Re: in_compat_syscall() on x86

[Al Viro]

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?