Commit d53d9bc0cf78 ("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 we 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: d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")
Reported-by: Kyle Huey <m...@kylehuey.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
v2: uncondtionally provide DR_STEP, at the very least WINE relies on this
arch/x86/kernel/traps.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -937,10 +937,13 @@ static __always_inline void exc_debug_us
instrumentation_begin();
/*
- * Clear the virtual DR6 value, ptrace() routines will set bits here
- * for things it wants signals for.
+ * Start the virtual/ptrace DR6 value with just the DR_STEP mask
+ * of the real DR6. ptrace_triggered() will set the DR_TRAPn bits.
+ *
+ * Userspace expects DR_STEP to be visible in ptrace_get_debugreg(6)
+ * even if it is not the result of PTRACE_SINGLESTEP.
*/
- current->thread.virtual_dr6 = 0;
+ current->thread.virtual_dr6 = (dr6 & DR_STEP);
/*
* The SDM says "The processor clears the BTF flag when it
