gdb: call post_create_inferior at end of follow_fork_inferior
GDB doesn't handle well the case of an inferior using the JIT interface
to register JIT-ed objfiles and forking. If an inferior registers a
code object using the JIT interface and then forks, the child process
conceptually has the same code object loaded, so GDB should look it up
and learn about it (it currently doesn't).
To achieve this, I think it would make sense to have the
inferior_created observable called when an inferior is created due to a
fork in follow_fork_inferior. The inferior_created observable is
currently called both after starting a new inferior and after attaching
to an inferior, allowing various sub-components to learn about that new
executing inferior. We can see handling a fork child just like
attaching to it, so any work done when attaching should also be done in
the case of a fork child.
Instead of just calling the inferior_created observable, this patch
makes follow_fork_inferior call the whole post_create_inferior function.
This way, the attach and follow-fork code code paths are more alike.
Given that post_create_inferior calls solib_create_inferior_hook,
follow_fork_inferior doesn't need to do it itself, so those calls to
solib_create_inferior_hook are removed.
One question you may have: why not just call post_create_inferior at the
places where solib_create_inferior_hook is currently called, instead of
after target_follow_fork?
- there's something fishy for the second solib_create_inferior_hook
call site: at this point we have switched the current program space
to the child's, but not the current inferior nor the current thread.
So solib_create_inferior_hook (and everything under, including
check_for_thread_db, for example) is called with inferior 1 as the
current inferior and inferior 2's program space as the current
program space. I think that's wrong, because at this point we are
setting up inferior 2, and all that code relies on the current
inferior. We could just add a switch_to_thread call before it to
make inferior 2 the current one, but there are other problems (see
below).
- solib_create_inferior_hook is currently not called on the
`follow_child && detach_fork` path. I think we need to call it,
because we still get a new inferior in that case (even though we
detach the parent). If we only call post_create_inferior where
solib_create_inferior_hook used to be called, then the JIT
subcomponent doesn't get informed about the new inferior, and that
introduces a failure in the new gdb.base/jit-elf-fork.exp test.
- if we try to put the post_create_inferior just after the
switch_to_thread that was originally at line 662, or just before the
call to target_follow_fork, we introduce a subtle failure in
gdb.threads/fork-thread-pending.exp. What happens then is that
libthread_db gets loaded (somewhere under post_create_inferior)
before the linux-nat target learns about the LWPs (which happens in
linux_nat_target::follow_fork). As a result, the ALL_LWPS loop in
try_thread_db_load_1 doesn't see the child LWP, and the thread-db
target doesn't have the chance to fill in thread_info::priv. A bit
later, when the test does "info threads", and
thread_db_target::pid_to_str is called, the thread-db target doesn't
recognize the thread as one of its own, and delegates the request to
the target below. Because the pid_to_str output is not the expected
one, the test fails.
This tells me that we need to call the process target's follow_fork
first, to make the process target create the necessary LWP and thread
structures. Then, we can call post_create_inferior to let the other
components of GDB do their thing.
But then you may ask: check_for_thread_db is already called today,
somewhere under solib_create_inferior_hook, and that is before
target_follow_fork, why don't we see this ordering problem!? Well,
because of the first bullet point: when check_for_thread_db /
thread_db_load are called, the current inferior is (erroneously)
inferior 1, the parent. Because libthread_db is already loaded for
the parent, thread_db_load early returns. check_for_thread_db later
gets called by linux_nat_target::follow_fork. At this point, the
current inferior is the correct one and the child's LWP exists, so
all is well.
Since we now call post_create_inferior after target_follow_fork, which
calls the inferior_created observable, which calls check_for_thread_db,
I don't think linux_nat_target needs to explicitly call
check_for_thread_db itself, so that is removed.
In terms of testing, this patch adds a new gdb.base/jit-elf-fork.exp
test. It makes an inferior register a JIT code object and then fork.
It then verifies that whatever the detach-on-fork and follow-fork-child
parameters are, GDB knows about the JIT code object in all the inferiors
that survive the fork. It verifies that the inferiors can unload that
code object.
There isn't currently a way to get visibility into GDB's idea of the JIT
code objects for each inferior. For the purpose of this test, add the
"maintenance info jit" command. There isn't much we can print about the
JIT code objects except their load address. So the output looks a bit
bare, but it's good enough for the test.
gdb/ChangeLog:
* NEWS: Mention "maint info jit" command.
* infrun.c (follow_fork_inferior): Don't call
solib_create_inferior_hook, call post_create_inferior if a new
inferior was created.
* jit.c (maint_info_jit_cmd): New.
(_initialize_jit): Register new command.
* linux-nat.c (linux_nat_target::follow_fork): Don't call
check_for_thread_db.
* linux-nat.h (check_for_thread_db): Remove declaration.
* linux-thread-db.c (check_thread_signals): Make static.
gdb/doc/ChangeLog:
* gdb.texinfo (Maintenance Commands): Mention "maint info jit".
gdb/testsuite/ChangeLog:
* gdb.base/jit-elf-fork-main.c: New test.
