Re: Using libthread_db.so with single-threaded programs, for TLS access

From 0edee198c4edb446bb5e676d6c49e1bef6e15ad1 Mon Sep 17 00:00:00 2001
From: Pedro Alves <[email protected]>
Date: Wed, 13 Sep 2017 11:43:15 +0100
Subject: [PATCH] Use libthread_db.so with non-threaded programs, for TLS

---
 gdb/linux-thread-db.c | 42 ++++++++++++++++++++++++++++++++----------
 gdb/proc-service.c    | 19 ++++++++++++++++---
 2 files changed, 48 insertions(+), 13 deletions(-)

diff --git a/gdb/linux-thread-db.c b/gdb/linux-thread-db.c
index 0e16f6a..4f627cb 100644
--- a/gdb/linux-thread-db.c
+++ b/gdb/linux-thread-db.c
@@ -104,7 +104,7 @@ set_libthread_db_search_path (char *ignored, int from_tty,
 
 /* If non-zero, print details of libthread_db processing.  */
 
-static unsigned int libthread_db_debug;
+unsigned int libthread_db_debug;
 
 static void
 show_libthread_db_debug (struct ui_file *file, int from_tty,
@@ -354,13 +354,19 @@ thread_from_lwp (ptid_t ptid)
   err = info->td_ta_map_lwp2thr_p (info->thread_agent, ptid_get_lwp (ptid),
 				   &th);
   if (err != TD_OK)
-    error (_("Cannot find user-level thread for LWP %ld: %s"),
-	   ptid_get_lwp (ptid), thread_db_err_str (err));
+    {
+      warning (_("Cannot find user-level thread for LWP %ld: %s"),
+	       ptid_get_lwp (ptid), thread_db_err_str (err));
+      return NULL;
+    }
 
   err = info->td_thr_get_info_p (&th, &ti);
   if (err != TD_OK)
-    error (_("thread_get_info_callback: cannot get thread info: %s"),
-	   thread_db_err_str (err));
+    {
+      warning (_("thread_get_info_callback: cannot get thread info: %s"),
+	       thread_db_err_str (err));
+      return NULL;
+    }
 
   /* Fill the cache.  */
   tp = find_thread_ptid (ptid);
@@ -1429,14 +1435,32 @@ thread_db_get_thread_local_address (struct target_ops *ops,
   if (thread_info != NULL && thread_info->priv == NULL)
     thread_info = thread_from_lwp (ptid);
 
-  if (thread_info != NULL && thread_info->priv != NULL)
+  if (thread_info != NULL)
     {
       td_err_e err;
       psaddr_t address;
       struct thread_db_info *info;
+      td_thrhandle_t main_thr_th;
 
       info = get_thread_db_info (ptid_get_pid (ptid));
 
+      /* Handle executables that don't link with pthread.
+	 Historically we didn't use to use libthread_db.so with those,
+	 but nowadays we do in order to be able to access TLS
+	 variables in non-threaded programs, like "errno".  If the
+	 program is not threaded, the initial thread's handle's tid
+	 field remains 0, and we won't have cached the handle /
+	 created the priv field.  */
+      td_thrhandle_t *th;
+      if (thread_info->priv == NULL)
+	{
+	  main_thr_th.th_ta_p = info->thread_agent;
+	  main_thr_th.th_unique = 0;
+	  th = &main_thr_th;
+	}
+      else
+	th = &thread_info->priv->th;
+
       /* Finally, get the address of the variable.  */
       if (lm != 0)
 	{
@@ -1448,8 +1472,7 @@ thread_db_get_thread_local_address (struct target_ops *ops,
 	  /* Note the cast through uintptr_t: this interface only works if
 	     a target address fits in a psaddr_t, which is a host pointer.
 	     So a 32-bit debugger can not access 64-bit TLS through this.  */
-	  err = info->td_thr_tls_get_addr_p (&thread_info->priv->th,
-					     (psaddr_t)(uintptr_t) lm,
+	  err = info->td_thr_tls_get_addr_p (th, (psaddr_t)(uintptr_t) lm,
 					     offset, &address);
 	}
       else
@@ -1466,8 +1489,7 @@ thread_db_get_thread_local_address (struct target_ops *ops,
 	     PR libc/16831 due to GDB PR threads/16954 LOAD_MODULE is also NULL.
 	     The constant number 1 depends on GNU __libc_setup_tls
 	     initialization of l_tls_modid to 1.  */
-	  err = info->td_thr_tlsbase_p (&thread_info->priv->th,
-					1, &address);
+	  err = info->td_thr_tlsbase_p (th, 1, &address);
 	  address = (char *) address + offset;
 	}
 
