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Message-ID: <CAH5fLghn2=Y-M-M7-17PhNvKN3qVTsa6wh=Df770kHj1Oxh8hQ@mail.gmail.com>
Date: Thu, 25 Jan 2024 17:15:13 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>,
Kees Cook <keescook@...omium.org>, Alexander Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Arve Hjønnevåg <arve@...roid.com>,
Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>,
Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
rust-for-linux@...r.kernel.org, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH 2/3] rust: add typed accessors for userspace pointers
On Thu, Jan 25, 2024 at 5:00 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Thu, Jan 25, 2024, at 13:37, Alice Ryhl wrote:
> > On Thu, Jan 25, 2024 at 1:27 PM Arnd Bergmann <arnd@...db.de> wrote:
> >> On Wed, Jan 24, 2024, at 12:20, Alice Ryhl wrote:
>
> >> > +EXPORT_SYMBOL_GPL(rust_helper_copy_to_user_unsafe_skip_check_object_size);
> >>
> >> These functions are almost identical to the ones in
> >> lib/usercopy.c for !defined(INLINE_COPY_TO_USER).
> >>
> >> That version has an extra memset() after a partial
> >> copy_from_user(), and you probably want to have the
> >> same thing here for consistency.
> >>
> >> I think ideally we should only have one out-of-line copy
> >> of these two functions and have that one shared between
> >> rust and architectures that want the C version out of line
> >> as well.
> >
> > I had a bit of trouble figuring out all of the copy_[to/from]_user
> > methods that are available. I was hoping that a better solution would
> > be available, and it sounds like one is. Is _copy_from_user always
> > available as an exported symbol? If it's always available and skips
> > the check, then I can just use that. I don't think the memset matters
> > for my case.
>
> At the moment, it appears that it's available on the few architectures
> that don't #define INLINE_COPY_FROM_USER: alpha, csky, powerpc,
> riscv and x86. On the other architectures it is always an inline
> function.
>
> > Otherwise, I can add a helper in rust/helpers.c that wraps
> > _copy_from_user only when INLINE_COPY_FROM_USER is defined, and call
> > the helper in those cases, and otherwise call the exported symbol
> > directly. (I need an exported symbol to call into C from Rust.)
> >
> > Would that make sense?
>
> I don't think we can have a perfect abstraction here, but rather
> than putting knowledge of INLINE_COPY_FROM_USER into the rust
> wrapper, I would suggest putting a bit of information about
> rust into lib/usercopy.c.
>
> I've tried to come up with an idea below, see if that works
> for you.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 3064314f4832..835aa175d0ee 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -138,13 +138,18 @@ __copy_to_user(void __user *to, const void *from, unsigned long n)
> return raw_copy_to_user(to, from, n);
> }
>
> -#ifdef INLINE_COPY_FROM_USER
> static inline __must_check unsigned long
> -_copy_from_user(void *to, const void __user *from, unsigned long n)
> +_inline_copy_from_user(void *to, const void __user *from, unsigned long n)
> {
> unsigned long res = n;
> might_fault();
> if (!should_fail_usercopy() && likely(access_ok(from, n))) {
> + /*
> + * Ensure that bad access_ok() speculation will not
> + * lead to nasty side effects *after* the copy is
> + * finished:
> + */
> + barrier_nospec();
> instrument_copy_from_user_before(to, from, n);
> res = raw_copy_from_user(to, from, n);
> instrument_copy_from_user_after(to, from, n, res);
> @@ -153,14 +158,11 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
> memset(to + (n - res), 0, res);
> return res;
> }
> -#else
> extern __must_check unsigned long
> _copy_from_user(void *, const void __user *, unsigned long);
> -#endif
>
> -#ifdef INLINE_COPY_TO_USER
> static inline __must_check unsigned long
> -_copy_to_user(void __user *to, const void *from, unsigned long n)
> +_inline_copy_to_user(void __user *to, const void *from, unsigned long n)
> {
> might_fault();
> if (should_fail_usercopy())
> @@ -171,25 +173,32 @@ _copy_to_user(void __user *to, const void *from, unsigned long n)
> }
> return n;
> }
> -#else
> extern __must_check unsigned long
> _copy_to_user(void __user *, const void *, unsigned long);
> -#endif
>
> static __always_inline unsigned long __must_check
> copy_from_user(void *to, const void __user *from, unsigned long n)
> {
> - if (check_copy_size(to, n, false))
> - n = _copy_from_user(to, from, n);
> - return n;
> + if (!check_copy_size(to, n, false))
> + return n;
> +#ifdef INLINE_COPY_FROM_USER
> + return _inline_copy_from_user(to, from, n);
> +#else
> + return _copy_from_user(to, from, n);
> +#endif
> }
>
> static __always_inline unsigned long __must_check
> copy_to_user(void __user *to, const void *from, unsigned long n)
> {
> - if (check_copy_size(from, n, true))
> - n = _copy_to_user(to, from, n);
> - return n;
> + if (!check_copy_size(from, n, true))
> + return n;
> +
> +#ifdef INLINE_COPY_TO_USER
> + return _inline_copy_to_user(to, from, n);
> +#else
> + return _copy_to_user(to, from, n);
> +#endif
> }
>
> #ifndef copy_mc_to_kernel
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index d29fe29c6849..503a064d79e2 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -7,40 +7,18 @@
>
> /* out-of-line parts */
>
> -#ifndef INLINE_COPY_FROM_USER
> +#if !defined(INLINE_COPY_FROM_USER) || defined(CONFIG_RUST)
> unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n)
> {
> - unsigned long res = n;
> - might_fault();
> - if (!should_fail_usercopy() && likely(access_ok(from, n))) {
> - /*
> - * Ensure that bad access_ok() speculation will not
> - * lead to nasty side effects *after* the copy is
> - * finished:
> - */
> - barrier_nospec();
> - instrument_copy_from_user_before(to, from, n);
> - res = raw_copy_from_user(to, from, n);
> - instrument_copy_from_user_after(to, from, n, res);
> - }
> - if (unlikely(res))
> - memset(to + (n - res), 0, res);
> - return res;
> + return _inline_copy_from_user(to, from, n);
> }
> EXPORT_SYMBOL(_copy_from_user);
> #endif
>
> -#ifndef INLINE_COPY_TO_USER
> +#if !defined(INLINE_COPY_TO_USER) || defined(CONFIG_RUST)
> unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
> {
> - might_fault();
> - if (should_fail_usercopy())
> - return n;
> - if (likely(access_ok(to, n))) {
> - instrument_copy_to_user(to, from, n);
> - n = raw_copy_to_user(to, from, n);
> - }
> - return n;
> + return _inline_copy_to_user(to, from, n);
> }
> EXPORT_SYMBOL(_copy_to_user);
> #endif
Sure, if that's okay with you, then I'm happy to do it that way in v2.
Thank you!
Alice
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