Re: [PATCHv5 perf/core 09/22] uprobes/x86: Add uprobe syscall to speed up uprobe

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 14 Jul 2025 11:39:03 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Jul 14, 2025 at 05:39:15PM +0900, Masami Hiramatsu wrote:
> 
> > > +	/*
> > > +	 * Some of the uprobe consumers has changed sp, we can do nothing,
> > > +	 * just return via iret.
> > > +	 */
> > 
> > Do we allow consumers to change the `sp`? It seems dangerous
> > because consumer needs to know whether it is called from
> > breakpoint or syscall. Note that it has to set up ax, r11
> > and cx on the stack correctly only if it is called from syscall,
> > that is not compatible with breakpoint mode.
> > 
> > > +	if (regs->sp != sp)
> > > +		return regs->ax;
> > 
> > Shouldn't we recover regs->ip? Or in this case does consumer has
> > to change ip (== return address from trampline) too?
> > 
> > IMHO, it should not allow to change the `sp` and `ip` directly
> > in syscall mode. In case of kprobes, kprobe jump optimization
> > must be disabled explicitly (e.g. setting dummy post_handler)
> > if the handler changes `ip`.
> > 
> > Or, even if allowing to modify `sp` and `ip`, it should be helped
> > by this function, e.g. stack up the dummy regs->ax/r11/cx on the
> > new stack at the new `regs->sp`. This will allow modifying those
> > registries transparently as same as breakpoint mode.
> > In this case, I think we just need to remove above 2 lines.
> 
> There are two syscall return paths; the 'normal' is sysret and for that
> you need to undo all things just right.
> 
> The other is IRET. At which point we can have whatever state we want,
> including modified SP.
> 
> See arch/x86/entry/syscall_64.c:do_syscall_64() and
> arch/x86/entry/entry_64.S:entry_SYSCALL_64
> 
> The IRET path should return pt_regs as is from an interrupt/exception
> very much like INT3.

OK, so SYSRET case, we need to follow;

sys_uprobe -> do_syscall_64 -> entry_SYSCALL_64 -> trampoline -> retaddr

But using IRET to return, we can skip returning to trampoline,

sys_uprobe -> do_syscall_64 -> entry_SYSCALL_64 -> regs->ip

Thus we have to check the way, or in both cases use trampoline hack to
change return address.

Thank you,
-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>