gdb/linux: consider ptrace_scope when building attach error message

On GNU/Linux, if an attach fails, then we end up calling
linux_ptrace_attach_fail_reason, which returns a string describing why
the attach might have failed.

This commit extends linux_ptrace_attach_fail_reason to check the
current /proc/sys/kernel/yama/ptrace_scope setting.  When this is set
to a value greater than 0 attaching can be restricted in some cases.

If I first set ptrace_scope to 1, like this:

  $ echo 1 | sudo tee /proc/sys/kernel/yama/ptrace_scope

Then use current master GDB to attach to a process, I see this:

  (gdb) attach 2476518
  Attaching to process 2476518
  ptrace: Operation not permitted.
  (gdb)

With this patch applied the output becomes:

  (gdb) attach 2476518
  Attaching to process 2476518
  warning: the /proc/sys/kernel/yama/ptrace_scope setting of 1 might prevent attaching, see 'man 2 ptrace'
  ptrace: Operation not permitted.
  (gdb)

In my (limited) experience, not every user is familiar with the
ptrace_scope setting.  Finding that attach just doesn't work often
leads users to believe that the only way to have GDB attach to another
process is to run GDB as root.

Though that is a solution to the problem, my hope is that, by giving
the users more information, they can make a more informed choice about
which solution is right for them (e.g. run as root, change the
setting, start the inferior directly under GDB, etc).

This change also works for gdbserver.  For example:

  (gdb) target extended-remote :54321
  Remote debugging using :54321
  (gdb) attach 2475389
  Attaching to process 2475389
  Attaching to process 2475389 failed: Cannot attach to process 2475389: \
     Operation not permitted (1), the /proc/sys/kernel/yama/ptrace_scope \
     setting of 1 might prevent attaching, see 'man 2 ptrace'
  (gdb)

There's no test for this as testing would require adjusting a system
wide setting.

I've linked to a semi-related bug.  It is discussing the same issue at
least.  This patch doesn't try to "fix" anything that the bug is
talking about, it just makes it (I think) slightly clearer when the
issue does arise.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31520

Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>

diff --git a/gdb/nat/linux-ptrace.c b/gdb/nat/linux-ptrace.c
index f73058be2eb23bc424d13218d62152458a8e008b..7df85c1c87fd1c43f179db7b455e54a526679e77 100644 (file)
--- a/gdb/nat/linux-ptrace.c
+++ b/gdb/nat/linux-ptrace.c
@@ -24,12 +24,46 @@
 #endif
 #include "gdbsupport/eintr.h"
 #include "gdbsupport/signals-state-save-restore.h"
+#include "gdbsupport/gdb_file.h"
 
 /* Stores the ptrace options supported by the running kernel.
    A value of -1 means we did not check for features yet.  A value
    of 0 means there are no supported features.  */
 static int supported_ptrace_options = -1;
 
+/* The file from which the kernel.yama.ptrace_scope setting is read.  */
+
+static constexpr char ptrace_scope_filename[]
+  = "/proc/sys/kernel/yama/ptrace_scope";
+
+/* Reads the yama ptrace_scope value from /proc.  Returns the integer value
+   of ptrace_scope (0, 1, 2, or 3) or -1 if the file cannot be read (e.g.,
+   yama is not enabled).  */
+
+static int
+get_ptrace_scope ()
+{
+  int ptrace_scope = -1;
+
+  /* The /proc file that contains the system-wide ptrace scope setting.  */
+  gdb_file_up fp (fopen (ptrace_scope_filename, "r"));
+  if (fp != nullptr)
+    {
+      if (fscanf (fp.get (), "%d", &ptrace_scope) != 1)
+       {
+         /* If fscanf fails then we couldn't parse the number.  But just
+            to be safe, set the scope back to -1 to indicate no value is
+            available.  */
+         ptrace_scope = -1;
+       }
+    }
+
+  /* If fp is NULL, it's likely because the yama security module is not
+     active.  In this case, we return -1 to indicate that the scope is
+     unknown or not applicable.  */
+  return ptrace_scope;
+}
+
 /* Find all possible reasons we could fail to attach PID and return these
    as a string.  An empty string is returned if we didn't find any reason.  */
 
@@ -44,6 +78,13 @@ linux_ptrace_attach_fail_reason (pid_t pid)
                    _("process %d is already traced by process %d"),
                    (int) pid, (int) tracerpid);
 
+  int ptrace_scope = get_ptrace_scope ();
+  if (ptrace_scope > 0)
+    string_appendf (result,
+                   _("the %s setting of %d might prevent attaching, "
+                     "see 'man 2 ptrace'"),
+                   ptrace_scope_filename, ptrace_scope);
+
   if (linux_proc_pid_is_zombie_nowarn (pid))
     string_appendf (result,
                    _("process %d is a zombie - the process has already "