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[binutils-gdb] Fix follow_exec latent problem


https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c4c17fb0f5879d3f58c733a4139fa59817e8155e

commit c4c17fb0f5879d3f58c733a4139fa59817e8155e
Author: Pedro Alves <palves@redhat.com>
Date:   Thu Nov 22 16:09:12 2018 +0000

    Fix follow_exec latent problem
    
    A following commit to make each inferior have its own thread list
    exposes a problem with bf93d7ba99 ("Add thread after updating gdbarch
    when exec'ing"), which is that we can't defer adding the thread
    because that breaks try_open_exec_file which deep inside ends up
    calling inferior_thread():
    
     #5  0x0000000000637c78 in internal_error(char const*, int, char const*, ...) (file=0xc151f8 "src/gdb/thread.c", line=165, fmt=0xc15180 "%s: Assertion `%s' failed.") at src/gdb/common/errors.c:55
     #6  0x00000000008a3d80 in inferior_thread() () at src/gdb/thread.c:165
     #7  0x0000000000456f91 in try_thread_db_load_1(thread_db_info*) (info=0x277eb00) at src/gdb/linux-thread-db.c:830
     #8  0x0000000000457554 in try_thread_db_load(char const*, int) (library=0xb01a4f "libthread_db.so.1", check_auto_load_safe=0)
         at src/gdb/linux-thread-db.c:1002
     #9  0x0000000000457861 in try_thread_db_load_from_sdir() () at src/gdb/linux-thread-db.c:1079
     #10 0x0000000000457b72 in thread_db_load_search() () at src/gdb/linux-thread-db.c:1134
     #11 0x0000000000457d29 in thread_db_load() () at src/gdb/linux-thread-db.c:1192
     #12 0x0000000000457e51 in check_for_thread_db() () at src/gdb/linux-thread-db.c:1244
     #13 0x0000000000457ed2 in thread_db_new_objfile(objfile*) (objfile=0x270ff60) at src/gdb/linux-thread-db.c:1273
     #14 0x000000000045a92e in std::_Function_handler<void (objfile*), void (*)(objfile*)>::_M_invoke(std::_Any_data const&, objfile*&&) (__functor=..., __args#0=@0x7ffef3efe140: 0x270ff60) at /usr/include/c++/7/bits/std_function.h:316
     #15 0x00000000007bbebf in std::function<void (objfile*)>::operator()(objfile*) const (this=0x24e1d18, __args#0=0x270ff60)
         at /usr/include/c++/7/bits/std_function.h:706
     #16 0x00000000007bba86 in gdb::observers::observable<objfile*>::notify(objfile*) const (this=0x117ce80 <gdb::observers::new_objfile>, args#0=0x270ff60) at src/gdb/common/observable.h:106
     #17 0x0000000000856000 in symbol_file_add_with_addrs(bfd*, char const*, symfile_add_flags, section_addr_info*, objfile_flags, objfile*) (abfd=0x1d7dae0, name=0x254bfc0 "/ho
    
    The problem is latent currently because inferior_thread() at that
    point manages to return a thread, even though it's the wrong one (of
    the old inferior).
    
    The problem originally fixed by bf93d7ba99 was:
    
        (...) we should avoid doing register reads
        after a process does an exec and before we've updated that inferior's
        gdbarch.  Otherwise, we may interpret the registers using the wrong
        architecture.
    
        (...) The call to "add_thread" done just after adding the inferior is
        problematic, because it ends up reading the registers (because the ptid
        is re-used, we end up doing a switch_to_thread to it, which tries to
        update stop_pc). (...)
    
    The register-reading issue is no longer a problem nowadays, ever since
    switch_to_thread stopped reading the stop_pc in git commit
    f2ffa92bbce9 ("gdb: Eliminate the 'stop_pc' global").
    
    So this commit basically reverts bf93d7ba99.
    
    gdb/ChangeLog:
    2018-11-22  Pedro Alves  <palves@redhat.com>
    
    	* infrun.c (follow_exec) <set follow-exec new>: Add thread and
    	switch to it before calling into try_open_exec_file.

Diff:
---
 gdb/ChangeLog | 5 +++++
 gdb/infrun.c  | 6 +-----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index a6738bd..4887e57 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2018-11-22  Pedro Alves  <palves@redhat.com>
 
+	* infrun.c (follow_exec) <set follow-exec new>: Add thread and
+	switch to it before calling into try_open_exec_file.
+
+2018-11-22  Pedro Alves  <palves@redhat.com>
+
 	* cli/cli-interp.c (cli_on_user_selected_context_changed): Use
 	inferior_thread instead of find_thread_ptid, and only when
 	inferior_ptid is not null_ptid.
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 3e9acb4..81f45be 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1202,6 +1202,7 @@ follow_exec (ptid_t ptid, char *exec_file_target)
 
       set_current_inferior (inf);
       set_current_program_space (inf->pspace);
+      add_thread (ptid);
     }
   else
     {
@@ -1231,11 +1232,6 @@ follow_exec (ptid_t ptid, char *exec_file_target)
      registers.  */
   target_find_description ();
 
-  /* The add_thread call ends up reading registers, so do it after updating the
-     target description.  */
-  if (follow_exec_mode_string == follow_exec_mode_new)
-    add_thread (ptid);
-
   solib_create_inferior_hook (0);
 
   jit_inferior_created_hook ();


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