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Subject: Re: [PATCH][v4] uprobes/x86: emulate push insns for uprobe on x86
To:     Oleg Nesterov <oleg@redhat.com>
CC:     <mingo@kernel.org>, <tglx@linutronix.de>, <peterz@infradead.org>,
        <linux-kernel@vger.kernel.org>, <x86@kernel.org>,
        <netdev@vger.kernel.org>, <ast@fb.com>, <kernel-team@fb.com>
References: <20171115175928.3821714-1-yhs@fb.com>
 <20171117172526.GA10949@redhat.com>
From:   Yonghong Song <yhs@fb.com>
Message-ID: <47697170-3737-052e-584f-811cf4e4dbf2@fb.com>
Date:   Fri, 17 Nov 2017 10:12:55 -0800
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On 11/17/17 9:25 AM, Oleg Nesterov wrote:
> On 11/15, Yonghong Song wrote:
>>
>> v3 -> v4:
>>    . Revert most of v3 change as 32bit emulation is not really working
>>      on x86_64 platform as among other issues, function emulate_push_stack()
>>      needs to account for 32bit app on 64bit platform.
>>      A separate effort is ongoing to address this issue.
> 
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> 
> 
> 
> Please test your patch with the fix below, in this particular case the
> TIF_IA32 check should be fine. Although this is not what we really want,
> we should probably use user_64bit_mode(regs) which checks ->cs. But this
> needs more changes and doesn't solve other problems (get_unmapped_area)
> so I still can't decide what should we do right now...

I tested the below change with my patch. On x86_64, both 64bit and 32bit 
program can be uprobe emulated properly. On x86_32, however, there is a 
compilation error like below:

In function ‘check_copy_size’,
     inlined from ‘copy_to_user’ at 
/home/yhs/work/tip/include/linux/uaccess.h:154:6,
     inlined from ‘emulate_push_stack.isra.9’ at 
/home/yhs/work/tip/arch/x86/kernel/uprobes.c:535:6:
/home/yhs/work/tip/include/linux/thread_info.h:139:4: error: call to 
‘__bad_copy_from’ declared with attribute error: copy source size is too 
small
     __bad_copy_from();

Basically, test_thread_flag(TIF_IA32) returns 0 on x86_32 system.

> 
> Oleg.
> 
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -516,7 +516,7 @@ struct uprobe_xol_ops {
>   
>   static inline int sizeof_long(void)
>   {
> -	return in_ia32_syscall() ? 4 : 8;
> +	return test_thread_flag(TIF_IA32) ? 4 : 8;
>   }
>   
>   static int default_pre_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
> 

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