Fix TBI handling for watchpoints

When inserting hw watchpoints, we take care of masking off the top byte
of the address (and sign-extending it if needed).  This guarantees we won't
pass tagged addresses to the kernel via ptrace.

However, from the kernel documentation on tagged pointers...

"Non-zero tags are not preserved when delivering signals. This means that
signal handlers in applications making use of tags cannot rely on the tag
information for user virtual addresses being maintained for fields inside
siginfo_t.

One exception to this rule is for signals raised in response to watchpoint
debug exceptions, where the tag information will be preserved."

So the stopped data address after a hw watchpoint hit can be potentially
tagged, and we don't handle this in GDB at the moment.  This results in
GDB missing a hw watchpoint hit and attempting to step over an unsteppable
hw watchpoint, causing it to spin endlessly.

The following patch fixes this by adjusting the stopped data address and adds
some tests to expose the problem.

gdb/ChangeLog:

2020-12-16  Luis Machado  <luis.machado@linaro.org>

	* aarch64-linux-nat.c
	(aarch64_linux_nat_target::stopped_data_address): Handle the TBI.

gdbserver/ChangeLog:

2020-12-16  Luis Machado  <luis.machado@linaro.org>

	* linux-aarch64-low.cc (address_significant): New function.
	(aarch64_target::low_stopped_data_address): Handle the TBI.

gdb/testsuite/ChangeLog:

2020-12-16  Luis Machado  <luis.machado@linaro.org>

	* gdb.arch/aarch64-tagged-pointer.c (main): Add a few more
	pointer-based memory accesses.
	* gdb.arch/aarch64-tagged-pointer.exp: Exercise additional
	hw watchpoint cases.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 979d26eee2a671f567af0dcb059fad657247add8..764329d52f6ee138c6d6a8eecd284a5f472b4a03 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2020-12-16  Luis Machado  <luis.machado@linaro.org>
+
+       * aarch64-linux-nat.c
+       (aarch64_linux_nat_target::stopped_data_address): Handle the TBI.
+
 2020-12-15  Rae Kim  <rae.kim@gmail.com>
 
        * cli/cli-script.c (do_document_command): Rename from

diff --git a/gdb/aarch64-linux-nat.c b/gdb/aarch64-linux-nat.c
index 77d5863a56aa2bd78779827fe58a1d4ba28fb577..b3bbde4b92c8a8d620fbc9fe5bc9664ceb1563c8 100644 (file)
--- a/gdb/aarch64-linux-nat.c
+++ b/gdb/aarch64-linux-nat.c
@@ -877,6 +877,13 @@ aarch64_linux_nat_target::stopped_data_address (CORE_ADDR *addr_p)
       || (siginfo.si_code & 0xffff) != TRAP_HWBKPT)
     return false;
 
+  /* Make sure to ignore the top byte, otherwise we may not recognize a
+     hardware watchpoint hit.  The stopped data addresses coming from the
+     kernel can potentially be tagged addresses.  */
+  struct gdbarch *gdbarch = thread_architecture (inferior_ptid);
+  const CORE_ADDR addr_trap
+    = address_significant (gdbarch, (CORE_ADDR) siginfo.si_addr);
+
   /* Check if the address matches any watched address.  */
   state = aarch64_get_debug_reg_state (inferior_ptid.pid ());
   for (i = aarch64_num_wp_regs - 1; i >= 0; --i)
@@ -884,7 +891,6 @@ aarch64_linux_nat_target::stopped_data_address (CORE_ADDR *addr_p)
       const unsigned int offset
        = aarch64_watchpoint_offset (state->dr_ctrl_wp[i]);
       const unsigned int len = aarch64_watchpoint_length (state->dr_ctrl_wp[i]);
-      const CORE_ADDR addr_trap = (CORE_ADDR) siginfo.si_addr;
       const CORE_ADDR addr_watch = state->dr_addr_wp[i] + offset;
       const CORE_ADDR addr_watch_aligned = align_down (state->dr_addr_wp[i], 8);
       const CORE_ADDR addr_orig = state->dr_addr_orig_wp[i];

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 549475a75f34d24aaca91ec181facbd31e3ac6ee..7249ae670d6450cf32877338b5860e8a21d0ad60 100644 (file)
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2020-12-16  Luis Machado  <luis.machado@linaro.org>
+
+       * gdb.arch/aarch64-tagged-pointer.c (main): Add a few more
+       pointer-based memory accesses.
+       * gdb.arch/aarch64-tagged-pointer.exp: Exercise additional
+       hw watchpoint cases.
+
 2020-12-15  Rae Kim  <rae.kim@gmail.com>
 
        * gdb.base/document.exp: New test.

