RE: [PATCH 02/17] x86, mpx: use new tsk_get_xsave_addr()

["Yu, Fenghua" <fenghua.yu@intel.com>]

> From: Dave Hansen [mailto:dave@sr71.net]
> Sent: Wednesday, April 22, 2015 11:28 AM
> To: linux-kernel@vger.kernel.org
> Cc: x86@kernel.org; tglx@linutronix.de; Dave Hansen;
> dave.hansen@linux.intel.com; oleg@redhat.com; bp@alien8.de;
> riel@redhat.com; sbsiddha@gmail.com; luto@amacapital.net;
> mingo@redhat.com; hpa@zytor.com; Yu, Fenghua
> Subject: [PATCH 02/17] x86, mpx: use new tsk_get_xsave_addr()
> 
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> The MPX registers (bndcsr/bndcfgu/bndstatus) are not directly accessible via
> normal instructions.  They essentially act as if they were floating point
> registers and are saved/restored along with those registers.
> 
> There are two main paths in the MPX code where we care about the
> contents of these registers:
> 	1. #BR (bounds) faults
> 	2. the prctl() code where we are setting MPX up
> 
> Both of those paths _might_ be called without the FPU having been used.
> That means that 'tsk->thread.fpu.state' might never be allocated.
> 
> Also, fpu_save_init() is not preempt-safe.  It was a bug to call it without
> disabling preemption.  The new
> tsk_get_xsave_addr() calls unlazy_fpu() instead and properly disables
> preemption.
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: bp@alien8.de
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Suresh Siddha <sbsiddha@gmail.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: the arch/x86 maintainers <x86@kernel.org>
> Cc: linux-kernel <linux-kernel@vger.kernel.org>
> ---
> 
>  b/arch/x86/include/asm/mpx.h |    8 ++++----
>  b/arch/x86/kernel/traps.c    |   10 ++++------
>  b/arch/x86/mm/mpx.c          |   23 +++++++++++------------
>  3 files changed, 19 insertions(+), 22 deletions(-)
> 
> diff -puN arch/x86/include/asm/mpx.h~use-new-tsk_get_xsave_addr
> arch/x86/include/asm/mpx.h
> --- a/arch/x86/include/asm/mpx.h~use-new-tsk_get_xsave_addr	2015-
> 04-22 11:16:18.188818959 -0700
> +++ b/arch/x86/include/asm/mpx.h	2015-04-22 11:16:18.195819274 -0700
> @@ -60,8 +60,8 @@
> 
>  #ifdef CONFIG_X86_INTEL_MPX
>  siginfo_t *mpx_generate_siginfo(struct pt_regs *regs,
> -				struct xsave_struct *xsave_buf);
> -int mpx_handle_bd_fault(struct xsave_struct *xsave_buf);
> +				struct task_struct *tsk);
> +int mpx_handle_bd_fault(struct task_struct *tsk);
>  static inline int kernel_managing_mpx_tables(struct mm_struct *mm)  {
>  	return (mm->bd_addr != MPX_INVALID_BOUNDS_DIR); @@ -78,11
> +78,11 @@ void mpx_notify_unmap(struct mm_struct *
>  		      unsigned long start, unsigned long end);  #else  static
> inline siginfo_t *mpx_generate_siginfo(struct pt_regs *regs,
> -					      struct xsave_struct *xsave_buf)
> +					      struct task_struct *tsk)
>  {
>  	return NULL;
>  }
> -static inline int mpx_handle_bd_fault(struct xsave_struct *xsave_buf)
> +static inline int mpx_handle_bd_fault(struct task_struct *tsk)
>  {
>  	return -EINVAL;
>  }
> diff -puN arch/x86/kernel/traps.c~use-new-tsk_get_xsave_addr
> arch/x86/kernel/traps.c
> --- a/arch/x86/kernel/traps.c~use-new-tsk_get_xsave_addr	2015-04-22
> 11:16:18.190819049 -0700
> +++ b/arch/x86/kernel/traps.c	2015-04-22 11:16:18.195819274 -0700
> @@ -61,6 +61,7 @@
>  #include <asm/mach_traps.h>
>  #include <asm/alternative.h>
>  #include <asm/mpx.h>
> +#include <asm/xsave.h>
> 
>  #ifdef CONFIG_X86_64
>  #include <asm/x86_init.h>
> @@ -373,7 +374,6 @@ dotraplinkage void do_double_fault(struc
> dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)  {
>  	struct task_struct *tsk = current;
> -	struct xsave_struct *xsave_buf;
>  	enum ctx_state prev_state;
>  	struct bndcsr *bndcsr;
>  	siginfo_t *info;
> @@ -397,9 +397,7 @@ dotraplinkage void do_bounds(struct pt_r
>  	 * It is not directly accessible, though, so we need to
>  	 * do an xsave and then pull it out of the xsave buffer.
>  	 */
> -	fpu_save_init(&tsk->thread.fpu);
> -	xsave_buf = &(tsk->thread.fpu.state->xsave);
> -	bndcsr = get_xsave_addr(xsave_buf, XSTATE_BNDCSR);
> +	bndcsr = get_xsave_field(XSTATE_BNDCSR);
>  	if (!bndcsr)
>  		goto exit_trap;
> 
> @@ -410,11 +408,11 @@ dotraplinkage void do_bounds(struct pt_r
>  	 */
>  	switch (bndcsr->bndstatus & MPX_BNDSTA_ERROR_CODE) {
>  	case 2:	/* Bound directory has invalid entry. */
> -		if (mpx_handle_bd_fault(xsave_buf))
> +		if (mpx_handle_bd_fault(tsk))
>  			goto exit_trap;
>  		break; /* Success, it was handled */
>  	case 1: /* Bound violation. */
> -		info = mpx_generate_siginfo(regs, xsave_buf);
> +		info = mpx_generate_siginfo(regs, tsk);
>  		if (IS_ERR(info)) {
>  			/*
>  			 * We failed to decode the MPX instruction.  Act as if
> diff -puN arch/x86/mm/mpx.c~use-new-tsk_get_xsave_addr
> arch/x86/mm/mpx.c
> --- a/arch/x86/mm/mpx.c~use-new-tsk_get_xsave_addr	2015-04-22
> 11:16:18.192819139 -0700
> +++ b/arch/x86/mm/mpx.c	2015-04-22 11:16:18.196819320 -0700
> @@ -273,7 +273,7 @@ bad_opcode:
>   * The caller is expected to kfree() the returned siginfo_t.
>   */
>  siginfo_t *mpx_generate_siginfo(struct pt_regs *regs,
> -				struct xsave_struct *xsave_buf)
> +				struct task_struct *tsk)
>  {

