Warn if /proc is not accessible

There's a buildroot where I want to debug a binary, and I tried to
connect to it from outside, but got very weird errors like
architecture mismatch or protocol errors.  At last, after switching on
'--debug' for gdbserver I found a message 'Can't open /proc/pid/'
message and suddenly found that I forgot to mount procfs in my
buildroot.

Make discovering the problem easier by making GDB / GDBserver warn
(even without --debug) if /proc can not be accessed.

Native debugging:

 (gdb) start
 Temporary breakpoint 1 at 0x400835: file test.c, line 10.
 Starting program: /tmp/test
 warning: /proc is not accessible.

GDBserver/remote debugging:

 $ ./gdbserver :9999 ./gdbserver
 gdbserver: /proc is not accessible.

gdb/ChangeLog:
2018-07-04  Vyacheslav Barinov  <v.barinov@samsung.com>
	    Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_init_ptrace): Rename to ...
	(linux_init_ptrace_procfs): ... this.  Call
	linux_proc_init_warnings.
	(linux_nat_target::post_attach)
	(linux_nat_target::post_startup_inferior): Adjust.
	* nat/linux-procfs.c (linux_proc_init_warnings): Define function.
	* nat/linux-procfs.h (linux_proc_init_warnings): Declare function.

gdb/gdbserver/ChangeLog:
2018-07-04  Vyacheslav Barinov  <v.barinov@samsung.com>
	    Pedro Alves  <palves@redhat.com>

	* linux-low.c (initialize_low): Call linux_proc_init_warnings.

gdb/ChangeLog

@@ -1,3 +1,14 @@
+2018-07-04  Vyacheslav Barinov  <v.barinov@samsung.com>
+	    Pedro Alves  <palves@redhat.com>
+
+	* linux-nat.c (linux_init_ptrace): Rename to ...
+	(linux_init_ptrace_procfs): ... this.  Call
+	linux_proc_init_warnings.
+	(linux_nat_target::post_attach)
+	(linux_nat_target::post_startup_inferior): Adjust.
+	* nat/linux-procfs.c (linux_proc_init_warnings): Define function.
+	* nat/linux-procfs.h (linux_proc_init_warnings): Declare function.
+
 2018-07-04  Tom de Vries  <tdevries@suse.de>
 
 	* dwarf2read.c (error_check_comp_unit_head): Move dwarf version

gdb/gdbserver/ChangeLog

@@ -1,3 +1,8 @@
+2018-07-04  Vyacheslav Barinov  <v.barinov@samsung.com>
+	    Pedro Alves  <palves@redhat.com>
+
+	* linux-low.c (initialize_low): Call linux_proc_init_warnings.
+
 2018-07-03  Tom Tromey  <tom@tromey.com>
 
 	* linux-low.c: Update.

gdb/gdbserver/linux-low.c

@@ -7558,6 +7558,7 @@ initialize_low (void)
   set_target_ops (&linux_target_ops);
 
   linux_ptrace_init_warnings ();
+  linux_proc_init_warnings ();
 
   sigchld_action.sa_handler = sigchld_handler;
   sigemptyset (&sigchld_action.sa_mask);

gdb/linux-nat.c

@@ -384,18 +384,19 @@ linux_nat_ptrace_options (int attached)
   return options;
 }
 
-/* Initialize ptrace warnings and check for supported ptrace
-   features given PID.
+/* Initialize ptrace and procfs warnings and check for supported
+   ptrace features given PID.
 
    ATTACHED should be nonzero iff we attached to the inferior.  */
 
 static void
-linux_init_ptrace (pid_t pid, int attached)
+linux_init_ptrace_procfs (pid_t pid, int attached)
 {
   int options = linux_nat_ptrace_options (attached);
 
   linux_enable_event_reporting (pid, options);
   linux_ptrace_init_warnings ();
+  linux_proc_init_warnings ();
 }
 
 linux_nat_target::~linux_nat_target ()
@@ -404,13 +405,13 @@ linux_nat_target::~linux_nat_target ()
 void
 linux_nat_target::post_attach (int pid)
 {
-  linux_init_ptrace (pid, 1);
+  linux_init_ptrace_procfs (pid, 1);
 }
 
 void
 linux_nat_target::post_startup_inferior (ptid_t ptid)
 {
-  linux_init_ptrace (ptid.pid (), 0);
+  linux_init_ptrace_procfs (ptid.pid (), 0);
 }
 
 /* Return the number of known LWPs in the tgid given by PID.  */

gdb/nat/linux-procfs.c

@@ -357,3 +357,20 @@ linux_proc_pid_to_exec_file (int pid)
 
   return buf;
 }
+
+/* See linux-procfs.h.  */
+
+void
+linux_proc_init_warnings ()
+{
+  static bool warned = false;
+
+  if (warned)
+    return;
+  warned = true;
+
+  struct stat st;
+
+  if (stat ("/proc/self", &st) != 0)
+    warning (_("/proc is not accessible."));
+}

gdb/nat/linux-procfs.h

@@ -80,4 +80,9 @@ extern int linux_proc_task_list_dir_exists (pid_t pid);
 
 extern char *linux_proc_pid_to_exec_file (int pid);
 
+/* Display possible problems on this system.  Display them only once
+   per GDB execution.  */
+
+extern void linux_proc_init_warnings ();
+
 #endif /* COMMON_LINUX_PROCFS_H */