diff --git a/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.c b/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.c
index 609d4f220e63fa7dc9320f2653bc4568616e4473..658c3093e873eb5e1fdda11c14f9e97fecc0df6d 100644 (file)
--- a/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.c
+++ b/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.c
@@ -53,5 +53,11 @@ main (void)
     }
 
   sp1->i = 8765;
-  i = 1;
+  sp2->i = 4321;
+  sp1->i = 8765;
+  sp2->i = 4321;
+  *p1 = 1;
+  *p2 = 2;
+  *p1 = 1;
+  *p2 = 2;
 }

diff --git a/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.exp b/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.exp
index 957571fdf99f1748efd38ab47ff28e8dcc585a95..01c2b577d53b61821a3583d54c776dee0195798b 100644 (file)
--- a/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.exp
+++ b/gdb/testsuite/gdb.arch/aarch64-tagged-pointer.exp
@@ -92,14 +92,21 @@ foreach_with_prefix bptype {"hbreak" "break"} {
 
 gdb_test "down"
 gdb_test "finish"
-# Watch on tagged pointer.
-gdb_test "watch *sp2"
-gdb_test "continue" \
-    "Continuing\\..*Hardware watchpoint \[0-9\]+.*" \
-    "run until watchpoint on s1"
-delete_breakpoints
 
-gdb_test "watch *p2"
-gdb_test "continue" \
-    "Continuing\\..*Hardware watchpoint \[0-9\]+.*" \
-    "run until watchpoint on i"
+# sp1 and p1 are untagged pointers, but sp2 and p2 are tagged pointers.
+# Cycle through all of them to make sure the following combinations work:
+#
+# hw watch on untagged address, hit on untagged address.
+# hw watch on tagged address, hit on untagged address.
+# hw watch on untagged address, hit on tagged address.
+# hw watch on tagged address, hit on tagged address.
+foreach symbol {"sp1" "sp2" "p1" "p2"} {
+    gdb_test "watch *${symbol}"
+    gdb_test "continue" \
+       "Continuing\\..*Hardware watchpoint \[0-9\]+.*" \
+       "run until watchpoint on ${symbol}"
+    gdb_test "continue" \
+       "Continuing\\..*Hardware watchpoint \[0-9\]+.*" \
+       "run until watchpoint on ${symbol}, 2nd hit"
+    delete_breakpoints
+}

diff --git a/gdbserver/ChangeLog b/gdbserver/ChangeLog
index 9756d4cadaa8bc0638f884b1253524cf61b030ff..16a9609bd0bf6ed36ed46e3ed6a69574b9fa05e7 100644 (file)
--- a/gdbserver/ChangeLog
+++ b/gdbserver/ChangeLog
@@ -1,3 +1,8 @@
+2020-12-16  Luis Machado  <luis.machado@linaro.org>
+
+       * linux-aarch64-low.cc (address_significant): New function.
+       (aarch64_target::low_stopped_data_address): Handle the TBI.
+
 2020-12-11  Andrew Burgess  <andrew.burgess@embecosm.com>
 
        * Makefile.in (IPA_LIB): Include libiberty library.

diff --git a/gdbserver/linux-aarch64-low.cc b/gdbserver/linux-aarch64-low.cc
index 08208ae4f4524a579b5f1bd7ea5afd2c103c9e24..f39d7c231646c83825f49d9d1b5e1feb9b4696f1 100644 (file)
--- a/gdbserver/linux-aarch64-low.cc
+++ b/gdbserver/linux-aarch64-low.cc
@@ -458,6 +458,23 @@ aarch64_target::low_remove_point (raw_bkpt_type type, CORE_ADDR addr,
   return ret;
 }
 
+/* Return the address only having significant bits.  This is used to ignore
+   the top byte (TBI).  */
+
+static CORE_ADDR
+address_significant (CORE_ADDR addr)
+{
+  /* Clear insignificant bits of a target address and sign extend resulting
+     address.  */
+  int addr_bit = 56;
+
+  CORE_ADDR sign = (CORE_ADDR) 1 << (addr_bit - 1);
+  addr &= ((CORE_ADDR) 1 << addr_bit) - 1;
+  addr = (addr ^ sign) - sign;
+
+  return addr;
+}
+
 /* Implementation of linux target ops method "low_stopped_data_address".  */
 
 CORE_ADDR
@@ -478,6 +495,12 @@ aarch64_target::low_stopped_data_address ()
       || (siginfo.si_code & 0xffff) != 0x0004 /* TRAP_HWBKPT */)
     return (CORE_ADDR) 0;
 
+  /* Make sure to ignore the top byte, otherwise we may not recognize a
+     hardware watchpoint hit.  The stopped data addresses coming from the
+     kernel can potentially be tagged addresses.  */
+  const CORE_ADDR addr_trap
+    = address_significant ((CORE_ADDR) siginfo.si_addr);
+
   /* Check if the address matches any watched address.  */
   state = aarch64_get_debug_reg_state (pid_of (current_thread));
   for (i = aarch64_num_wp_regs - 1; i >= 0; --i)
@@ -485,7 +508,6 @@ aarch64_target::low_stopped_data_address ()
       const unsigned int offset
        = aarch64_watchpoint_offset (state->dr_ctrl_wp[i]);
       const unsigned int len = aarch64_watchpoint_length (state->dr_ctrl_wp[i]);
-      const CORE_ADDR addr_trap = (CORE_ADDR) siginfo.si_addr;
       const CORE_ADDR addr_watch = state->dr_addr_wp[i] + offset;
       const CORE_ADDR addr_watch_aligned = align_down (state->dr_addr_wp[i], 8);
       const CORE_ADDR addr_orig = state->dr_addr_orig_wp[i];