RE: [PATCH 3/4] exec: simplify the compat syscall handling

[David Laight <David.Laight@ACULAB.COM>]

From: Al Viro
> Sent: 26 March 2021 16:12
> 
> On Fri, Mar 26, 2021 at 03:38:30PM +0100, Christoph Hellwig wrote:
> 
> > +static const char __user *
> > +get_user_arg_ptr(const char __user *const __user *argv, int nr)
> >  {
> > +	if (in_compat_syscall()) {
> > +		const compat_uptr_t __user *compat_argv =
> > +			compat_ptr((unsigned long)argv);
> >  		compat_uptr_t compat;
> >
> > +		if (get_user(compat, compat_argv + nr))
> >  			return ERR_PTR(-EFAULT);
> >  		return compat_ptr(compat);
> > +	} else {
> > +		const char __user *native;
> >
> > +		if (get_user(native, argv + nr))
> > +			return ERR_PTR(-EFAULT);
> > +		return native;
> > +	}
> >  }
> 
> Yecchhh....  So you have in_compat_syscall() called again and again, for
> each argument in the list?  I agree that current version is fucking ugly,
> but I really hate that approach ;-/

Especially since in_compat_syscall() isn't entirely trivial on x86-64.
Probably all in the noise for 'exec', but all the bits do add up.

You may not want separate get_user() on some architectures either.
The user_access_begin/end aren't cheap.

OTOH if you call copy_from_user() you get hit by the stupid
additional costs of 'user copy hardening'.

	David

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