Subject: Re: [PATCH] x86/entry: Improve system call entry comments
To: Andy Lutomirski <luto@amacapital.net>
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Cc: Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@kernel.org>,
        X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>, Oleg Nesterov <oleg@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Brian Gerst <brgerst@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <56DF20FE.4040607@zytor.com>
Date: Tue, 8 Mar 2016 10:59:10 -0800
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On 03/08/16 10:50, Andy Lutomirski wrote:
> On Tue, Mar 8, 2016 at 10:47 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 03/08/16 10:45, Andy Lutomirski wrote:
>>>
>>> s/modern/most, perhaps?
>>>
>>> I'm hoping that some day Bionic goes away and gets replaced by musl.
>>>
>>> Of course, musl doesn't always use fast syscalls because it needs a
>>> vdso facility that doesn't currently exist.  I'll deal with that
>>> eventually.
>>>
>>
>> You don't actually need actual DSO support to support fast system calls
>> on i386.  Even klibc uses them now, and the additional code to support
>> it is trivial.
> 
> That's not the issue.  The issue is that musl does something
> crazy^Wclever to support POSIX pthread cancellation, and it involves
> being able to tell whether a signal's ucontext points to a syscall
> and, if so, what the return address is.  This is straightforward with
> an inlined int $0x80, but doing it reliably with the current vdso
> design would requiring parsing the DWARF data, and I can't really
> blame musl for not wanting to do that.
> 
> There was a thread awhile back about adding a new vdso helper to do
> this.  I think I even had some code for it.  If I find time, I'll try
> to send patches for 4.7.
> 

As far as I know, when we get a signal the EIP always points to int
$0x80 as we don't support system call restart (being a rare case) for
the fast system calls.

	-hpa