Debugging through exec() (Linux MAY_FOLLOW_EXEC)
Jan Kratochvil
jan.kratochvil@redhat.com
Mon Aug 14 15:07:00 GMT 2006
Hi,
please review the proposed functionality to be able to debug though exec().
In fact I am not aware of any change until you type "catch exec".
Daniel Jacobowitz has objections to the correctness of the patch.
Some demo of the attached patch functionality:
(gdb) tcatch exec
Catchpoint 1 (exec)
(gdb) run
Starting program: /tmp/execve
Executing new program: /tmp/hello
[Switching to process 1588]
### Notice here the symbol is no longer resolved as gdb still did not detect
### right after exec() that it has shared library loaded.
0x44031840 in ?? ()
(gdb) tbreak main
Breakpoint 2 at 0x8048395: file hello.c, line 6.
(gdb) continue
Continuing.
[Switching to process 1588]
main () at hello.c:6
6 puts("hello world");
(gdb) continue
Continuing.
hello world
Program exited normally.
(gdb) run
### New messages notifying the loaded executable changed.
Restoring the program name before exec().
Restoring the symbol table before exec().
Starting program: /tmp/execve
Executing new program: /tmp/hello
hello world
Program exited normally.
(gdb)
Original behavior was:
(gdb) tcatch exec
Catchpoint 1 (exec)
(gdb) run
Starting program: /tmp/execve
### Symbol is resolved here but the shared library is in fact no longer valid.
0x44031840 in _start () from /lib/ld-linux.so.2
(gdb) tbreak main
### Break address/sourcefile is stale here and thus disfunctional.
Breakpoint 2 at 0x80483b4: file execve.c, line 7.
(gdb) continue
Continuing.
hello world
Program exited normally.
(gdb)
Please read the mails from Daniel Jacobowitz below:
On Tue, 08 Aug 2006 18:01:13 +0200, Daniel Jacobowitz wrote:
> On Sat, Aug 05, 2006 at 06:41:44PM +0200, Jan Kratochvil wrote:
> > Hi Mark,
> >
> > On Mon, 31 Jul 2006 22:38:43 +0200, Mark Kettenis wrote:
> > ...
> > > That WNOHANG is wrong;
> >
> > In fact yes, the patch is more correct without that WNOHANG hack there.
> >
> >
> > 2006-07-29 Jan Kratochvil <jan.kratochvil@redhat.com>
> >
> > * inf-ptrace.c (inf_ptrace_mourn_inferior): waitpid(2) only if there
> > is valid inferior_ptid to wait for.
> > * linux-fork.c (linux_fork_mourn_inferior): Ditto.
> > * infrun.c (follow_exec): Unconditionally enabled by MAY_FOLLOW_EXEC.
> > Provide restoration of exec_bfd and symfile_objfile for any new "run".
> > * linux-thread-db.c (thread_db_wait): Handle TARGET_WAITKIND_EXECD.
> > * linux-thread-db.c (thread_db_mourn_inferior): Turn off threading.
> > * foll-exec.exp: Uncoditionally enabled for all platforms.
> > Relaxed regex to apply besides HP-UX also for GNU/Linux backtrace.
>
> Sorry, but I can't approve this patch. I think someone needs to
> discuss the concept and interface on gdb@ first.
>
> You're using make_run_cleanup to restore the inferior file. This means
> it happens before "run". Any time we choose to change the file
> silently will be surprising (especially to front ends like Eclipse) but
> that time will be pretty surprising to the user too:
>
> ... info files shows first prog
> (gdb) catch exec
> (gdb) run
> ... info files shows second prog
> (gdb) continue
> ... exits.
> ... info files shows second prog
> (gdb) run
> ... starts first prog!
>
> I also think that changing inferior_ptid before mourning is pretty
> strange. The whole way follow_exec works is a hack and not
> particularly well defined.
>
> This is a hard problem to solve; I think a half-finished solution would
> be worse than leaving it unsolved.
>
> --
> Daniel Jacobowitz
> CodeSourcery
Daniel's earlier mail:
On Mon, 24 Jul 2006 21:03:32 +0200, Daniel Jacobowitz wrote:
> Jan Kratochvil wrote:
> > On Fri, 21 Jul 2006 20:44:21 +0200, Daniel Jacobowitz wrote:
> > ...
> > > Turning on MAY_FOLLOW_EXEC is not a good idea. No one really knows how
> > > that behavior works, a lot of it doesn't, and the way it implicitly
> > > changes the symbol file is very disorienting. Please don't mix it up
> > > with the fix for your current bug.
> >
> > Still I am for MAY_FOLLOW_EXEC as it improves the user experience and makes
> > debugging of exec()ing processes much more convenient - without having to find
> > out how each child gets executed and replay such conditions by hand.
