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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Sun, 22 Mar 2015 15:15:08 +0100
Message-ID: <CAK1hOcNp28VHfWfYtOxyom+qw-8aqkLg0Vt1fu2sRj857RxzJQ@mail.gmail.com>
Subject: Re: [tip:x86/asm] x86/asm/entry/64: Always allocate a complete "
 struct pt_regs" on the kernel stack
To: Brian Gerst <brgerst@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, Will Drewry <wad@chromium.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Alexei Starovoitov <ast@plumgrid.com>, Oleg Nesterov <oleg@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-tip-commits@vger.kernel.org
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>> @@ -492,7 +496,6 @@ GLOBAL(stub32_clone)
>>
>>         ALIGN
>>  ia32_ptregs_common:
>> -       popq %r11
>>         CFI_ENDPROC
>>         CFI_STARTPROC32 simple
>>         CFI_SIGNAL_FRAME
>> @@ -507,9 +510,9 @@ ia32_ptregs_common:
>>  /*     CFI_REL_OFFSET  rflags,EFLAGS-ARGOFFSET*/
>>         CFI_REL_OFFSET  rsp,RSP-ARGOFFSET
>>  /*     CFI_REL_OFFSET  ss,SS-ARGOFFSET*/
>> -       SAVE_REST
>> +       SAVE_EXTRA_REGS 8
>>         call *%rax
>> -       RESTORE_REST
>> -       jmp  ia32_sysret        /* misbalances the return cache */
>> +       RESTORE_EXTRA_REGS 8
>> +       ret
>>         CFI_ENDPROC
>>  END(ia32_ptregs_common)
>
> This change is causing Wine to fail when it's built to support both
> 32-bit and 64-bit code:
>
> [  523.657406] wineserver[1605]: segfault at 453a7c90 ip
> 00000000453a7c90 sp 00000000b3c6ad10 error 14 in
> ld-2.21.so[7fad453a7000+21000]
>
> It tries to exec the 64-bit wineserver from 32-bit code and faults
> because %rip is truncated.  This is because simply returning to the
> caller will exit via sysretl which doesn't change %cs to 64-bit mode.
> We need to set a thread flag to enter the slow path and return via
> iret instead.
>
> Patch coming to fix it.

Something like adding

    orl $_TIF_NOTIFY_RESUME,TI_flags+THREAD_INFO(%rsp,RIP)

in stub32_execve[at] will probably do it
Please add a comment why it is needed.