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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
---
arch/x86/kernel/hw_breakpoint.c | 2 +-
arch/x86/kernel/ptrace.c | 4 ++--
arch/x86/kernel/traps.c | 5 ++---
3 files changed, 5 insertions(+), 6 deletions(-)
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -500,7 +500,7 @@ 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);
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -601,7 +601,7 @@ static unsigned long ptrace_get_debugreg
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 ta
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);
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -745,9 +745,8 @@ static __always_inline unsigned long deb
* 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
