[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20210104165827.GJ3579531@ZenIV.linux.org.uk>
Date: Mon, 4 Jan 2021 16:58:27 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: David Laight <David.Laight@...lab.com>
Cc: Christoph Hellwig <hch@....de>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: in_compat_syscall() on x86
On Mon, Jan 04, 2021 at 12:16:56PM +0000, David Laight wrote:
> On x86 in_compat_syscall() is defined as:
> in_ia32_syscall() || in_x32_syscall()
>
> Now in_ia32_syscall() is a simple check of the TS_COMPAT flag.
> However in_x32_syscall() is a horrid beast that has to indirect
> through to the original %eax value (ie the syscall number) and
> check for a bit there.
>
> So on a kernel with x32 support (probably most distro kernels)
> the in_compat_syscall() check is rather more expensive than
> one might expect.
>
> It would be muck better if both checks could be done together.
> I think this would require the syscall entry code to set a
> value in both the 64bit and x32 entry paths.
> (Can a process make both 64bit and x32 system calls?)
Yes, it bloody well can.
And I see no benefit in pushing that logics into syscall entry,
since anything that calls in_compat_syscall() more than once
per syscall execution is doing the wrong thing. Moreover,
in quite a few cases we don't call the sucker at all, and for
all of those pushing that crap into syscall entry logics is
pure loss.
What's the point, really?
Powered by blists - more mailing lists