On Mon, 14 Jul 2025 11:39:03 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxx>