Do you need the parameter "tsk"? It's not used in this function.
Seems this function only needs to take one parameter "regs". Other parameters
are unnecessary.

>  	struct bndreg *bndregs, *bndreg;
>  	siginfo_t *info = NULL;
> @@ -295,8 +295,8 @@ siginfo_t *mpx_generate_siginfo(struct p
>  		err = -EINVAL;
>  		goto err_out;
>  	}
> -	/* get the bndregs _area_ of the xsave structure */
> -	bndregs = get_xsave_addr(xsave_buf, XSTATE_BNDREGS);
> +	/* get bndregs field from urrent task's xsave area */
> +	bndregs = get_xsave_field(XSTATE_BNDREGS);
>  	if (!bndregs) {
>  		err = -EINVAL;
>  		goto err_out;
> @@ -358,8 +358,7 @@ static __user void *task_get_bounds_dir(
>  	 * The bounds directory pointer is stored in a register
>  	 * only accessible if we first do an xsave.
>  	 */
> -	fpu_save_init(&tsk->thread.fpu);
> -	bndcsr = get_xsave_addr(&tsk->thread.fpu.state->xsave,
> XSTATE_BNDCSR);
> +	bndcsr = get_xsave_field(XSTATE_BNDCSR);
>  	if (!bndcsr)
>  		return MPX_INVALID_BOUNDS_DIR;
> 
> @@ -390,9 +389,9 @@ int mpx_enable_management(struct task_st
>  	 * directory into XSAVE/XRSTOR Save Area and enable MPX through
>  	 * XRSTOR instruction.
>  	 *
> -	 * fpu_xsave() is expected to be very expensive. Storing the bounds
> -	 * directory here means that we do not have to do xsave in the
> unmap
> -	 * path; we can just use mm->bd_addr instead.
> +	 * xsaves are expected to be very expensive.  Storing the bounds
> +	 * directory here means that we do not have to do xsave in the
> +	 * unmap path; we can just use mm->bd_addr instead.
>  	 */
>  	bd_base = task_get_bounds_dir(tsk);
>  	down_write(&mm->mmap_sem);
> @@ -498,12 +497,12 @@ out_unmap:
>   * bound table is 16KB. With 64-bit mode, the size of BD is 2GB,
>   * and the size of each bound table is 4MB.
>   */
> -static int do_mpx_bt_fault(struct xsave_struct *xsave_buf)
> +static int do_mpx_bt_fault(struct task_struct *tsk)

Do you need the parameter "tsk"? It's not used in this function.
Seems this function doesn't need to take any parameter.

>  {
>  	unsigned long bd_entry, bd_base;
>  	struct bndcsr *bndcsr;
> 
> -	bndcsr = get_xsave_addr(xsave_buf, XSTATE_BNDCSR);
> +	bndcsr = get_xsave_field(XSTATE_BNDCSR);
>  	if (!bndcsr)
>  		return -EINVAL;
>  	/*
> @@ -526,7 +525,7 @@ static int do_mpx_bt_fault(struct xsave_
>  	return allocate_bt((long __user *)bd_entry);  }
> 
> -int mpx_handle_bd_fault(struct xsave_struct *xsave_buf)
> +int mpx_handle_bd_fault(struct task_struct *tsk)
>  {
>  	/*
>  	 * Userspace never asked us to manage the bounds tables, @@ -
> 535,7 +534,7 @@ int mpx_handle_bd_fault(struct xsave_str
>  	if (!kernel_managing_mpx_tables(current->mm))
>  		return -EINVAL;
> 
> -	if (do_mpx_bt_fault(xsave_buf)) {
> +	if (do_mpx_bt_fault(tsk)) {
>  		force_sig(SIGSEGV, current);
>  		/*
>  		 * The force_sig() is essentially "handling" this _