Commit-ID: 73a3aeb3ac5312122d5a261c7acb0ab9be93857a
Gitweb: http://git.kernel.org/tip/73a3aeb3ac5312122d5a261c7acb0ab9be93857a
Author: Ingo Molnar <mingo@xxxxxxxxxx>
AuthorDate: Fri, 24 Apr 2015 11:32:59 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 19 May 2015 15:47:36 +0200
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@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/kernel/fpu/xsave.c | 36 +++++++++++++++++++++++-------------
1 file changed, 23 insertions(+), 13 deletions(-)
diff --git a/arch/x86/kernel/fpu/xsave.c b/arch/x86/kernel/fpu/xsave.c
index 467e463..f3d30f0 100644
--- a/arch/x86/kernel/fpu/xsave.c
+++ b/arch/x86/kernel/fpu/xsave.c
@@ -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);
}
--
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