Dominik Brodowski linux at dominikbrodowski.net
Mon Mar 26 17:24:49 AEDT 2018

On Mon, Mar 26, 2018 at 04:47:50AM +0100, Al Viro wrote:
> 	* 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...

Nice catch. A similar thing is needed already for non-compat syscalls like
sys_close(), which takes pt_regs->bx on IA32_EMULATION and pt_regs->di on
native x86-64. Therefore, I propose to generate all the stubs we need within
SYSCALL_DEFINEx() and COMPAT_SYSCALL_DEFINEx() (actually, within the
arch-provided version of these macros). See

	https://git.kernel.org/pub/scm/linux/kernel/git/brodo/linux.git	syscalls-WIP

for details on my current plans.


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