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x86/debug: Fix DR_STEP vs ptrace_get_debugreg(6)
Commitd53d9bc0cf
("x86/debug: Change thread.debugreg6 to thread.virtual_dr6") changed the semantics of the variable from random collection of bits, to exactly only those bits that ptrace() needs. Unfortunately this lost DR_STEP for PTRACE_{BLOCK,SINGLE}STEP. Furthermore, it turns out that userspace expects DR_STEP to be unconditionally available, even for manual TF usage outside of PTRACE_{BLOCK,SINGLE}_STEP. Fixes:d53d9bc0cf
("x86/debug: Change thread.debugreg6 to thread.virtual_dr6") Reported-by: Kyle Huey <me@kylehuey.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Kyle Huey <me@kylehuey.com> Link: https://lore.kernel.org/r/20201027183330.GM2628@hirez.programming.kicks-ass.net
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1 changed files with 6 additions and 3 deletions
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@ -937,10 +937,13 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
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instrumentation_begin();
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/*
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* Clear the virtual DR6 value, ptrace() routines will set bits here
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* for things it wants signals for.
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* Start the virtual/ptrace DR6 value with just the DR_STEP mask
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* of the real DR6. ptrace_triggered() will set the DR_TRAPn bits.
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*
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* Userspace expects DR_STEP to be visible in ptrace_get_debugreg(6)
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* even if it is not the result of PTRACE_SINGLESTEP.
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*/
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current->thread.virtual_dr6 = 0;
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current->thread.virtual_dr6 = (dr6 & DR_STEP);
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/*
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* The SDM says "The processor clears the BTF flag when it
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