diff --git a/gdb/proc-service.c b/gdb/proc-service.c
index 4620fea..940993c 100644
--- a/gdb/proc-service.c
+++ b/gdb/proc-service.c
@@ -102,6 +102,8 @@ ps_xfer_memory (const struct ps_prochandle *ph, psaddr_t addr,
 }
 
 
+extern unsigned int libthread_db_debug;
+
 /* Search for the symbol named NAME within the object named OBJ within
    the target process PH.  If the symbol is found the address of the
    symbol is stored in SYM_ADDR.  */
@@ -119,9 +121,20 @@ ps_pglobal_lookup (gdb_ps_prochandle_t ph, const char *obj,
   /* FIXME: kettenis/2000-09-03: What should we do with OBJ?  */
   bound_minimal_symbol ms = lookup_minimal_symbol (name, NULL, NULL);
   if (ms.minsym == NULL)
-    return PS_NOSYM;
-
-  *sym_addr = core_addr_to_ps_addr (BMSYMBOL_VALUE_ADDRESS (ms));
+    {
+      if (libthread_db_debug)
+	fprintf_unfiltered (gdb_stdlog,
+			    "ps_pglobal_lookup: name=\"%s\" => PS_NOSYM\n",
+			    name);
+      return PS_NOSYM;
+    }
+
+  CORE_ADDR ms_addr = BMSYMBOL_VALUE_ADDRESS (ms);
+  if (libthread_db_debug)
+    fprintf_unfiltered (gdb_stdlog,
+			"ps_pglobal_lookup: name=\"%s\" => PS_OK, %s\n", name,
+			paddress (target_gdbarch (), ms_addr));
+  *sym_addr = core_addr_to_ps_addr (ms_addr);
   return PS_OK;
 }
 
-- 
2.5.5

From 77e31bd04a937264436ddaa63ebceeb9a6c9b906 Mon Sep 17 00:00:00 2001
From: Pedro Alves <[email protected]>
Date: Wed, 13 Sep 2017 23:58:03 +0100
Subject: [PATCH] Allow loading libthread_db.so with non-threaded processes

---
 elf/dl-tls.c                | 13 +++++++++++++
 nptl_db/structs.def         |  6 ++++--
 nptl_db/td_ta_map_lwp2thr.c | 27 +++++++++++++++++++--------
 nptl_db/td_thr_get_info.c   | 12 ++++++++++--
 4 files changed, 46 insertions(+), 12 deletions(-)

diff --git a/elf/dl-tls.c b/elf/dl-tls.c
index 5aba33b..8d43e38 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -951,3 +951,16 @@ cannot create TLS data structures"));
   listp->slotinfo[idx].map = l;
   listp->slotinfo[idx].gen = GL(dl_tls_generation) + 1;
 }
+
+/* Information for libthread_db.  */
+
+#include <version.h>
+#include <ldsodefs.h>
+
+/* Version of the library, used in libthread_db to detect mismatches.  */
+static const char nptl_version[] __attribute_used__ = VERSION;
+
+/* Likely could have better #if checks in structs.def instead.  */
+#define DB_RTLD_SYMS 1
+#include "../nptl_db/db_info.c"
+
diff --git a/nptl_db/structs.def b/nptl_db/structs.def
index 8a65afd..f9f9ab7 100644
--- a/nptl_db/structs.def
+++ b/nptl_db/structs.def
@@ -31,7 +31,7 @@
 #endif
 
 #ifndef DB_RTLD_GLOBAL_FIELD
-# if !IS_IN (libpthread)
+# if !IS_IN (libpthread) && !defined DB_RTLD_SYMS
 #  define DB_RTLD_GLOBAL_FIELD(field)		\
   DB_STRUCT_FIELD (rtld_global, _##field)	\
   DB_MAIN_VARIABLE (_##field)
@@ -76,8 +76,10 @@ DB_FUNCTION (__nptl_create_event)
 DB_FUNCTION (__nptl_death_event)
 DB_SYMBOL (__nptl_threads_events)
 DB_VARIABLE (__nptl_nthreads)
+#if !defined DB_RTLD_SYMS
 DB_VARIABLE (__nptl_last_event)
 DB_VARIABLE (__nptl_initial_report_events)
+#endif
 
