linux-kernel - Re: [PATCH RFCv2 12/18] uprobes/x86: Add support to optimize uprobes

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Message-ID: <Z8JAHHM4xqEQA2f3@krava>
Date: Sat, 1 Mar 2025 00:00:44 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii@...nel.org>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, x86@...nel.org,
	Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Alan Maguire <alan.maguire@...cle.com>,
	David Laight <David.Laight@...lab.com>,
	Thomas Weißschuh <thomas@...ch.de>
Subject: Re: [PATCH RFCv2 12/18] uprobes/x86: Add support to optimize uprobes

On Mon, Feb 24, 2025 at 03:01:44PM +0100, Jiri Olsa wrote:

SNIP

> @@ -1523,15 +1698,23 @@ arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr, struct pt_regs
>  {
>  	int rasize = sizeof_long(regs), nleft;
>  	unsigned long orig_ret_vaddr = 0; /* clear high bits for 32-bit apps */
> +	unsigned long off = 0;
> +
> +	/*
> +	 * Optimized uprobe goes through uprobe trampoline which adds 4 8-byte
> +	 * values on stack, check uprobe_trampoline_entry for details.
> +	 */
> +	if (!swbp)
> +		off = 4*8;

ok, now when I started to add the missing register modifications in uprobe syscall,
I realized we will modify the regs->sp appropriately already in the uprobe syscall
(before the code above is hit)

so we don't need this code and we can get rid of the swbp flag and patch#7 completely

jirka

>  
> -	if (copy_from_user(&orig_ret_vaddr, (void __user *)regs->sp, rasize))
> +	if (copy_from_user(&orig_ret_vaddr, (void __user *)regs->sp + off, rasize))
>  		return -1;
>  
>  	/* check whether address has been already hijacked */
>  	if (orig_ret_vaddr == trampoline_vaddr)
>  		return orig_ret_vaddr;
>  
> -	nleft = copy_to_user((void __user *)regs->sp, &trampoline_vaddr, rasize);
> +	nleft = copy_to_user((void __user *)regs->sp + off, &trampoline_vaddr, rasize);
>  	if (likely(!nleft)) {
>  		if (shstk_update_last_frame(trampoline_vaddr)) {
>  			force_sig(SIGSEGV);

SNIP