Linux/ptrace: don't convert ptids when asking inf-ptrace layer to resume LWP

Ref: https://sourceware.org/ml/gdb-patches/2015-03/msg00060.html

The record-btrace target can hit an assertion here:

 Breakpoint 1, record_btrace_fetch_registers (ops=0x974bfc0 <record_btrace_ops>,
     regcache=0x9a0a798, regno=8) at gdb/record-btrace.c:1202
 1202	  gdb_assert (tp != NULL);

 (gdb) p regcache->ptid
 $3 = {pid = 23856, lwp = 0, tid = 0}

The problem is that the linux-nat layer converts the ptid to a
single-process ptid before passing the request down to the inf-ptrace
layer, which loses information, and then record-btrace can't find the
corresponding thread in GDB's thread list:

 (gdb) bt
 #0  record_btrace_fetch_registers (ops=0x974bfc0 <record_btrace_ops>, regcache=0x9a0a798, regno=8)
     at gdb/record-btrace.c:1202
 #1  0x083f4ee2 in delegate_fetch_registers (self=0x974bfc0 <record_btrace_ops>, arg1=0x9a0a798,
     arg2=8) at gdb/target-delegates.c:149
 #2  0x08406562 in target_fetch_registers (regcache=0x9a0a798, regno=8)
     at gdb/target.c:3279
 #3  0x08355255 in regcache_raw_read (regcache=0x9a0a798, regnum=8,
     buf=0xbfffe6c0 "¨\003\222\tÀ8kIøæÿ¿HO5\b\035]")
     at gdb/regcache.c:643
 #4  0x083558a7 in regcache_cooked_read (regcache=0x9a0a798, regnum=8,
     buf=0xbfffe6c0 "¨\003\222\tÀ8kIøæÿ¿HO5\b\035]")
     at gdb/regcache.c:734
 #5  0x08355de3 in regcache_cooked_read_unsigned (regcache=0x9a0a798, regnum=8, val=0xbfffe738)
     at gdb/regcache.c:838
 #6  0x0827a106 in i386_linux_resume (ops=0x9737ca0 <linux_ops_saved>, ptid=..., step=1,
     signal=GDB_SIGNAL_0) at gdb/i386-linux-nat.c:670
 #7  0x08280c12 in linux_resume_one_lwp (lp=0x9a0a5b8, step=1, signo=GDB_SIGNAL_0)
     at gdb/linux-nat.c:1529
 #8  0x08281281 in linux_nat_resume (ops=0x98da608, ptid=..., step=1, signo=GDB_SIGNAL_0)
     at gdb/linux-nat.c:1708
 #9  0x0850738e in record_btrace_resume (ops=0x98da608, ptid=..., step=1, signal=GDB_SIGNAL_0)
     at gdb/record-btrace.c:1760
 ...

The fix is just to not lose information, and let the intact ptid reach
record-btrace.c.

Tested on x86-64 Fedora 20, -m32.

gdb/ChangeLog:
2015-03-03  Pedro Alves  <palves@redhat.com>

	* i386-linux-nat.c (i386_linux_resume): Get the ptrace PID out of
	the lwp field of ptid.  Pass the full ptid to get_thread_regcache.
	* inf-ptrace.c (get_ptrace_pid): New function.
	(inf_ptrace_resume): Use it.
	* linux-nat.c (linux_resume_one_lwp): Pass the LWP's ptid ummodified
	to the lower layer.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2015-03-03  Pedro Alves  <palves@redhat.com>
+
+	* i386-linux-nat.c (i386_linux_resume): Get the ptrace PID out of
+	the lwp field of ptid.  Pass the full ptid to get_thread_regcache.
+	* inf-ptrace.c (get_ptrace_pid): New function.
+	(inf_ptrace_resume): Use it.
+	* linux-nat.c (linux_resume_one_lwp): Pass the LWP's ptid ummodified
+	to the lower layer.
+
 2015-03-03  Markus Metzger  <markus.t.metzger@intel.com>
 
 	* nat/linux-btrace.c: Include sys/utsname.h.

gdb/i386-linux-nat.c

@@ -648,8 +648,7 @@ static void
 i386_linux_resume (struct target_ops *ops,
 		   ptid_t ptid, int step, enum gdb_signal signal)
 {
-  int pid = ptid_get_pid (ptid);
+  int pid = ptid_get_lwp (ptid);
-
   int request;
 
   if (catch_syscall_enabled () > 0)
@@ -659,7 +658,7 @@ i386_linux_resume (struct target_ops *ops,
 
   if (step)
     {
-      struct regcache *regcache = get_thread_regcache (pid_to_ptid (pid));
+      struct regcache *regcache = get_thread_regcache (ptid);
       struct gdbarch *gdbarch = get_regcache_arch (regcache);
       enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
       ULONGEST pc;

gdb/inf-ptrace.c

@@ -289,6 +289,22 @@ inf_ptrace_stop (struct target_ops *self, ptid_t ptid)
   kill (-inferior_process_group (), SIGINT);
 }
 
+/* Return which PID to pass to ptrace in order to observe/control the
+   tracee identified by PTID.  */
+
+static pid_t
+get_ptrace_pid (ptid_t ptid)
+{
+  pid_t pid;
+
+  /* If we have an LWPID to work with, use it.  Otherwise, we're
+     dealing with a non-threaded program/target.  */
+  pid = ptid_get_lwp (ptid);
+  if (pid == 0)
+    pid = ptid_get_pid (ptid);
+  return pid;
+}
+
 /* Resume execution of thread PTID, or all threads if PTID is -1.  If
    STEP is nonzero, single-step it.  If SIGNAL is nonzero, give it
    that signal.  */
@@ -297,13 +313,16 @@ static void
 inf_ptrace_resume (struct target_ops *ops,
 		   ptid_t ptid, int step, enum gdb_signal signal)
 {
-  pid_t pid = ptid_get_pid (ptid);
+  pid_t pid;
+
   int request;
 
-  if (pid == -1)
+  if (ptid_equal (minus_one_ptid, ptid))
     /* Resume all threads.  Traditionally ptrace() only supports
        single-threaded processes, so simply resume the inferior.  */
-    pid = ptid_get_pid (inferior_ptid);
+    pid = get_ptrace_pid (inferior_ptid);
+  else
+    pid = get_ptrace_pid (ptid);
 
   if (catch_syscall_enabled () > 0)
     request = PT_SYSCALL;

gdb/linux-nat.c

@@ -1506,8 +1506,6 @@ linux_nat_detach (struct target_ops *ops, const char *args, int from_tty)
 static void
 linux_resume_one_lwp (struct lwp_info *lp, int step, enum gdb_signal signo)
 {
-  ptid_t ptid;
-
   lp->step = step;
 
   /* stop_pc doubles as the PC the LWP had when it was last resumed.
@@ -1524,9 +1522,7 @@ linux_resume_one_lwp (struct lwp_info *lp, int step, enum gdb_signal signo)
 
   if (linux_nat_prepare_to_resume != NULL)
     linux_nat_prepare_to_resume (lp);
-  /* Convert to something the lower layer understands.  */
+  linux_ops->to_resume (linux_ops, lp->ptid, step, signo);
-  ptid = pid_to_ptid (ptid_get_lwp (lp->ptid));
-  linux_ops->to_resume (linux_ops, ptid, step, signo);
   lp->stop_reason = LWP_STOPPED_BY_NO_REASON;
   lp->stopped = 0;
   registers_changed_ptid (lp->ptid);