Re: [PATCH v3] eventpoll: Convert epoll_put_uevent() to scoped user access

[Eric Dumazet]

On Sun, Mar 8, 2026 at 12:45 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sat, 7 Mar 2026 at 15:33, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Oh well. This makes one little piece of the kernel better, even if
> > other horrors lurk everywhere...
>
> Side note: if you want to improve code generation just a *tiny* bit
> more, you should make that epoll_put_uevent() return a success value
> instead ot the updated pointer value.
>
> Because right now the caller does this:
>
>                 events = epoll_put_uevent(revents, epi->event.data, events);
>                 if (!events) {
>
> and that "if (!events)" test triggers even for the success case,
> because the compiler doesn't know whether the "uevent+1" might
> overflow (and we really *do* want to make sure the compiler doesn't
> remove overflow checks even if it causes these kinds of issues).
>
> If epoll_put_uevent() just returns a success/fail marker (or 0/-EFAULT
> or whatever), we wouldn't have that issue.
>
> HOWEVER - the same magical ARM case would need to be dealt with - the
> real size of the user space notion of "struct epoll_event" isn't
> actually known to generic code, because it ends up depending on
> architecture-specific packing issues.
>
> So you'd have to pass in 'events' by reference and epoll_put_uevent()
> would still update it.
>
> Probably not worth it - it would mainly get rid of a perfectly
> predicted conditional branch, and might possibly help a tiny bit with
> register liveness analysis. But it's unlikely to have any really
> visible effects - it's not like the clac/stac overhead.
>
> But since I looked at the generated code, I thought I'd mention it.
>
>                   Linus

Thanks Linus

BTW, not really a bug, but we could remove these extra semicolons
added in af4e9ef3d784 ("uaccess: Fix scoped_user_read_access() for
'pointer to const'")

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 809e4f7dfdbd4d4426ef38457ae108f119cb1bee..cdce14d864d6b71ea87396aee35acd6c9f188ae0
100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -654,15 +654,15 @@ static inline void user_access_restore(unsigned
long flags) { }
 static __always_inline void __scoped_user_read_access_end(const void *p)
 {
        user_read_access_end();
-};
+}
 static __always_inline void __scoped_user_write_access_end(const void *p)
 {
        user_write_access_end();
-};
+}
 static __always_inline void __scoped_user_rw_access_end(const void *p)
 {
        user_access_end();
-};
+}

 /**
  * __scoped_user_access_begin - Start a scoped user access