[binutils-gdb] Fix follow-exec regression / crash

[Pedro Alves <palves at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=ecdc3a72c89e43e0e13c5478723b4f70b3964e9f

commit ecdc3a72c89e43e0e13c5478723b4f70b3964e9f
Author: Pedro Alves <palves@redhat.com>
Date:   Thu Jun 28 16:57:18 2018 +0100

    Fix follow-exec regression / crash
    
    After commit 00431a78b28f ("Use thread_info and inferior pointers more
    throughout"), following an exec can result in gdb crashing.  On some
    systems, this is visible with gdb.multi/multi-arch-exec.exp and
    gdb.base/foll-exec-mode.exp.  E.g.:
    
      $ make check TESTS="gdb.multi/multi-arch-exec.exp gdb.base/foll-exec-mode.exp"
      [snip]
      FAIL: gdb.multi/multi-arch-exec.exp: first_arch=1: selected_thread=1: follow_exec_mode=new: continue across exec that changes architecture (GDB internal error)
      ERROR: : spawn id exp10 not open
          while executing
    
    Running multi-arch-exec under Valgrind we easily spot the problem:
    
      process 16305 is executing new program: ..../gdb.multi/multi-arch-exec/1-multi-arch-exec-hello
      [New inferior 2 (process 0)]
      [New process 16305]
      ==16129== Invalid read of size 8
      ==16129==    at 0x7FA14D: get_thread_regcache(thread_info*) (regcache.c:399)
      ==16129==    by 0x75E54B: handle_inferior_event_1(execution_control_state*) (infrun.c:5292)
      ==16129==    by 0x75E82D: handle_inferior_event(execution_control_state*) (infrun.c:5382)
      ==16129==    by 0x75BC6A: fetch_inferior_event(void*) (infrun.c:3918)
      ==16129==    by 0x748DA3: inferior_event_handler(inferior_event_type, void*) (inf-loop.c:43)
      ==16129==    by 0x464B5D: handle_target_event(int, void*) (linux-nat.c:4359)
      ==16129==    by 0x7047E0: handle_file_event(file_handler*, int) (event-loop.c:733)
      ==16129==    by 0x704D83: gdb_wait_for_event(int) (event-loop.c:859)
      ==16129==    by 0x703BF6: gdb_do_one_event() (event-loop.c:322)
      ==16129==    by 0x703CA2: start_event_loop() (event-loop.c:371)
      ==16129==    by 0x791D95: captured_command_loop() (main.c:330)
      ==16129==    by 0x79311C: captured_main(void*) (main.c:1157)
      ==16129==  Address 0x15a5bad0 is 32 bytes inside a block of size 600 free'd
      ==16129==    at 0x4C2E1E8: operator delete(void*) (vg_replace_malloc.c:576)
      ==16129==    by 0x8A15D0: delete_thread_1(thread_info*, bool) (thread.c:465)
      ==16129==    by 0x8A15FA: delete_thread(thread_info*) (thread.c:476)
      ==16129==    by 0x8A0D43: add_thread_silent(ptid_t) (thread.c:291)
      ==16129==    by 0x8A0DF0: add_thread_with_info(ptid_t, private_thread_info*) (thread.c:317)
      ==16129==    by 0x8A0E79: add_thread(ptid_t) (thread.c:331)
      ==16129==    by 0x75764C: follow_exec(ptid_t, char*) (infrun.c:1233)
      ==16129==    by 0x75E534: handle_inferior_event_1(execution_control_state*) (infrun.c:5290)
      ==16129==    by 0x75E82D: handle_inferior_event(execution_control_state*) (infrun.c:5382)
      ==16129==    by 0x75BC6A: fetch_inferior_event(void*) (infrun.c:3918)
      ==16129==    by 0x748DA3: inferior_event_handler(inferior_event_type, void*) (inf-loop.c:43)
      ==16129==    by 0x464B5D: handle_target_event(int, void*) (linux-nat.c:4359)
    
    The problem is that handle_inferior_event_1 is reading the stop_pc off
    of a thread that was deleted by follow_exec.  Before commit
    00431a78b28f, we didn't crash because we were passing down a ptid to
    get_thread_regcache instead of ecs->event_thread.
    
    Fix this by simply moving the stop_pc reading until after
    ecs->event_thread is refreshed.
    
    gdb/ChangeLog:
    2018-06-28  Pedro Alves  <palves@redhat.com>
    
    	* infrun.c (handle_inferior_event_1) <TARGET_WAITKIND_EXECD>:
    	Moving fetching stop_pc until after ecs->event_thread is refreshed.

Diff:
---
 gdb/ChangeLog | 5 +++++
 gdb/infrun.c  | 4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 81fae49..417c563 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2018-06-28  Pedro Alves  <palves@redhat.com>
+
+	* infrun.c (handle_inferior_event_1) <TARGET_WAITKIND_EXECD>:
+	Moving fetching stop_pc until after ecs->event_thread is refreshed.
+
 2018-06-28  Tom Tromey  <tom@tromey.com>
 
 	* coffread.c (coff_symfile_finish): Update.
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 9548f9c..4c73234 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -5289,13 +5289,13 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
          stop.  */
       follow_exec (inferior_ptid, ecs->ws.value.execd_pathname);
 
-      stop_pc = regcache_read_pc (get_thread_regcache (ecs->event_thread));
-
       /* In follow_exec we may have deleted the original thread and
 	 created a new one.  Make sure that the event thread is the
 	 execd thread for that case (this is a nop otherwise).  */
       ecs->event_thread = inferior_thread ();
 
+      stop_pc = regcache_read_pc (get_thread_regcache (ecs->event_thread));
+
       ecs->event_thread->control.stop_bpstat
 	= bpstat_stop_status (get_current_regcache ()->aspace (),
 			      stop_pc, ecs->event_thread, &ecs->ws);