Factorize killing the children in linux-ptrace.c, and fix a 'process leak'.

Running the gdb testsuite under Valgrind started to fail after 100+ tests,
due to out of memory caused by lingering processes.

The lingering processes are caused by the combination
of a limitation in Valgrind signal handling when using PTRACE_TRACEME
and a (minor) bug in GDB.

The Valgrind limitation is : when a process is ptraced and raises
a signal, Valgrind will replace the raised signal by SIGSTOP as other
signals are masked by Valgrind when executing a system call.
Removing this limitation seems far to be trivial, valgrind signal
handling is very complex.

Due to this valgrind limitation, GDB linux_ptrace_test_ret_to_nx gets
a SIGSTOP signal instead of the expected SIGTRAP or SIGSEGV.
In such a case, linux_ptrace_test_ret_to_nx does an early return, but
does not kill the child (running under valgrind), child stays in a STOP-ped
state.
These lingering processes then eat the available system memory,
till launching a new process starts to fail.

This patch fixes the GDB minor bug by killing the child in case
linux_ptrace_test_ret_to_nx does an early return.

nat/linux-ptrace.c has 3 different logics to kill a child process.
So, this patch factorizes killing a child in the function kill_child.

The 3 different logics are:
* linux_ptrace_test_ret_to_nx is calling both kill (child, SIGKILL)
  and ptrace (PTRACE_KILL, child, ...), and then is calling once
  waitpid.
* linux_check_ptrace_features is calling ptrace (PTRACE_KILL, child, ...)
  + my_waitpid in a loop, as long as the waitpid status was WIFSTOPPED.
* linux_test_for_tracefork is calling once ptrace (PTRACE_KILL, child, ...)
  + my_waitpid.

The linux ptrace documentation indicates that PTRACE_KILL is deprecated,
and tells to not use it, as it might return success but not kill the tracee.
The documentation indicates to send SIGKILL directly.

I suspect that linux_ptrace_test_ret_to_nx calls both kill and ptrace just
to be sure ...
I suspect that linux_check_ptrace_features calls ptrace in a loop
to bypass the PTRACE_KILL limitation.
And it looks like linux_test_for_tracefork does not handle the PTRACE_KILL
limitation.
Also, 2 of the 3 logics are calling my_waitpid, which seems better,
as this is protecting the waitpid syscall against EINTR.

So, the logic in kill_child is just using kill (child, SIGKILL)
+ my_waitpid, and then does a few verifications to see everything worked
accordingly to the plan.

Tested on Debian/x86_64.

2018-12-16  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* nat/linux-ptrace.c (kill_child): New function.
	(linux_ptrace_test_ret_to_nx): Use kill_child instead of local code.
	Add a call to kill_child in case of early return after fork.
	(linux_check_ptrace_features): Use kill_child instead of local code.
	(linux_test_for_tracefork): Likewise.

2 files changed, 48 insertions, 37 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index f3248cd..24d0e64 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2018-12-16  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
+
+	* nat/linux-ptrace.c (kill_child): New function.
+	(linux_ptrace_test_ret_to_nx): Use kill_child instead of local code.
+	Add a call to kill_child in case of early return after fork.
+	(linux_check_ptrace_features): Use kill_child instead of local code.
+	(linux_test_for_tracefork): Likewise.
+
 2018-12-14  Tom Tromey  <tom@tromey.com>
 
 	* Makefile.in: Remove "alloca" comment.
diff --git a/gdb/nat/linux-ptrace.c b/gdb/nat/linux-ptrace.c
index 1f21ef0..49a1d01 100644
--- a/gdb/nat/linux-ptrace.c
+++ b/gdb/nat/linux-ptrace.c
@@ -79,6 +79,39 @@ EXTERN_C void linux_ptrace_test_ret_to_nx_instr (void);
 
