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x86/entry/traps: Clear TIF_BLOCKSTEP on all debug exceptions
The SDM says that debug exceptions clear BTF, and we need to keep TIF_BLOCKSTEP in sync with BTF. Clear it unconditionally and improve the comment. I suspect that the fact that kmemcheck could cause TIF_BLOCKSTEP not to be cleared was just an oversight. Signed-off-by: Andy Lutomirski <luto@kernel.org> Cc: Andrew Cooper <andrew.cooper3@citrix.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.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> Link: http://lkml.kernel.org/r/fa86e55d196e6dde5b38839595bde2a292c52fdc.1457578375.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -597,6 +597,13 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
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/* Filter out all the reserved bits which are preset to 1 */
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dr6 &= ~DR6_RESERVED;
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/*
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* The SDM says "The processor clears the BTF flag when it
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* generates a debug exception." Clear TIF_BLOCKSTEP to keep
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* TIF_BLOCKSTEP in sync with the hardware BTF flag.
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*/
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clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);
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/*
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* If dr6 has no reason to give us about the origin of this trap,
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* then it's very likely the result of an icebp/int01 trap.
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@ -612,11 +619,6 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
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/* DR6 may or may not be cleared by the CPU */
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set_debugreg(0, 6);
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/*
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* The processor cleared BTF, so don't mark that we need it set.
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*/
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clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);
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/* Store the virtualized DR6 value */
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tsk->thread.debugreg6 = dr6;
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