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

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

On Mon, 14 Jul 2025 11:39:03 +0200
Peter Zijlstra [off-list ref] wrote:

quoted

On Mon, Jul 14, 2025 at 05:39:15PM +0900, Masami Hiramatsu wrote:

quoted

quoted

+	/*
+	 * 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.

quoted

+	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);