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

[Oleg Nesterov <oleg@redhat.com>]

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.