On Wed, Oct 11, 2023 at 11:27:23PM +0800, Leon Hwang wrote:
> From commit ebf7d1f508a73871 ("bpf, x64: rework pro/epilogue and tailcall
> handling in JIT"), the tailcall on x64 works better than before.
>
> From commit e411901c0b775a3a ("bpf: allow for tailcalls in BPF subprograms
> for x64 JIT"), tailcall is able to run in BPF subprograms on x64.
>
> How about:
>
> 1. More than 1 subprograms are called in a bpf program.
> 2. The tailcalls in the subprograms call the bpf program.
>
> Because of missing tail_call_cnt back-propagation, a tailcall hierarchy
> comes up. And MAX_TAIL_CALL_CNT limit does not work for this case.
>
> As we know, in tail call context, the tail_call_cnt propagates by stack
> and rax register between BPF subprograms. So, propagating tail_call_cnt
> pointer by stack and rax register makes tail_call_cnt as like a global
> variable, in order to make MAX_TAIL_CALL_CNT limit works for tailcall
> hierarchy cases.
>
> Before jumping to other bpf prog, load tail_call_cnt from the pointer
> and then compare with MAX_TAIL_CALL_CNT. Finally, increment
> tail_call_cnt by its pointer.
>
> But, where does tail_call_cnt store?
>
> It stores on the stack of bpf prog's caller, like
>
> | STACK |
> | |
> | rip |
> +->| tcc |
> | | rip |
> | | rbp |
> | +---------+ RBP
> | | |
> | | |
> | | |
> +--| tcc_ptr |
> | rbx |
> +---------+ RSP
>
> And tcc_ptr is unnecessary to be popped from stack at the epilogue of bpf
> prog, like the way of commit d207929d97ea028f ("bpf, x64: Drop "pop %rcx"
> instruction on BPF JIT epilogue").
>
> Why not back-propagate tail_call_cnt?
>
> It's because it's vulnerable to back-propagate it. It's unable to work
> well with the following case.
>
> int prog1();
> int prog2();
>
> prog1 is tail caller, and prog2 is tail callee. If we do back-propagate
> tail_call_cnt at the epilogue of prog2, can prog2 run standalone at the
> same time? The answer is NO. Otherwise, there will be a register to be
> polluted, which will make kernel crash.
Sorry but I keep on reading this explanation and I'm lost what is being
fixed here.
You want to limit the total amount of tail calls that entry prog can do to
MAX_TAIL_CALL_CNT. Although I was working on that, my knowledge here is
rusty, therefore my view might be distorted :) to me MAX_TAIL_CALL_CNT is
to protect us from overflowing kernel stack and endless loops. As long a
single call chain doesn't go over 8kB program is fine. Verifier has a
limit of 256 subprogs from what I see.
Can you elaborate a bit more about the kernel crash you mention in the
last paragraph?
I also realized that verifier assumes MAX_TAIL_CALL_CNT as 32 which has
changed in the meantime to 33 and we should adjust the max allowed stack
depth of subprogs? I believe this was brought up at LPC?
