Re: [RFC PATCH bpf-next v3 1/2] bpf, x64: Fix tailcall infinite loop

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Fri, Aug 25, 2023 at 10:52:15PM +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.
> 
> From commit 5b92a28aae4dd0f8 ("bpf: Support attaching tracing BPF program
> to other BPF programs"), BPF program is able to trace other BPF programs.
> 
> How about combining them all together?
> 
> 1. FENTRY/FEXIT on a BPF subprogram.
> 2. A tailcall runs in the BPF subprogram.
> 3. The tailcall calls itself.

I would be interested in seeing broken asm code TBH :)

> 
> As a result, a tailcall infinite loop comes up. And the loop would halt
> the machine.
> 
> As we know, in tail call context, the tail_call_cnt propagates by stack
> and rax register between BPF subprograms. So do it in trampolines.
> 
> Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 32 ++++++++++++++++++++++++++------
>  include/linux/bpf.h         |  5 +++++
>  kernel/bpf/trampoline.c     |  4 ++--
>  kernel/bpf/verifier.c       | 30 +++++++++++++++++++++++-------
>  4 files changed, 56 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index a5930042139d3..2846c21d75bfa 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -303,8 +303,12 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
>  	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. */

While these comments are helpful I have mixed feelings about them residing
in this patch - rule of thumb to me is to keep the fixes as small as
possible.

>  			EMIT2(0x66, 0x90); /* nop2 */
>  	}
>  	EMIT1(0x55);             /* push rbp */
> @@ -1018,6 +1022,10 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)
>  
>  #define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))
>  
> +/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
> +#define RESTORE_TAIL_CALL_CNT(stack)				\
> +	EMIT3_off32(0x48, 0x8B, 0x85, -round_up(stack, 8) - 8)
> +
>  static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,
>  		  int oldproglen, struct jit_context *ctx, bool jmp_padding)
>  {
> @@ -1623,9 +1631,7 @@ st:			if (is_imm8(insn->off))
>  
>  			func = (u8 *) __bpf_call_base + imm32;
>  			if (tail_call_reachable) {
> -				/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
> -				EMIT3_off32(0x48, 0x8B, 0x85,
> -					    -round_up(bpf_prog->aux->stack_depth, 8) - 8);
> +				RESTORE_TAIL_CALL_CNT(bpf_prog->aux->stack_depth);
>  				if (!imm32)
>  					return -EINVAL;
>  				offs = 7 + x86_call_depth_emit_accounting(&prog, func);
> @@ -2400,6 +2406,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>  	 *                     [ ...        ]
>  	 *                     [ stack_arg2 ]
>  	 * RBP - arg_stack_off [ stack_arg1 ]
> +	 * RSP                 [ tail_call_cnt ] BPF_TRAMP_F_TAIL_CALL_CTX
>  	 */
>  
>  	/* room for return value of orig_call or fentry prog */
> @@ -2464,6 +2471,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>  	else
>  		/* sub rsp, stack_size */
>  		EMIT4(0x48, 0x83, 0xEC, stack_size);
> +	if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
> +		EMIT1(0x50);		/* push rax */
>  	/* mov QWORD PTR [rbp - rbx_off], rbx */
>  	emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_6, -rbx_off);
>  
> @@ -2516,9 +2525,15 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>  		restore_regs(m, &prog, regs_off);
>  		save_args(m, &prog, arg_stack_off, true);
>  
> +		if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
> +			/* Before calling the original function, restore the
> +			 * tail_call_cnt from stack to rax.
> +			 */
> +			RESTORE_TAIL_CALL_CNT(stack_size);
> +
>  		if (flags & BPF_TRAMP_F_ORIG_STACK) {
> -			emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
> -			EMIT2(0xff, 0xd0); /* call *rax */
> +			emit_ldx(&prog, BPF_DW, BPF_REG_6, BPF_REG_FP, 8);
> +			EMIT2(0xff, 0xd3); /* call *rbx */
>  		} else {
>  			/* call original function */
>  			if (emit_rsb_call(&prog, orig_call, prog)) {
> @@ -2569,7 +2584,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>  			ret = -EINVAL;
>  			goto cleanup;
>  		}
> -	}
> +	} else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
> +		/* Before running the original function, restore the
> +		 * tail_call_cnt from stack to rax.
> +		 */
> +		RESTORE_TAIL_CALL_CNT(stack_size);
> +
>  	/* restore return value of orig_call or fentry prog back into RAX */
>  	if (save_ret)
>  		emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index cfabbcf47bdb8..c8df257ea435d 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1028,6 +1028,11 @@ struct btf_func_model {
>   */
>  #define BPF_TRAMP_F_SHARE_IPMODIFY	BIT(6)
>  
> +/* Indicate that current trampoline is in a tail call context. Then, it has to
> + * cache and restore tail_call_cnt to avoid infinite tail call loop.
> + */
> +#define BPF_TRAMP_F_TAIL_CALL_CTX	BIT(7)
> +
>  /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
>   * bytes on x86.
>   */
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 78acf28d48732..16ab5da7161f2 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -415,8 +415,8 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
>  		goto out;
>  	}
>  
> -	/* clear all bits except SHARE_IPMODIFY */
> -	tr->flags &= BPF_TRAMP_F_SHARE_IPMODIFY;
> +	/* clear all bits except SHARE_IPMODIFY and TAIL_CALL_CTX */
> +	tr->flags &= (BPF_TRAMP_F_SHARE_IPMODIFY | BPF_TRAMP_F_TAIL_CALL_CTX);
>  
>  	if (tlinks[BPF_TRAMP_FEXIT].nr_links ||
>  	    tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links) {
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 4ccca1f6c9981..6f290bc6f5f19 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -19246,6 +19246,21 @@ static int check_non_sleepable_error_inject(u32 btf_id)
>  	return btf_id_set_contains(&btf_non_sleepable_error_inject, btf_id);
>  }
>  
> +static inline int find_subprog_index(const struct bpf_prog *prog,