 #endif /* defined __i386__ || defined __x86_64__ */
 
+/* Kill CHILD.  WHO is used to report warnings.  */
+
+static void
+kill_child (pid_t child, const char *who)
+{
+  pid_t got_pid;
+  int kill_status;
+
+  if (kill (child, SIGKILL) != 0)
+    {
+      warning (_("%s: failed to kill child pid %ld %s\n"),
+	       who, (long) child, safe_strerror (errno));
+      return;
+    }
+
+  errno = 0;
+  got_pid = my_waitpid (child, &kill_status, 0);
+  if (got_pid != child)
+    {
+      warning (_("%s: "
+		 "kill waitpid returned %ld: %s"),
+	       who, (long) got_pid, safe_strerror (errno));
+      return;
+    }
+  if (!WIFSIGNALED (kill_status))
+    {
+      warning (_("%s: "
+		 "kill status %d is not WIFSIGNALED!"),
+	       who, kill_status);
+      return;
+    }
+}
+
 /* Test broken off-trunk Linux kernel patchset for NX support on i386.  It was
    removed in Fedora kernel 88fa1f0332d188795ed73d7ac2b1564e11a0b4cd.
 
@@ -91,7 +124,7 @@ linux_ptrace_test_ret_to_nx (void)
   pid_t child, got_pid;
   gdb_byte *return_address, *pc;
   long l;
-  int status, kill_status;
+  int status;
   elf_gregset_t regs;
 
   return_address
@@ -169,6 +202,7 @@ linux_ptrace_test_ret_to_nx (void)
     {
       warning (_("linux_ptrace_test_ret_to_nx: status %d is not WIFSTOPPED!"),
 	       status);
+      kill_child (child, "linux_ptrace_test_ret_to_nx");
       return;
     }
 
@@ -178,6 +212,7 @@ linux_ptrace_test_ret_to_nx (void)
       warning (_("linux_ptrace_test_ret_to_nx: "
 		 "WSTOPSIG %d is neither SIGTRAP nor SIGSEGV!"),
 	       (int) WSTOPSIG (status));
+      kill_child (child, "linux_ptrace_test_ret_to_nx");
       return;
     }
 
@@ -195,26 +230,7 @@ linux_ptrace_test_ret_to_nx (void)
 # error "!__i386__ && !__x86_64__"
 #endif
 
-  kill (child, SIGKILL);
-  ptrace (PTRACE_KILL, child, (PTRACE_TYPE_ARG3) NULL,
-	  (PTRACE_TYPE_ARG4) NULL);
-
-  errno = 0;
-  got_pid = waitpid (child, &kill_status, 0);
-  if (got_pid != child)
-    {
-      warning (_("linux_ptrace_test_ret_to_nx: "
-		 "PTRACE_KILL waitpid returned %ld: %s"),
-	       (long) got_pid, safe_strerror (errno));
-      return;
-    }
-  if (!WIFSIGNALED (kill_status))
-    {
-      warning (_("linux_ptrace_test_ret_to_nx: "
-		 "PTRACE_KILL status %d is not WIFSIGNALED!"),
-	       status);
-      return;
-    }
+  kill_child (child, "linux_ptrace_test_ret_to_nx");
 
   /* + 1 is there as x86* stops after the 'int3' instruction.  */
   if (WSTOPSIG (status) == SIGTRAP && pc == return_address + 1)
@@ -352,16 +368,8 @@ linux_check_ptrace_features (void)
 
   linux_test_for_exitkill (child_pid);
 
-  /* Clean things up and kill any pending children.  */
-  do
-    {
-      ret = ptrace (PTRACE_KILL, child_pid, (PTRACE_TYPE_ARG3) 0,
-		    (PTRACE_TYPE_ARG4) 0);
-      if (ret != 0)
-	warning (_("linux_check_ptrace_features: failed to kill child"));
-      my_waitpid (child_pid, &status, 0);
-    }
-  while (WIFSTOPPED (status));
+  /* Kill child_pid.  */
+  kill_child (child_pid, "linux_check_ptrace_features");
 }
 
 /* Determine if PTRACE_O_TRACESYSGOOD can be used to catch
@@ -444,12 +452,7 @@ linux_test_for_tracefork (int child_pid)
 
 	  /* Do some cleanup and kill the grandchild.  */
 	  my_waitpid (second_pid, &second_status, 0);
-	  ret = ptrace (PTRACE_KILL, second_pid, (PTRACE_TYPE_ARG3) 0,
-			(PTRACE_TYPE_ARG4) 0);
-	  if (ret != 0)
-	    warning (_("linux_test_for_tracefork: "
-		       "failed to kill second child"));
-	  my_waitpid (second_pid, &status, 0);
+	  kill_child (second_pid, "linux_test_for_tracefork");
 	}
     }
   else