>
> Fixes: ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall handling in JIT")
> Fixes: e411901c0b77 ("bpf: allow for tailcalls in BPF subprograms for x64 JIT")
> Signed-off-by: Leon Hwang <hffilwlqm@xxxxxxxxx>
> ---
> arch/x86/net/bpf_jit_comp.c | 40 ++++++++++++++++++++++---------------
> 1 file changed, 24 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index c2a0465d37da4..36631129cc800 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -256,7 +256,7 @@ 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)
> {
> @@ -340,14 +340,21 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
> EMIT_ENDBR();
> 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);
> + }
> }
> /* Exception callback receives FP as third parameter */
> if (is_exception_cb) {
> @@ -373,6 +380,7 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
> if (stack_depth)
> EMIT3_off32(0x48, 0x81, 0xEC, round_up(stack_depth, 8));
> if (tail_call_reachable)
> + /* Here, rax is tail_call_cnt_ptr. */
> EMIT1(0x50); /* push rax */
> *pprog = prog;
> }
> @@ -528,7 +536,7 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
> u32 stack_depth, u8 *ip,
> struct jit_context *ctx)
> {
> - int tcc_off = -4 - round_up(stack_depth, 8);
> + int tcc_ptr_off = -8 - round_up(stack_depth, 8);
> u8 *prog = *pprog, *start = *pprog;
> int offset;
>
> @@ -553,13 +561,12 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
> * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
> * goto out;
> */
> - EMIT2_off32(0x8B, 0x85, tcc_off); /* mov eax, dword ptr [rbp - tcc_off] */
> - EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT); /* cmp eax, MAX_TAIL_CALL_CNT */
> + EMIT3_off32(0x48, 0x8B, 0x85, tcc_ptr_off); /* mov rax, qword ptr [rbp - tcc_ptr_off] */
> + EMIT3(0x83, 0x38, MAX_TAIL_CALL_CNT); /* cmp dword ptr [rax], MAX_TAIL_CALL_CNT */
>
> offset = ctx->tail_call_indirect_label - (prog + 2 - start);
> EMIT2(X86_JAE, offset); /* jae out */
> - EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
> - EMIT2_off32(0x89, 0x85, tcc_off); /* mov dword ptr [rbp - tcc_off], eax */
> + EMIT3(0x83, 0x00, 0x01); /* add dword ptr [rax], 1 */
>
> /* prog = array->ptrs[index]; */
> EMIT4_off32(0x48, 0x8B, 0x8C, 0xD6, /* mov rcx, [rsi + rdx * 8 + offsetof(...)] */
> @@ -581,6 +588,7 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
> pop_callee_regs(&prog, callee_regs_used);
> }
>
> + /* pop tail_call_cnt_ptr */
> EMIT1(0x58); /* pop rax */
> if (stack_depth)
> EMIT3_off32(0x48, 0x81, 0xC4, /* add rsp, sd */
> @@ -609,7 +617,7 @@ static void emit_bpf_tail_call_direct(struct bpf_prog *bpf_prog,
> bool *callee_regs_used, u32 stack_depth,
> struct jit_context *ctx)
> {
> - int tcc_off = -4 - round_up(stack_depth, 8);
> + int tcc_ptr_off = -8 - round_up(stack_depth, 8);
> u8 *prog = *pprog, *start = *pprog;
> int offset;
>
> @@ -617,13 +625,12 @@ static void emit_bpf_tail_call_direct(struct bpf_prog *bpf_prog,
> * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
> * goto out;
> */
> - EMIT2_off32(0x8B, 0x85, tcc_off); /* mov eax, dword ptr [rbp - tcc_off] */
> - EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT); /* cmp eax, MAX_TAIL_CALL_CNT */
> + EMIT3_off32(0x48, 0x8B, 0x85, tcc_ptr_off); /* mov rax, qword ptr [rbp - tcc_ptr_off] */
> + EMIT3(0x83, 0x38, MAX_TAIL_CALL_CNT); /* cmp dword ptr [rax], MAX_TAIL_CALL_CNT */
>
> offset = ctx->tail_call_direct_label - (prog + 2 - start);
> EMIT2(X86_JAE, offset); /* jae out */
> - EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
> - EMIT2_off32(0x89, 0x85, tcc_off); /* mov dword ptr [rbp - tcc_off], eax */
> + EMIT3(0x83, 0x00, 0x01); /* add dword ptr [rax], 1 */
>
> poke->tailcall_bypass = ip + (prog - start);
> poke->adj_off = X86_TAIL_CALL_OFFSET;
> @@ -640,6 +647,7 @@ static void emit_bpf_tail_call_direct(struct bpf_prog *bpf_prog,
> pop_callee_regs(&prog, callee_regs_used);
> }
>
> + /* pop tail_call_cnt_ptr */
> EMIT1(0x58); /* pop rax */
> if (stack_depth)
> EMIT3_off32(0x48, 0x81, 0xC4, round_up(stack_depth, 8));
> --
> 2.41.0
>
>
