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

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Date: Tue, 15 Aug 2023 17:35:50 +0300
From: Eduard Zingerman <eddyz87@...il.com>
To: Leon Hwang <hffilwlqm@...il.com>, bpf@...r.kernel.org
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev,  song@...nel.org, yonghong.song@...ux.dev,
 john.fastabend@...il.com,  kpsingh@...nel.org, sdf@...gle.com,
 haoluo@...gle.com, jolsa@...nel.org,  x86@...nel.org, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de,  dave.hansen@...ux.intel.com,
 hpa@...or.com, mykolal@...com, shuah@...nel.org,  davem@...emloft.net,
 dsahern@...nel.org, tangyeechou@...il.com,  kernel-patches-bot@...com,
 maciej.fijalkowski@...el.com, netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next 1/2] bpf, x64: Fix tailcall infinite loop
 bug

On Tue, 2023-08-15 at 11:01 +0800, Leon Hwang wrote:
[...]
> a) Initial value of RAX is in emit_prologue().
> 	if (!ebpf_from_cbpf) {
> 		if (tail_call_reachable && !is_subprog)
> 			/* When it's the entry of the whole
> 			 * tailcall context, zeroing the RAX
> 			 * means init tail_call_cnt.
> 			 */
> 			EMIT2(0x31, 0xC0); /* xor eax, eax */
> 		else
> 			// Keep the same asm layout.
> 			EMIT2(0x66, 0x90); /* nop2 */
> 	}
>    I'd like to add this comment to emit_prologue().

Got it, thank you.


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