> >
> > As gdb-6.5 has been released and the MAY_FOLLOW_EXEC feature IMO generally
> > works for GNU/Linux - isn't appropriate to enable it and settle it down?
> > I would even like to fix any issues possibly roaring its head.
>
> Does anyone else have an opinion on this? I'm starting to think you're
> right - we should turn it on, invite people to use it, and see what
> happens.
>
> The reason I find it so disorienting is this:
[ silent changing of the loaded exec + symbol file ]
Thanks,
Jan Kratochvil
-------------- next part --------------
Index: gdb/inf-ptrace.c
===================================================================
RCS file: /cvs/src/src/gdb/inf-ptrace.c,v
retrieving revision 1.32
diff -u -p -r1.32 inf-ptrace.c
--- gdb/inf-ptrace.c 24 Jan 2006 22:34:34 -0000 1.32
+++ gdb/inf-ptrace.c 14 Aug 2006 15:04:53 -0000
@@ -167,7 +167,8 @@ inf_ptrace_mourn_inferior (void)
Do not check whether this succeeds though, since we may be
dealing with a process that we attached to. Such a process will
only report its exit status to its original parent. */
- waitpid (ptid_get_pid (inferior_ptid), &status, 0);
+ if (! ptid_equal (inferior_ptid, null_ptid) && target_has_execution)
+ waitpid (ptid_get_pid (inferior_ptid), &status, 0);
unpush_target (ptrace_ops_hack);
generic_mourn_inferior ();
Index: gdb/infrun.c
===================================================================
RCS file: /cvs/src/src/gdb/infrun.c,v
retrieving revision 1.213
diff -u -p -r1.213 infrun.c
--- gdb/infrun.c 22 Jul 2006 14:48:03 -0000 1.213
+++ gdb/infrun.c 14 Aug 2006 15:04:56 -0000
@@ -47,6 +47,7 @@
#include "language.h"
#include "solib.h"
#include "main.h"
+#include "objfiles.h"
#include "gdb_assert.h"
#include "mi/mi-common.h"
@@ -109,10 +110,10 @@ int sync_execution = 0;
static ptid_t previous_inferior_ptid;
/* This is true for configurations that may follow through execl() and
- similar functions. At present this is only true for HP-UX native. */
+ similar functions. */
#ifndef MAY_FOLLOW_EXEC
-#define MAY_FOLLOW_EXEC (0)
+#define MAY_FOLLOW_EXEC (1)
#endif
static int may_follow_exec = MAY_FOLLOW_EXEC;
@@ -375,6 +376,45 @@ follow_inferior_reset_breakpoints (void)
insert_breakpoints ();
}
+static void
+follow_exec_restore_execfile (void *filename_new_untyped)
+{
+ char *filename_new = filename_new_untyped;
+
+ /* filename_new == NULL is not expected. */
+ if (filename_new == NULL && exec_bfd != NULL)
+ exec_file_clear (0);
+ /* exec_bfd == NULL is not expected. */
+ if (filename_new != NULL &&
+ (exec_bfd == NULL || strcmp (get_exec_file (0), filename_new)))
+ {
+ /* The filename will be printed already below: Starting program: %s */
+ printf_unfiltered (_("Restoring the program name before exec().\n"));
+ exec_file_attach (filename_new, 0);
+ }
+
+ free (filename_new);
+}
+
+static void
+follow_exec_restore_symfile (void *filename_new_untyped)
+{
+ char *filename_new = filename_new_untyped;
+
+ /* symfile_objfile == NULL is not expected. */
+ if (filename_new == NULL && symfile_objfile != NULL)
+ symbol_file_clear (0);
+ if (filename_new != NULL && (symfile_objfile == NULL
+ || strcmp (symfile_objfile->name, filename_new)))
+ {
+ /* The filename will be printed already below: Starting program: %s */
+ printf_unfiltered (_("Restoring the symbol table before exec().\n"));
+ symbol_file_add_main (filename_new, 0);
+ }
+
+ free (filename_new);
+}
+
/* EXECD_PATHNAME is assumed to be non-NULL. */
static void
@@ -382,6 +422,7 @@ follow_exec (int pid, char *execd_pathna
{
int saved_pid = pid;
struct target_ops *tgt;
+ struct objfile *objfile, *objfile_temp;
if (!may_follow_exec)
return;
@@ -427,6 +468,32 @@ follow_exec (int pid, char *execd_pathna
error (_("Could find run target to save before following exec"));
gdb_flush (gdb_stdout);
+
+ /* During the common "run" bare command we should run again the original
+ * program spawning us. Stacking order is correct this way. */
+ make_run_cleanup (follow_exec_restore_symfile, (!symfile_objfile ? NULL :
+ xstrdup (symfile_objfile->name)));
+ make_run_cleanup (follow_exec_restore_execfile,
+ (!exec_bfd ? NULL : xstrdup (exec_bfd->filename)));
+
+ /* As symbol_file_add_main()->thread_db_new_objfile()->check_for_thread_db()
+ * would fine already loaded libpthread.so while the threading structures
+ * would not be yet initialized for this early inferior.
