RE: in_compat_syscall() on x86

[David Laight]

From: Eric W. Biederman
> Sent: 04 January 2021 20:41
> 
> Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:
> 
> > 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.
> 
> I suggest you check the distro kernels.  I suspect they don't compile in
> support for x32.  As far as I can tell x32 is an undead beast of a
> subarchitecture that just enough people use that it can't be removed,
> but few enough people use it likely has a few lurking scary bugs.

It is defined in the Ubuntu kernel configs I've got lurking:
Both 3.8.0-19_generic (Ubuntu 13.04) and 5.4.0-56_generic (probably 20.04).
Which is probably why it is in my test builds (I've just cut out
a lot of modules).

> >> 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.
> 
> The x32 system calls have their own system call table and it would be
> trivial to set a flag like TS_COMPAT when looking up a system call from
> that table.  I expect such a change would be purely in the noise.

Certainly a write of 0/1/2 into a dirtied cache line of 'current'
could easily cost absolutely nothing.
Especially if current has already been read.

I also wondered about resetting it to zero when an x32 system call
exits (rather than entry to a 64bit one).

For ia32 the flag is set (with |=) on every syscall entry.
Even though I'm pretty sure it can only change during exec.

> > What's the point, really?
> 
> Before we came up with the current games with __copy_siginfo_to_user
> and x32_copy_siginfo_to_user I was wondering if we should make such
> a change.  The delivery of compat signal frames and core dumps which
> do not go through the system call entry path could almost benefit from
> a flag that could be set/tested when on those paths.

For signal delivery it should (probably) depend on the system call
that setup the signal handler.
Although I'm sure I remember one kernel where some of it was done
in libc (with a single entrypoint for all hadlers).

> The fact that only SIGCHLD (which can not trigger a coredump) is
> different saves the coredump code from needing such a test.
> 
> The fact that the signal frame code is simple enough it can directly
> call x32_copy_siginfo_to_user or __copy_siginfo_to_user saves us there.
> 
> So I don't think we have any cases where we actually need a flag that
> is independent of the system call but we have come very close.

If a program can do both 64bit and x32 system calls you probably
need to generate a 64bit core dump if it has ever made a 64bit
system call??

> For people who want to optimize I suggest tracking down the handful of
> users of x32 and see if x32 can be made to just go away.

Unlikely since Ubuntu seem to have enabled it for years.

	David

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