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Commit 73a3aeb3 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

x86/fpu: Improve the __sanitize_i387_state() documentation 



Improve the comments and add new ones, as this code isn't very obvious.

Reviewed-by: default avatarBorislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent e783e816

arch/x86/kernel/fpu/xsave.c

+23 −13
Original line number Diff line number Diff line
@@ -30,19 +30,23 @@ static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask)*8]; 
static unsigned int xfeatures_nr;



/*

 * If a processor implementation discern that a processor state component is

 * in its initialized state it may modify the corresponding bit in the

 * header.xfeatures as '0', with out modifying the corresponding memory

 * layout in the case of xsaveopt. While presenting the xstate information to

 * the user, we always ensure that the memory layout of a feature will be in

 * the init state if the corresponding header bit is zero. This is to ensure

 * that the user doesn't see some stale state in the memory layout during

 * signal handling, debugging etc.

 * When executing XSAVEOPT (optimized XSAVE), if a processor implementation

 * detects that an FPU state component is still (or is again) in its

 * initialized state, it may clear the corresponding bit in the header.xfeatures

 * field, and can skip the writeout of registers to the corresponding memory layout.

 *

 * This means that when the bit is zero, the state component might still contain

 * some previous - non-initialized register state.

 *

 * Before writing xstate information to user-space we sanitize those components,

 * to always ensure that the memory layout of a feature will be in the init state

 * if the corresponding header bit is zero. This is to ensure that user-space doesn't

 * see some stale state in the memory layout during signal handling, debugging etc.

 */

void __sanitize_i387_state(struct task_struct *tsk)

{

	struct i387_fxsave_struct *fx = &tsk->thread.fpu.state->fxsave;

	int feature_bit = 0x2;

	int feature_bit;

	u64 xfeatures;



	if (!fx)
@@ -76,19 +80,25 @@ void __sanitize_i387_state(struct task_struct *tsk) 
	if (!(xfeatures & XSTATE_SSE))

		memset(&fx->xmm_space[0], 0, 256);



	/*

	 * First two features are FPU and SSE, which above we handled

	 * in a special way already:

	 */

	feature_bit = 0x2;

	xfeatures = (xfeatures_mask & ~xfeatures) >> 2;



	/*

	 * Update all the other memory layouts for which the corresponding

	 * header bit is in the init state.

	 * Update all the remaining memory layouts according to their

	 * standard xstate layout, if their header bit is in the init

	 * state:

	 */

	while (xfeatures) {

		if (xfeatures & 0x1) {

			int offset = xstate_offsets[feature_bit];

			int size = xstate_sizes[feature_bit];



			memcpy(((void *) fx) + offset,

			       ((void *) init_xstate_buf) + offset,

			memcpy((void *)fx + offset,

			       (void *)init_xstate_buf + offset,

			       size);

		}