linux-kernel - Re: [PATCH] x86/entry: Improve system call entry comments

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Date:	Tue, 8 Mar 2016 10:59:10 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/entry: Improve system call entry comments

On 03/08/16 10:50, Andy Lutomirski wrote:
> On Tue, Mar 8, 2016 at 10:47 AM, H. Peter Anvin <hpa@...or.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