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Message-ID: <20150728004528.GQ16376@brightrain.aerifal.cx>
Date: Mon, 27 Jul 2015 20:45:28 -0400
From: Rich Felker <dalias@...c.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: musl@...ts.openwall.com,
Alexander Larsson <alexander.larsson@...il.com>
Subject: Re: Re: Using direct socket syscalls on x86_32 where
available?
On Mon, Jul 27, 2015 at 04:56:51PM -0700, Andy Lutomirski wrote:
> On 07/26/2015 09:59 AM, Rich Felker wrote:
> >On Sat, Jul 25, 2015 at 10:54:28AM -0700, Andy Lutomirski wrote:
> >>On x86_32, the only way to call socket(2), etc is using socketcall.
> >>This is slated to change in Linux 4.3:
> >>
> >>https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=x86/asm&id=9dea5dc921b5f4045a18c63eb92e84dc274d17eb
> >>
> >>If userspace adapts by preferring the direct syscalls when available,
> >>it'll make it easier for seccomp to filter new userspace programs
> >>(and, ideally, eventually disallow socketcall for sandbox-aware code).
> >>
> >>Would musl be willing to detect these syscalls and use them if available?
> >>
> >>(Code to do this probably shouldn't be committed until that change
> >>lands in Linus' tree, just in case the syscall numbers change in the
> >>mean time.)
> >
> >My preference would be not to do this, since it seems to be enlarging
> >the code and pessimizing normal usage for the sake of a very special
> >usage scenario. At the very least there would be at least one extra
> >syscall to probe at first usage, and that probe could generate a
> >termination on existing seccomp setups. :-p
>
> There will be some tiny performance benefit for newer kernels: it
> avoids a silly indirection that has a switch statement along six
> stores into memory, validation of the userspace address, and then
> six loads to pull the syscall args back out of memory. It's not a
> big deal, but the new syscalls really will be slightly faster.
Unless you're going to try the new syscalls first and fallback on
ENOSYS every time...
> >So far we don't probe and
> >store results for any fallbacks though; we just do the fallback on
> >error every time. This is because all of the existing fallbacks are in
> >places where we actually want new functionality a new syscall offers,
> >and the old ones are not able to provide it precisely but require poor
> >emulation, and in these cases it's expected that the user not be using
> >old kernels that can't give correct semantics. But in the case of
> >these socket calls there's no semantic difference or reason for us to
> >be preferring the 'new' calls. It's just a duplicate API for the same
> >thing.
>
> One way to implement it would be to favor the new syscalls but to
> set some variable the first time one of them returns ENOSYS. Once
> that happens, either all of them could fall back to socketcall or
> just that one syscall could.
...right, a global. Which requires a barrier to access it. A barrier
costs a lot more than a few loads or a switch.
> Or you could just avoid implementing it and see if anyone complains.
> It's plausible that xdg-app might start requiring the new syscalls
> (although it would presumably not kill you if tried to use
> socketcall).
>
> Alex, if glibc started using the new syscalls, would you want to
> require them inside xdg-app?
I don't see any reason to require them except forcing policy. And I
don't see any reason for adding them to the kernel to begin with.
While we would have been better off with proper syscalls for each one
rather than this multiplexed mess if it had been done right from the
beginning, having to support both is even worse than the existing
multiplexed socketcall.
Rich
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