linux-kernel - Re: [PATCH] uprobes/x86: fix detection of 32-bit user mode

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Date:   Fri, 23 Aug 2019 17:13:25 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Mayr <me@....st>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] uprobes/x86: fix detection of 32-bit user mode



> On Aug 23, 2019, at 5:03 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Sat, 24 Aug 2019, Thomas Gleixner wrote:
>> On Fri, 23 Aug 2019, Andy Lutomirski wrote:
>>>> On Aug 23, 2019, at 4:44 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>>> 
>>>>>> On Sat, 24 Aug 2019, Thomas Gleixner wrote:
>>>>>> On Sun, 28 Jul 2019, Sebastian Mayr wrote:
>>>>>> 
>>>>>> -static inline int sizeof_long(void)
>>>>>> +static inline int sizeof_long(struct pt_regs *regs)
>>>>>> {
>>>>>> -    return in_ia32_syscall() ? 4 : 8;
>>>>> 
>>>>> This wants a comment.
>>>>> 
>>>>>> +    return user_64bit_mode(regs) ? 8 : 4;
>>>> 
>>>> The more simpler one liner is to check
>>>> 
>>>>   test_thread_flag(TIF_IA32)
>>> 
>>> I still want to finish killing TIF_IA32 some day.  Let’s please not add new users.
>> 
>> Well, yes and no. This needs to be backported ....
> 
> And TBH the magic in user_64bit_mode() is not pretty either.
> 
> 

It’s only magic on Xen. I should probably stick a cpu_feature_enabled(X86_FEATURE_XENPV) in there instead.