Re: [RFC] new SYSCALL_DEFINE/COMPAT_SYSCALL_DEFINE wrappers

Al Viro
2018-03-26T03:47:50Z

On Mon, Mar 26, 2018 at 01:40:17AM +0100, Al Viro wrote:

> Kinda-sorta part:
> * asmlinkage_protect is taken out for now, so m68k has problems.
> * syscalls that run out of 6 slots barf violently. For mips it's
> wrong (there we have 8 slots); for stuff like arm and ppc it's right, but
> it means that things like e.g. compat sync_file_range() should not even
> be compiled on those. __ARCH_WANT_SYS_SYNC_FILE_RANGE, presumably...
> In any case, we *can't* do pt_regs-based wrappers for those syscalls on
> such architectures, so ifdefs around those puppies are probably the right
> thing to do.
> * s390 macrology in compat_wrapper.c not even touched; it needs
> a trivial update to keep working (__MAP callbacks take an extra argument,
> unused for those users).
> * sys_... and compat_sys_... aliases are unchanged; if we kill
> direct callers, we can trivially rename SyS##name and compat_SyS##name
> to sys##name and compat_sys##name and get rid of aliases.

* mips n32 and x86 x32 can become an extra source of headache.
That actually applies to any plans of passing struct pt_regs *. As it
is, e.g. syscall 515 on amd64 is compat_sys_readv(). Dispatched via
this:
/*
* NB: Native and x32 syscalls are dispatched from the same
* table. The only functional difference is the x32 bit in
* regs->orig_ax, which changes the behavior of some syscalls.
*/
if (likely((nr & __SYSCALL_MASK) < NR_syscalls)) {
nr = array_index_nospec(nr & __SYSCALL_MASK, NR_syscalls);
regs->ax = sys_call_table[nr](
regs->di, regs->si, regs->dx,
regs->r10, regs->r8, regs->r9);
}
Now, syscall 145 via 32bit call is *also* compat_sys_readv(), dispatched
via
nr = array_index_nospec(nr, IA32_NR_syscalls);
/*
* It's possible that a 32-bit syscall implementation
* takes a 64-bit parameter but nonetheless assumes that
* the high bits are zero. Make sure we zero-extend all
* of the args.
*/
regs->ax = ia32_sys_call_table[nr](
(unsigned int)regs->bx, (unsigned int)regs->cx,
(unsigned int)regs->dx, (unsigned int)regs->si,
(unsigned int)regs->di, (unsigned int)regs->bp);
Right now it works - we call the same function, passing it arguments picked
from different set of registers (di/si/dx in x32 case, bx/cx/dx in i386 one).
But if we switch to passing struct pt_regs * and have the wrapper fetch
regs->{bx,cx,dx}, we have a problem. It won't work for both entry points.

IMO it's a good reason to have dispatcher(s) handle extraction from pt_regs
and let the wrapper deal with the resulting 6 u64 or 6 u32, normalizing
them and arranging them into arguments expected by syscall body.

Linus, Dominik - how do you plan dealing with that fun? Regardless of the
way we generate the glue, the issue remains. We can't get the same
struct pt_regs *-taking function for both; we either need to produce
a separate chunk of glue for each compat_sys_... involved (either making
COMPAT_SYSCALL_DEFINE generate both, or having duplicate X32_SYSCALL_DEFINE
for each of those COMPAT_SYSCALL_DEFINE - with identical body, at that)
or we need to have the registers-to-slots mapping done in dispatcher...