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SubjectRe: [PATCH 09/11] x86, mpx: on-demand kernel allocation of bounds tables
On Wed, 12 Nov 2014, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> Thomas, I know you're not a huge fan of using mm->mmap_sem for serializing
> this stuff. But, now that we are not adding an additional lock a la
> mm->bd_sem, I can't quite justify adding another lock and trying to
> reconcile the interactions and ording with mmap_sem.
>
> We are only adding two spots where we acquire mmap_sem and did not. All of
> the other "use" is in places where it is held already. Those two points
> of new use are *tiny* and can easily be replaced in the future.

I'm fine with that as long as we dont have the "drop, reacquire, handle
races of all sorts" dance.


> +static inline void arch_bprm_mm_init(struct mm_struct *mm,
> + struct vm_area_struct *vma)
> +{
> +#ifdef CONFIG_X86_INTEL_MPX
> + mm->bd_addr = MPX_INVALID_BOUNDS_DIR;
> +#endif
> +}
> +

I'd rather have in mpx.h

static inline void mpx_mm_init(struct mm_struct *mm)
{
#ifdef CONFIG_X86_INTEL_MPX
mm->bd_addr = MPX_INVALID_BOUNDS_DIR;
#endif
}

and make this

static inline void arch_bprm_mm_init(struct mm_struct *mm,
struct vm_area_struct *vma)
{
mpx_mm_init(mm);
}

So this #ifdef can be replaced

> +++ b/arch/x86/kernel/setup.c 2014-11-12 08:49:26.494916477 -0800
> @@ -959,6 +959,13 @@ void __init setup_arch(char **cmdline_p)
> init_mm.end_code = (unsigned long) _etext;
> init_mm.end_data = (unsigned long) _edata;
> init_mm.brk = _brk_end;
> +#ifdef CONFIG_X86_INTEL_MPX
> + /*
> + * NULL is theoretically a valid place to put the bounds
> + * directory, so point this at an invalid address.
> + */
> + init_mm.bd_addr = MPX_INVALID_BOUNDS_DIR;
> +#endif

with

mpx_mm_init(&init_mm);

> +dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
> +{
> + enum ctx_state prev_state;
> + struct bndcsr *bndcsr;
> + struct xsave_struct *xsave_buf;
> + struct task_struct *tsk = current;
> + siginfo_t *info;
> +
> + prev_state = exception_enter();
> + if (notify_die(DIE_TRAP, "bounds", regs, error_code,
> + X86_TRAP_BR, SIGSEGV) == NOTIFY_STOP)
> + goto exit;
> + conditional_sti(regs);
> +
> + if (!user_mode(regs))
> + die("bounds", regs, error_code);
> +
> + if (!cpu_feature_enabled(X86_FEATURE_MPX)) {
> + /* The exception is not from Intel MPX */
> + goto exit_trap;
> + }
> +
> + fpu_save_init(&tsk->thread.fpu);

That lacks a comment why we need to do an xsave here.

> + xsave_buf = &(tsk->thread.fpu.state->xsave);
> + bndcsr = get_xsave_addr(xsave_buf, XSTATE_BNDCSR);
> + if (!bndcsr)
> + goto exit_trap;

...
> +exit:
> + exception_exit(prev_state);

And this lacks a:

return;

Otherwise you can avoid the whole exercise above and just jump to
exit_trap :)

> +exit_trap:
> + /*
> + * This path out is for all the cases where we could not
> + * handle the exception in some way (like allocating a
> + * table or telling userspace about it. We will also end
> + * up here if the kernel has MPX turned off at compile
> + * time..
> + */
> + do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, NULL);
> + exception_exit(prev_state);
> +}

> +int mpx_handle_bd_fault(struct xsave_struct *xsave_buf)
> +{
> + int ret = 0;
> + /*
> + * Userspace never asked us to manage the bounds tables,
> + * so refuse to help.
> + */
> + if (!kernel_managing_mpx_tables(current->mm)) {
> + ret = -EINVAL;
> + goto out;
> + }
> +
> + ret = do_mpx_bt_fault(xsave_buf);
> + if (ret) {
> + force_sig(SIGSEGV, current);
> + /*
> + * The force_sig() is essentially "handling" this
> + * exception. Return 0 so that the traps.c code
> + * does not take any further action.
> + */
> + ret = 0;
> + }
> +out:
> + return ret;

Wee. That's convoluted.

if (!kernel_managing_mpx_tables(current->mm))
return -EINVAL;
if (do_mpx_bt_fault(xsave_buf)) {
/* Add comment */
force_sig(SIGSEGV, current);
}
return 0;
Does the same thing in a readable form :)

Thanks,

tglx


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