On Tue, Jan 14, 2025 at 12:21:07PM +0100, Oleg Nesterov wrote:

On 01/14, Masami Hiramatsu wrote:

quoted

On Tue, 14 Jan 2025 10:22:20 +0100
Jiri Olsa [off-list ref] wrote:

quoted

@@ -418,6 +439,9 @@ SYSCALL_DEFINE0(uretprobe)
 	regs->r11 = regs->flags;
 	regs->cx  = regs->ip;

+	/* zero rbx to signal trampoline that uretprobe syscall was executed */
+	regs->bx  = 0;

Can we just return -ENOSYS as like as other syscall instead of
using rbx as a side channel?
We can carefully check the return address is not -ERRNO when set up
and reserve the -ENOSYS for this use case.

Not sure I understand...

But please not that the uretprobed function can return any value
including -ENOSYS, and this is what sys_uretprobe() has to return.

right, uretprobe syscall returns value of the uretprobed function,
so we can't use any reserved value

jirka