Re: [LSF/MM/BPF TOPIC] faster uprobes

From: Oleg Nesterov <oleg@redhat.com>
To: Jiri Olsa <olsajiri@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	yunwei356@gmail.com, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	lsf-pc <lsf-pc@lists.linux-foundation.org>,
	Yonghong Song <yonghong.song@linux.dev>,
	Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [LSF/MM/BPF TOPIC] faster uprobes
Date: Mon, 11 Mar 2024 16:06:59 +0100	[thread overview]
Message-ID: <20240311150658.GA28588@redhat.com> (raw)
In-Reply-To: <Ze7kGP5aAYGseRuM@krava>

I forgot everything about the low-level x86_64 code, but...

On 03/11, Jiri Olsa wrote:
>
>  #ifdef CONFIG_X86_64
> +
> +asm (
> +       ".pushsection .rodata\n"
> +       ".global uretprobe_syscall_entry\n"
> +       "uretprobe_syscall_entry:\n"
> +       "push %rax\n"
> +       "mov $462, %rax\n"
> +       "syscall\n"

Hmm... I think you need to save/restore more registers clobbered by
syscall/entry_SYSCALL_64 ?

> +SYSCALL_DEFINE1(uprobe, unsigned long, cmd)
> +{
> +	struct pt_regs *regs = task_pt_regs(current);
> +	unsigned long ax, err;
> +
> +	/*
> +	 * We get invoked from the trampoline that pushed rax
> +	 * value on stack, read and restore the value.
> +	 */
> +	err = copy_from_user((void*) &ax, (void *) regs->sp, sizeof(ax));
> +	WARN_ON_ONCE(err);
> +
> +	regs->ax = ax;

probably not strictly needed, we are going to return ax...

> +	regs->orig_ax = ax;

This doesn't look right. I think you need

	regs->orig_ax = -1;

Say, to avoid the "Did we come from a system call" checks in
arch_do_signal_or_restart() or handle_signal().

Oleg.