linux-kernel - Re: [PATCH bpf-next] uprobes: Fix the xol slots reserved for uretprobe trampoline

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Date: Thu, 20 Jun 2024 11:06:29 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: "Liao, Chang" <liaochang1@...wei.com>, rostedt@...dmis.org,
	mhiramat@...nel.org, ast@...nel.org, daniel@...earbox.net,
	andrii@...nel.org, nathan@...nel.org, peterz@...radead.org,
	mingo@...hat.com, mark.rutland@....com,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next] uprobes: Fix the xol slots reserved for
 uretprobe trampoline

On Thu, Jun 20, 2024 at 10:36:02AM +0200, Oleg Nesterov wrote:
> On 06/20, Liao, Chang wrote:
> >
> > However, when i asm porting uretprobe trampoline to arm64
> > to explore its benefits on that architecture, i discovered the problem that
> > single slot is not large enought for trampoline code.

ah ok, makes sense now.. x86_64 has the slot big enough for the trampoline,
but arm64 does not

> 
> Ah, but then I'd suggest to make the changelog more clear. It looks as
> if the problem was introduced by the patch from Jiri. Note that we was
> confused as well ;)
> 
> And,
> 
> 	+	/* Reserve enough slots for the uretprobe trampoline */
> 	+	for (slot_nr = 0;
> 	+	     slot_nr < max((insns_size / UPROBE_XOL_SLOT_BYTES), 1);
> 	+	     slot_nr++)
> 
> this doesn't look right. Just suppose that insns_size = UPROBE_XOL_SLOT_BYTES + 1.
> I'd suggest DIV_ROUND_UP(insns_size, UPROBE_XOL_SLOT_BYTES).
> 
> And perhaps it would be better to send this change along with
> uretprobe_trampoline_for_arm64 ?

+1, also I'm curious what's the gain on arm64?

thanks,
jirka