Re: [PATCH bpf-next v2 1/2] bpf, x64: Fix tailcall hierarchy

[Leon Hwang <hffilwlqm@gmail.com>]

On 2024/2/22 16:52, 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.
> 

[SNIP]

> 
> 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@gmail.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 128 ++++++++++++++++++++----------------
>  1 file changed, 71 insertions(+), 57 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index e1390d1e331b5..3d1498a13b04c 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -18,6 +18,7 @@
>  #include <asm/text-patching.h>
>  #include <asm/unwind.h>
>  #include <asm/cfi.h>
> +#include <asm/percpu.h>
>  

[SNIP]

> +
>  /*
>   * Generate the following code:

nit: the "tail_call_cnt++" of the comment should be updated too.

>   *
> @@ -594,7 +641,6 @@ 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);
>  	u8 *prog = *pprog, *start = *pprog;
>  	int offset;
>  
> @@ -615,17 +661,14 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
>  	offset = ctx->tail_call_indirect_label - (prog + 2 - start);
>  	EMIT2(X86_JBE, offset);                   /* jbe out */
>  
> -	/*
> -	 * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
> +	/* if (bpf_tail_call_cnt_fetch_and_inc() >= MAX_TAIL_CALL_CNT)
>  	 *	goto out;
>  	 */
> -	EMIT2_off32(0x8B, 0x85, tcc_off);         /* mov eax, dword ptr [rbp - tcc_off] */
> +	emit_call(&prog, bpf_tail_call_cnt_fetch_and_inc, ip + (prog - start));
>  	EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT);     /* cmp eax, MAX_TAIL_CALL_CNT */

[SNIP]

Thanks,
Leon