Message-ID: <CAK8P3a0_uJQXshn5N0o4J_8dVPNCw885xGHqnKj3i5kYB+GtBg@mail.gmail.com>
Date:   Fri, 18 Feb 2022 10:20:38 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "dalias@...c.org" <dalias@...c.org>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "jcmvbkbc@...il.com" <jcmvbkbc@...il.com>,
        "guoren@...nel.org" <guoren@...nel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-hexagon@...r.kernel.org" <linux-hexagon@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "will@...nel.org" <will@...nel.org>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "bcain@...eaurora.org" <bcain@...eaurora.org>,
        "deller@....de" <deller@....de>, "x86@...nel.org" <x86@...nel.org>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>,
        "hca@...ux.ibm.com" <hca@...ux.ibm.com>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        "linux-um@...ts.infradead.org" <linux-um@...ts.infradead.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "openrisc@...ts.librecores.org" <openrisc@...ts.librecores.org>,
        "green.hu@...il.com" <green.hu@...il.com>,
        "shorne@...il.com" <shorne@...il.com>,
        "monstr@...str.eu" <monstr@...str.eu>,
        "tsbogend@...ha.franken.de" <tsbogend@...ha.franken.de>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "nickhu@...estech.com" <nickhu@...estech.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "dinguyen@...nel.org" <dinguyen@...nel.org>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        "richard@....at" <richard@....at>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH v2 00/18] clean up asm/uaccess.h, kill set_fs for good

n Fri, Feb 18, 2022 at 3:21 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Thu, Feb 17, 2022 at 08:49:59AM +0100, Arnd Bergmann wrote:
>
> > Same here: architectures can already provide a __put_user_fn()
> > and __get_user_fn(), to get the generic versions of the interface,
> > but few architectures use that. You can actually get all the interfaces
> > by just providing raw_copy_from_user() and raw_copy_to_user(),
> > but the get_user/put_user versions you get from that are fairly
> > inefficient.
>
> FWIW, __{get,put}_user_{8,16,32,64} would probably make it easier to
> unify.  That's where the really variable part tends to be, anyway.
> IMO __get_user_fn() had been a mistake.

I've prototyped this now, to see what this might look like, see
https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git/commit/?h=generic-get_user-prototype

This adds generic inline version of {__get,get,__put,put}_user()
and converts x86 to (optionally) use it. This builds with gcc-5
through gcc-11 on 32-bit and 64-bit x86, using asm-goto with
outputs where possible, and requiring a minimum set of macro
definitions from the architecture. Compiling with clang produces
no warnings but does cause a linker issue at the moment, so
there is probably at least one bug in it.

Aside from compile-testing, I have not tried to verify if this
is correct or efficient, but let me know if you think this is headed
in the right direction.

> One thing I somewhat dislike about the series is the boilerplate in
> asm/uaccess.h instances - #include <asm-generic/access-ok.h> in
> a lot of them might make sense as a transitory state, but getting
> stuck with those indefinitely...

Christoph also complained about it, the problem for now is that
asm-generic/access_ok.h must first see the macro definitions for
architectures that override any of the contents, but access_ok()
itself is used at least in some of the asm/uaccess.h files as well,
so it must be included in the middle of it, until more of the uaccess.h
implementation is moved to linux/uaccess.h in an architecture
independent way.

Would you prefer having an asm/access_ok.h that falls back to
the asm-generic version but can have an architecture specific
override when needed (ia64, arm64, x86, um)?

>         BTW, do we need user_addr_max() anymore?  The definition in
> asm-generic/access-ok.h is the only one, so ifndef around it is pointless.

Right, the v2 changes got rid of the last override, so it could get
hardcoded to TASK_SIZE_MAX, or we can convert the five
references to just use that instead and remove it altogether:

arch/arm64/kernel/traps.c:      if (address >= user_addr_max()) {
                 \
arch/parisc/kernel/signal.c:    if (start >= user_addr_max() - sigframe_size)
arch/parisc/kernel/signal.c:            if (A(&usp[0]) >=
user_addr_max() - 5 * sizeof(int))
lib/strncpy_from_user.c:        max_addr = user_addr_max();
lib/strnlen_user.c:     max_addr = user_addr_max();

user_addr_max() first showed up in architecture-independent code in
c5389831cda3 ("sparc: Fix user_addr_max() definition."), and from that
I think the original intent is no longer useful.

          Arnd

Powered by blists - more mailing lists