dword ptr [rax], 0 */
> + } else {
> + /* Keep the same instruction layout. */
> + emit_nops(&prog, 3);
> + emit_nops(&prog, 6);
> + emit_nops(&prog, 6);
Extra 15 nops in the prologue of every bpf program (tailcall or not)
is too high a price to pay.
Think of a simple fix other on verifier side or
simple approach that all JITs can easily do.
