[binutils-gdb] [gdb/tdep] Fix s390_linux_nat_target::stopped_by_watchpoint

[binutils-gdb] [gdb/tdep] Fix s390_linux_nat_target::stopped_by_watchpoint

[Tom de Vries]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=ea6929aaac370daddf0999faa553231c8b806b20

commit ea6929aaac370daddf0999faa553231c8b806b20
Author: Tom de Vries <tdevries@suse.de>
Date:   Tue Dec 13 18:41:12 2022 +0100

    [gdb/tdep] Fix s390_linux_nat_target::stopped_by_watchpoint
    
    On s390x-linux, I run into:
    ...
    (gdb) continue^M
    Continuing.^M
    breakpoint.c:5784: internal-error: bpstat_stop_status_nowatch: \
      Assertion `!target_stopped_by_watchpoint ()' failed.^M
    A problem internal to GDB has been detected,^M
    further debugging may prove unreliable.^M
    FAIL: gdb.threads/watchpoint-fork.exp: parent: singlethreaded: \
      breakpoint after the first fork (GDB internal error)
    ...
    
    What happens is the follow:
    - a watchpoint event triggers
    - the event is processed, s390_linux_nat_target::stopped_by_watchpoint is called and
      it returns true, as expected
    - the watchpoint event is reported by gdb, and gdb stops
    - we issue a continue command
    - a fork event triggers
    - the event is processed, and during processing that event
      s390_linux_nat_target::stopped_by_watchpoint is called again, and returns
      true
    - the assertion fails, because the function is expected to return false
    
    The function s390_linux_nat_target::stopped_by_watchpoint returns true the
    second time, because it looks at the exact same data that was looked at when
    it was called the first time, and that data hasn't changed.
    
    There's code in the same function that intends to prevent that from happening:
    ...
          /* Do not report this watchpoint again.  */
          memset (&per_lowcore, 0, sizeof (per_lowcore));
          if (ptrace (PTRACE_POKEUSR_AREA, s390_inferior_tid (), &parea, 0) < 0)
           perror_with_name (_("Couldn't clear watchpoint status"));
    ...
    and that probably used to work for older kernels, but no longer does since
    linux kernel commit 5e9a26928f55 ("[S390] ptrace cleanup").
    
    Fix this by copying this:
    ...
      siginfo_t siginfo;
      if (!linux_nat_get_siginfo (inferior_ptid, &siginfo))
        return false;
      if (siginfo.si_signo != SIGTRAP
          || (siginfo.si_code & 0xffff) != TRAP_HWBKPT)
        return false;
    ...
    from aarch64_linux_nat_target::stopped_data_address and remove the
    obsolete watchpoint status clearing code.
    
    Tested on s390x-linux.
    
    Approved-By: Ulrich Weigand <uweigand@de.ibm.com>

Diff:
---
 gdb/s390-linux-nat.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/gdb/s390-linux-nat.c b/gdb/s390-linux-nat.c
index 96833e804e9..f85e8941655 100644
--- a/gdb/s390-linux-nat.c
+++ b/gdb/s390-linux-nat.c
@@ -673,6 +673,13 @@ s390_linux_nat_target::stopped_by_watchpoint ()
   if (state->watch_areas.empty ())
     return false;
 
+  siginfo_t siginfo;
+  if (!linux_nat_get_siginfo (inferior_ptid, &siginfo))
+    return false;
+  if (siginfo.si_signo != SIGTRAP
+      || (siginfo.si_code & 0xffff) != TRAP_HWBKPT)
+    return false;
+
   parea.len = sizeof (per_lowcore);
   parea.process_addr = (addr_t) & per_lowcore;
   parea.kernel_addr = offsetof (struct user_regs_struct, per_info.lowcore);
@@ -682,14 +689,6 @@ s390_linux_nat_target::stopped_by_watchpoint ()
   bool result = (per_lowcore.perc_storage_alteration == 1
 		 && per_lowcore.perc_store_real_address == 0);
 
-  if (result)
-    {
-      /* Do not report this watchpoint again.  */
-      memset (&per_lowcore, 0, sizeof (per_lowcore));
-      if (ptrace (PTRACE_POKEUSR_AREA, s390_inferior_tid (), &parea, 0) < 0)
-	perror_with_name (_("Couldn't clear watchpoint status"));
-    }
-
   return result;
 }