x86/debug: Support negative polarity DR6 bits (f4956cf8) · Commits · 戴 / test

DR6 has a whole bunch of bits that have negative polarity; they were architecturally reserved and defined to be 1 and are now getting used. Since they're 1 by default, 0 becomes the signal value. Handle this by xor'ing the read DR6 value by the reserved mask, this will flip them around such that 1 is the signal value (positive polarity). Current Linux doesn't yet support any of these bits, but there's two defined: - DR6[11] Bus Lock Debug Exception (ISEr39) - DR6[16] Restricted Transactional Memory (SDM) Update ptrace_{set,get}_debugreg() to provide/consume the value in architectural polarity. Although afaict ptrace_set_debugreg(6) is pointless, the value is not consumed anywhere. Change hw_breakpoint_restore() to alway write the DR6_RESERVED value to DR6, again, no consumer for that write. Suggested-by:

Andrew Cooper <Andrew.Cooper3@citrix.com> Signed-off-by:

Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by:

Thomas Gleixner <tglx@linutronix.de> Tested-by:

Daniel Thompson <daniel.thompson@linaro.org> Link: https://lore.kernel.org/r/20200902133201.354220797@infradead.org

arch/x86/kernel/hw_breakpoint.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -464,7 +464,7 @@ void hw_breakpoint_restore(void)
		set_debugreg(__this_cpu_read(cpu_debugreg[1]), 1);
		set_debugreg(__this_cpu_read(cpu_debugreg[2]), 2);
		set_debugreg(__this_cpu_read(cpu_debugreg[3]), 3);
		set_debugreg(current->thread.debugreg6, 6);
		set_debugreg(DR6_RESERVED, 6);
		set_debugreg(__this_cpu_read(cpu_dr7), 7);
		}
		EXPORT_SYMBOL_GPL(hw_breakpoint_restore);

arch/x86/kernel/ptrace.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -601,7 +601,7 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
		if (bp)
		val = bp->hw.info.address;
		} else if (n == 6) {
		val = thread->debugreg6;
		val = thread->debugreg6 ^ DR6_RESERVED; /* Flip back to arch polarity */
		} else if (n == 7) {
		val = thread->ptrace_dr7;
		}
		@@ -657,7 +657,7 @@ static int ptrace_set_debugreg(struct task_struct *tsk, int n,
		if (n < HBP_NUM) {
		rc = ptrace_set_breakpoint_addr(tsk, n, val);
		} else if (n == 6) {
		thread->debugreg6 = val;
		thread->debugreg6 = val ^ DR6_RESERVED; /* Flip to positive polarity */
		rc = 0;
		} else if (n == 7) {
		rc = ptrace_write_dr7(tsk, val);

arch/x86/kernel/traps.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -745,9 +745,8 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
		* Keep it simple: clear DR6 immediately.
		*/
		get_debugreg(dr6, 6);
		set_debugreg(0, 6);
		/* Filter out all the reserved bits which are preset to 1 */
		dr6 &= ~DR6_RESERVED;
		set_debugreg(DR6_RESERVED, 6);
		dr6 ^= DR6_RESERVED; /* Flip to positive polarity */

		/*
		* The SDM says "The processor clears the BTF flag when it

Admin message