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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>
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@ -30,19 +30,23 @@ static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask)*8];
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static unsigned int xfeatures_nr;
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/*
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* If a processor implementation discern that a processor state component is
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* in its initialized state it may modify the corresponding bit in the
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* header.xfeatures as '0', with out modifying the corresponding memory
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* layout in the case of xsaveopt. While presenting the xstate information to
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* the user, we always ensure that the memory layout of a feature will be in
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* the init state if the corresponding header bit is zero. This is to ensure
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* that the user doesn't see some stale state in the memory layout during
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* signal handling, debugging etc.
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* When executing XSAVEOPT (optimized XSAVE), if a processor implementation
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* detects that an FPU state component is still (or is again) in its
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* initialized state, it may clear the corresponding bit in the header.xfeatures
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* field, and can skip the writeout of registers to the corresponding memory layout.
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*
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* This means that when the bit is zero, the state component might still contain
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* some previous - non-initialized register state.
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*
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* Before writing xstate information to user-space we sanitize those components,
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* to always ensure that the memory layout of a feature will be in the init state
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* if the corresponding header bit is zero. This is to ensure that user-space doesn't
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* see some stale state in the memory layout during signal handling, debugging etc.
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*/
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void __sanitize_i387_state(struct task_struct *tsk)
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{
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struct i387_fxsave_struct *fx = &tsk->thread.fpu.state->fxsave;
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int feature_bit = 0x2;
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int feature_bit;
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u64 xfeatures;
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if (!fx)
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@ -76,19 +80,25 @@ void __sanitize_i387_state(struct task_struct *tsk)
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if (!(xfeatures & XSTATE_SSE))
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memset(&fx->xmm_space[0], 0, 256);
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/*
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* First two features are FPU and SSE, which above we handled
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* in a special way already:
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*/
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feature_bit = 0x2;
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xfeatures = (xfeatures_mask & ~xfeatures) >> 2;
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/*
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* Update all the other memory layouts for which the corresponding
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* header bit is in the init state.
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* Update all the remaining memory layouts according to their
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* standard xstate layout, if their header bit is in the init
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* state:
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*/
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while (xfeatures) {
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if (xfeatures & 0x1) {
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int offset = xstate_offsets[feature_bit];
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int size = xstate_sizes[feature_bit];
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memcpy(((void *) fx) + offset,
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((void *) init_xstate_buf) + offset,
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memcpy((void *)fx + offset,
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(void *)init_xstate_buf + offset,
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size);
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}
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