FWIW please no inlines in source files, but I don't currently follow the
need for that routine.

> +				     u32 btf_id)
> +{
> +	struct bpf_prog_aux *aux = prog->aux;
> +	int i, subprog = -1;
> +
> +	for (i = 0; i < aux->func_info_cnt; i++)
> +		if (aux->func_info[i].type_id == btf_id) {
> +			subprog = i;
> +			break;
> +		}
> +
> +	return subprog;
> +}
> +
>  int bpf_check_attach_target(struct bpf_verifier_log *log,
>  			    const struct bpf_prog *prog,
>  			    const struct bpf_prog *tgt_prog,
> @@ -19254,9 +19269,9 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  {
>  	bool prog_extension = prog->type == BPF_PROG_TYPE_EXT;
>  	const char prefix[] = "btf_trace_";
> -	int ret = 0, subprog = -1, i;
>  	const struct btf_type *t;
>  	bool conservative = true;
> +	int ret = 0, subprog;
>  	const char *tname;
>  	struct btf *btf;
>  	long addr = 0;
> @@ -19291,11 +19306,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  			return -EINVAL;
>  		}
>  
> -		for (i = 0; i < aux->func_info_cnt; i++)
> -			if (aux->func_info[i].type_id == btf_id) {
> -				subprog = i;
> -				break;
> -			}
> +		subprog = find_subprog_index(tgt_prog, btf_id);
>  		if (subprog == -1) {
>  			bpf_log(log, "Subprog %s doesn't exist\n", tname);
>  			return -EINVAL;
> @@ -19559,7 +19570,7 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>  	struct bpf_attach_target_info tgt_info = {};
>  	u32 btf_id = prog->aux->attach_btf_id;
>  	struct bpf_trampoline *tr;
> -	int ret;
> +	int ret, subprog;
>  	u64 key;
>  
>  	if (prog->type == BPF_PROG_TYPE_SYSCALL) {
> @@ -19629,6 +19640,11 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>  	if (!tr)
>  		return -ENOMEM;
>  
> +	if (tgt_prog && tgt_prog->aux->tail_call_reachable) {
> +		subprog = find_subprog_index(tgt_prog, btf_id);
> +		tr->flags = subprog > 0 ? BPF_TRAMP_F_TAIL_CALL_CTX : 0;
> +	}

I kinda forgot trampoline internals so please bear with me.
Here you are checking actually...what? That current program is a subprog
of tgt prog? My knee jerk reaction would be to propagate the
BPF_TRAMP_F_TAIL_CALL_CTX based on just tail_call_reachable, but I need
some more time to get my head around it again, sorry :<

> +
>  	prog->aux->dst_trampoline = tr;
>  	return 0;
>  }
> -- 
> 2.41.0
>