+ * Call before target_mourn_inferior() as it will breakpoint_re_set(). */
+#ifdef CLEAR_SOLIB
+ CLEAR_SOLIB ();
+#else
+ clear_solib ();
+#endif
+ /* Do not: symbol_file_clear()->clear_symtab_users()->breakpoint_re_set(). */
+ ALL_OBJFILES_SAFE (objfile, objfile_temp)
+ {
+ free_objfile (objfile);
+ }
+ symfile_objfile = NULL;
+
+ /* Avoid stucked waitpid(2) as PID inferior_ptid is still running. */
+ inferior_ptid = null_ptid;
target_mourn_inferior ();
inferior_ptid = pid_to_ptid (saved_pid);
/* Because mourn_inferior resets inferior_ptid. */
Index: gdb/linux-fork.c
===================================================================
RCS file: /cvs/src/src/gdb/linux-fork.c,v
retrieving revision 1.7
diff -u -p -r1.7 linux-fork.c
--- gdb/linux-fork.c 27 Apr 2006 23:03:41 -0000 1.7
+++ gdb/linux-fork.c 14 Aug 2006 15:04:56 -0000
@@ -366,7 +366,8 @@ linux_fork_mourn_inferior (void)
only report its exit status to its original parent. */
int status;
- waitpid (ptid_get_pid (inferior_ptid), &status, 0);
+ if (! ptid_equal (inferior_ptid, null_ptid) && target_has_execution)
+ waitpid (ptid_get_pid (inferior_ptid), &status, 0);
/* OK, presumably inferior_ptid is the one who has exited.
We need to delete that one from the fork_list, and switch
Index: gdb/linux-thread-db.c
===================================================================
RCS file: /cvs/src/src/gdb/linux-thread-db.c,v
retrieving revision 1.19
diff -u -p -r1.19 linux-thread-db.c
--- gdb/linux-thread-db.c 2 Aug 2006 10:24:00 -0000 1.19
+++ gdb/linux-thread-db.c 14 Aug 2006 15:04:57 -0000
@@ -884,6 +884,10 @@ thread_db_wait (ptid_t ptid, struct targ
return pid_to_ptid (GET_PID (ptid));
}
+ /* Threading structures got reset. Return as nonthreaded. */
+ if (ourstatus->kind == TARGET_WAITKIND_EXECD)
+ return pid_to_ptid (GET_PID (ptid));
+
if (ourstatus->kind == TARGET_WAITKIND_STOPPED
&& ourstatus->value.sig == TARGET_SIGNAL_TRAP)
/* Check for a thread event. */
@@ -975,6 +979,9 @@ thread_db_mourn_inferior (void)
the inferior, so that we don't try to uninsert them. */
remove_thread_event_breakpoints ();
+ /* Destroy thread info; it's no longer valid. */
+ init_thread_list ();
+
/* Detach thread_db target ops. */
unpush_target (&thread_db_ops);
using_thread_db = 0;
Index: gdb/testsuite/gdb.base/foll-exec.exp
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.base/foll-exec.exp,v
retrieving revision 1.3
diff -u -p -r1.3 foll-exec.exp
--- gdb/testsuite/gdb.base/foll-exec.exp 10 Aug 2006 05:27:20 -0000 1.3
+++ gdb/testsuite/gdb.base/foll-exec.exp 14 Aug 2006 15:04:58 -0000
@@ -47,12 +47,6 @@ if { [gdb_compile "${srcdir}/${subdir}/
}
-# Until "catch exec" is implemented on other targets...
-#
-if ![istarget "hppa*-hp-hpux*"] then {
- continue
-}
-
proc zap_session {} {
global gdb_prompt
global binfile
@@ -214,7 +208,9 @@ proc do_exec_tests {} {
setup_xfail hppa2.0w-hp-hpux* CLLbs16760
send_gdb "continue\n"
gdb_expect {
- -re ".*Executing new program:.*${testfile2}.*Catchpoint .*(exec\'d .*${testfile2}).*in .START..*$gdb_prompt $"\
+ # It is OS dependent and no symbols may be found, GNU/Linux has "_start"
+ # while HP-UX has " in .START..*$gdb_prompt" etc.
+ -re ".*Executing new program:.*${testfile2}.*Catchpoint .*(exec\'d .*${testfile2}).*in .*$gdb_prompt $"\
{pass "hit catch exec"}
-re "$gdb_prompt $" {fail "hit catch exec"}
timeout {fail "(timeout) hit catch exec"}
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