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

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jul 14, 2025 at 07:19:35PM +0900, Masami Hiramatsu wrote:
> 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

the handler gets the original breakpoint address, it's set in:

        regs->ip  = ax_r11_cx_ip[3] - 5;

and at the point we do:

        /*
         * Some of the uprobe consumers has changed sp, we can do nothing,
         * just return via iret.
         */
        if (regs->sp != sp)
                return regs->ax;


.. regs->ip value wasn't restored for the trampoline's return address,
so iret will skip the trampoline

but perhaps we could do the extra check below to land on the next instruction?

jirka


---
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 043d826295a3..4318517aa852 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -817,8 +817,12 @@ SYSCALL_DEFINE0(uprobe)
 	 * Some of the uprobe consumers has changed sp, we can do nothing,
 	 * just return via iret.
 	 */
-	if (regs->sp != sp)
+	if (regs->sp != sp) {
+		/* skip the trampoline call */
+		if (ax_r11_cx_ip[3] - 5 == regs->ip)
+			regs->ip += 5;
 		return regs->ax;
+	}
 
 	regs->sp -= sizeof(ax_r11_cx_ip);