 DB_ARRAY_VARIABLE (__pthread_keys)
 DB_STRUCT (pthread_key_struct)
@@ -101,7 +103,7 @@ DB_STRUCT_FIELD (dtv_t, counter)
 DB_STRUCT_FIELD (pthread, dtvp)
 #endif
 
-#if !(IS_IN (libpthread) && !defined SHARED)
+#if !(IS_IN (libpthread) && !defined SHARED) && !defined DB_RTLD_SYMS
 DB_STRUCT (rtld_global)
 DB_RTLD_VARIABLE (_rtld_global)
 #endif
diff --git a/nptl_db/td_ta_map_lwp2thr.c b/nptl_db/td_ta_map_lwp2thr.c
index d34711d..798c6e9 100644
--- a/nptl_db/td_ta_map_lwp2thr.c
+++ b/nptl_db/td_ta_map_lwp2thr.c
@@ -168,6 +168,20 @@ __td_ta_lookup_th_unique (const td_thragent_t *ta_arg,
   return TD_OK;
 }
 
+/* If LWPID is the thread group leader, return the fake special
+   descriptor for the initial thread.  Otherwise, return error.  */
+
+static td_err_e
+initial_thread_desc (td_thragent_t *const ta,
+		     lwpid_t lwpid, td_thrhandle_t *th)
+{
+  if (ps_getpid (ta->ph) != lwpid)
+    return TD_ERR;
+  th->th_ta_p = ta;
+  th->th_unique = 0;
+  return TD_OK;
+}
+
 td_err_e
 td_ta_map_lwp2thr (const td_thragent_t *ta_arg,
 		   lwpid_t lwpid, td_thrhandle_t *th)
@@ -189,20 +203,17 @@ td_ta_map_lwp2thr (const td_thragent_t *ta_arg,
   psaddr_t list;
   td_err_e err = DB_GET_SYMBOL (list, ta, __stack_user);
   if (err != TD_OK)
-    return err;
+    {
+      /* Looks like this is a non-threaded program.  */
+      return initial_thread_desc (ta, lwpid, th);
+    }
 
   err = DB_GET_FIELD (list, ta, list, list_t, next, 0);
   if (err != TD_OK)
     return err;
 
   if (list == 0)
-    {
-      if (ps_getpid (ta->ph) != lwpid)
-	return TD_ERR;
-      th->th_ta_p = ta;
-      th->th_unique = 0;
-      return TD_OK;
-    }
+    return initial_thread_desc (ta, lwpid, th);
 
   return __td_ta_lookup_th_unique (ta_arg, lwpid, th);
 }
diff --git a/nptl_db/td_thr_get_info.c b/nptl_db/td_thr_get_info.c
index 48979b8..e92bd1f 100644
--- a/nptl_db/td_thr_get_info.c
+++ b/nptl_db/td_thr_get_info.c
@@ -43,6 +43,14 @@ td_thr_get_info (const td_thrhandle_t *th, td_thrinfo_t *infop)
       tid = 0;
       err = DB_GET_VALUE (report_events, th->th_ta_p,
 			  __nptl_initial_report_events, 0);
+      if (err != TD_OK)
+	{
+	  /* If the program is non-threaded,
+	     __nptl_initial_report_events is nowhere to be found (it
+	     only exists in libpthread.so).  */
+	  report_events = 0;
+	  err = TD_OK;
+	}
     }
   else
     {
@@ -75,9 +83,9 @@ td_thr_get_info (const td_thrhandle_t *th, td_thrinfo_t *infop)
 	return err;
       err = DB_GET_FIELD_LOCAL (report_events, th->th_ta_p, copy, pthread,
 				report_events, 0);
+      if (err != TD_OK)
+	return err;
     }
-  if (err != TD_OK)
-    return err;
 
   /* Fill in information.  Clear first to provide reproducable
      results for the fields we do not fill in.  */
-- 
2.5.5