linux-kernel - Re: [RESEND][PATCH v2 1/3] syscalls,x86 Expose arch

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Date:   Wed, 14 Sep 2016 18:01:18 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     Dmitry Safonov <0x7f454c46@...il.com>
Cc:     "Robert O'Callahan" <robert@...llahan.org>,
        open list <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Milosz Tanski <milosz@...in.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        David Howells <dhowells@...hat.com>,
        Zach Brown <zab@...hat.com>,
        Eric B Munson <emunson@...mai.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Borislav Petkov <bp@...en8.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Mateusz Guzik <mguzik@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [RESEND][PATCH v2 1/3] syscalls,x86 Expose arch_prctl on x86-32.

On Wed, Sep 14, 2016 at 3:29 PM, Dmitry Safonov <0x7f454c46@...il.com> wrote:
> 2016-09-15 1:08 GMT+03:00 Kyle Huey <me@...ehuey.com>:
>> On Wed, Sep 14, 2016 at 2:59 PM, Dmitry Safonov <0x7f454c46@...il.com> wrote:
>>> 2016-09-15 0:08 GMT+03:00 Kyle Huey <me@...ehuey.com>:
>>>> Signed-off-by: Kyle Huey <khuey@...ehuey.com>
>>>> ---
>>>>  arch/x86/entry/syscalls/syscall_32.tbl |  1 +
>>>>  arch/x86/kernel/process.c              | 80 ++++++++++++++++++++++++++++++++++
>>>>  arch/x86/kernel/process_64.c           | 66 ----------------------------
>>>>  3 files changed, 81 insertions(+), 66 deletions(-)
>>>>
>>>> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
>>>> index f848572..3b6965b 100644
>>>> --- a/arch/x86/entry/syscalls/syscall_32.tbl
>>>> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
>>>> @@ -386,3 +386,4 @@
>>>>  377    i386    copy_file_range         sys_copy_file_range
>>>>  378    i386    preadv2                 sys_preadv2                     compat_sys_preadv2
>>>>  379    i386    pwritev2                sys_pwritev2                    compat_sys_pwritev2
>>>> +380    i386    arch_prctl              sys_arch_prctl
>>>
>>> Why not define it as other 32-bit syscalls with compat_sys_ prefix
>>> with the help of COMPAT_SYSCALL_DEFINE() macro?
>>> Then you could omit code moving, drop is_32 helper.
>>> I miss something obvious?
>>
>> The code will have to move regardless, because right now do_arch_prctl
>> is in process-64.c which is only compiled on a 64 bit kernel.
>
> Why? This code will not work anyway for 32-bit in your patches
> by obscuring it with is_32.
>
>> As I told Dave Hansen in the non-RESEND thread (not sure why
>> git-send-email didn't put him in this one ...) I considered doing a
>> compat_sys_arch_prctl that would reject the relevant arch_prctls that
>> don't apply on 32 bit but I didn't see any prior art for it (in my
>> admittedly non-exhaustive search).
>
> Well, you could just add to 64-bit do_arch_prctl() new cases for your
> prctls - that would be just a two-lines for each new prctl.
> Also add compat_sys_ and define *only* what's needed there for you,
> do not add there ARCH_{SET,GET}_{FS,GS}.
> Does this make sense?

Yeah, I should have spoken more clearly.  We'll need some
implementation of the syscall outside of process_64.c.  But we could
leave the 64 bit specific stuff behind in it.   Dave Hansen suggested
something similar (though without the compat_sys_bit)

>FWIW, I don't think it would be horrible to leave the existing
> do_arch_prctl() code in process_64.h and call it
> do_64_bit_only_something_arch_prctl(), and only call in to it from the
> generic do_arch_prctl().  You really have one reason for all the "if
> (is_32)"'s and it would be nice to document why in one single place.

- Kyle