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x86/fpu: Improve the __sanitize_i387_state() documentation 

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

Reviewed-by: Borislav 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2015-04-24 11:32:59 +02:00
parent e783e8167d
commit 73a3aeb3ac
1 changed files with 23 additions and 13 deletions
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36
arch/x86/kernel/fpu/xsave.c
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@ -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);
}