linux-kernel - Re: [PATCH RFCv2 08/18] uprobes/x86: Add uprobe syscall to speed up uprobe

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Message-ID: <CAADnVQJ_-7cB3OaeFWaupcq0fRPh3uP62HBGxq0QbyZsx3aHqA@mail.gmail.com>
Date: Mon, 24 Feb 2025 11:22:42 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, X86 ML <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 08/18] uprobes/x86: Add uprobe syscall to speed up uprobe

On Mon, Feb 24, 2025 at 6:08 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> +SYSCALL_DEFINE0(uprobe)
> +{
> +       struct pt_regs *regs = task_pt_regs(current);
> +       unsigned long bp_vaddr;
> +       int err;
> +
> +       err = copy_from_user(&bp_vaddr, (void __user *)regs->sp + 3*8, sizeof(bp_vaddr));
> +       if (err) {
> +               force_sig(SIGILL);
> +               return -1;
> +       }
> +
> +       /* Allow execution only from uprobe trampolines. */
> +       if (!in_uprobe_trampoline(regs->ip)) {
> +               force_sig(SIGILL);
> +               return -1;
> +       }
> +
> +       handle_syscall_uprobe(regs, bp_vaddr - 5);
> +       return 0;
> +}
> +
> +asm (
> +       ".pushsection .rodata\n"
> +       ".balign " __stringify(PAGE_SIZE) "\n"
> +       "uprobe_trampoline_entry:\n"
> +       "endbr64\n"

why endbr is there?
The trampoline is called with a direct call.

> +       "push %rcx\n"
> +       "push %r11\n"
> +       "push %rax\n"
> +       "movq $" __stringify(__NR_uprobe) ", %rax\n"

To avoid introducing a new syscall for a very similar operation
can we disambiguate uprobe vs uretprobe via %rdi or
some other way?
imo not too late to change uretprobe api.
Maybe it was discussed already.

> +       "syscall\n"
> +       "pop %rax\n"
> +       "pop %r11\n"
> +       "pop %rcx\n"
> +       "ret\n"

In later patches I see nop5 is replaced with a call to
uprobe_trampoline_entry, but which part saves
rdi and other regs?
Compiler doesn't automatically spill/fill around USDT's nop/nop5.
Selftest is doing:
+__naked noinline void uprobe_test(void)
so just lucky ?