Re: in_compat_syscall() on x86

[Andy Lutomirski <luto@amacapital.net>]

> On Jan 4, 2021, at 2:36 PM, David Laight <David.Laight@aculab.com> wrote:
> 
> From: Eric W. Biederman
>> Sent: 04 January 2021 20:41
>> 
>> Al Viro <viro@zeniv.linux.org.uk> 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.

It can change for every syscall. I have tests that do this.

> 
>>> 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.

I think it has worked this way for some time now.

> 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??

I think core dump should (and does) depend on the execution mode at the time of the crash.

It’s worth noting that GCC’s understanding of mixed bitness is horrible.