* gdb.base/jit-elf-fork-solib.c: New test.
* gdb.base/jit-elf-fork.exp: New test.
Change-Id: I9a192e55b8a451c00e88100669283fc9ca60de5c
2021-04-07 02:31:50 +08:00
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/* This test program is part of GDB, the GNU debugger.
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2022-01-01 22:56:03 +08:00
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Copyright 2011-2022 Free Software Foundation, Inc.
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gdb: call post_create_inferior at end of follow_fork_inferior
GDB doesn't handle well the case of an inferior using the JIT interface
to register JIT-ed objfiles and forking. If an inferior registers a
code object using the JIT interface and then forks, the child process
conceptually has the same code object loaded, so GDB should look it up
and learn about it (it currently doesn't).
To achieve this, I think it would make sense to have the
inferior_created observable called when an inferior is created due to a
fork in follow_fork_inferior. The inferior_created observable is
currently called both after starting a new inferior and after attaching
to an inferior, allowing various sub-components to learn about that new
executing inferior. We can see handling a fork child just like
attaching to it, so any work done when attaching should also be done in
the case of a fork child.
Instead of just calling the inferior_created observable, this patch
makes follow_fork_inferior call the whole post_create_inferior function.
This way, the attach and follow-fork code code paths are more alike.
Given that post_create_inferior calls solib_create_inferior_hook,
follow_fork_inferior doesn't need to do it itself, so those calls to
solib_create_inferior_hook are removed.
One question you may have: why not just call post_create_inferior at the
places where solib_create_inferior_hook is currently called, instead of
after target_follow_fork?
- there's something fishy for the second solib_create_inferior_hook
call site: at this point we have switched the current program space
to the child's, but not the current inferior nor the current thread.
So solib_create_inferior_hook (and everything under, including
check_for_thread_db, for example) is called with inferior 1 as the
current inferior and inferior 2's program space as the current
program space. I think that's wrong, because at this point we are
setting up inferior 2, and all that code relies on the current
inferior. We could just add a switch_to_thread call before it to
make inferior 2 the current one, but there are other problems (see
below).
- solib_create_inferior_hook is currently not called on the
`follow_child && detach_fork` path. I think we need to call it,
because we still get a new inferior in that case (even though we
detach the parent). If we only call post_create_inferior where
solib_create_inferior_hook used to be called, then the JIT
subcomponent doesn't get informed about the new inferior, and that
introduces a failure in the new gdb.base/jit-elf-fork.exp test.
- if we try to put the post_create_inferior just after the
switch_to_thread that was originally at line 662, or just before the
call to target_follow_fork, we introduce a subtle failure in
gdb.threads/fork-thread-pending.exp. What happens then is that
libthread_db gets loaded (somewhere under post_create_inferior)
before the linux-nat target learns about the LWPs (which happens in
linux_nat_target::follow_fork). As a result, the ALL_LWPS loop in
try_thread_db_load_1 doesn't see the child LWP, and the thread-db
target doesn't have the chance to fill in thread_info::priv. A bit
later, when the test does "info threads", and
thread_db_target::pid_to_str is called, the thread-db target doesn't
recognize the thread as one of its own, and delegates the request to
the target below. Because the pid_to_str output is not the expected
one, the test fails.
This tells me that we need to call the process target's follow_fork
first, to make the process target create the necessary LWP and thread
structures. Then, we can call post_create_inferior to let the other
components of GDB do their thing.
But then you may ask: check_for_thread_db is already called today,
somewhere under solib_create_inferior_hook, and that is before
target_follow_fork, why don't we see this ordering problem!? Well,
because of the first bullet point: when check_for_thread_db /
thread_db_load are called, the current inferior is (erroneously)
inferior 1, the parent. Because libthread_db is already loaded for
the parent, thread_db_load early returns. check_for_thread_db later
gets called by linux_nat_target::follow_fork. At this point, the
current inferior is the correct one and the child's LWP exists, so
all is well.
Since we now call post_create_inferior after target_follow_fork, which
calls the inferior_created observable, which calls check_for_thread_db,
I don't think linux_nat_target needs to explicitly call
check_for_thread_db itself, so that is removed.
In terms of testing, this patch adds a new gdb.base/jit-elf-fork.exp
test. It makes an inferior register a JIT code object and then fork.
It then verifies that whatever the detach-on-fork and follow-fork-child
parameters are, GDB knows about the JIT code object in all the inferiors
that survive the fork. It verifies that the inferiors can unload that
code object.
There isn't currently a way to get visibility into GDB's idea of the JIT
code objects for each inferior. For the purpose of this test, add the
"maintenance info jit" command. There isn't much we can print about the
JIT code objects except their load address. So the output looks a bit
bare, but it's good enough for the test.
gdb/ChangeLog:
* NEWS: Mention "maint info jit" command.
* infrun.c (follow_fork_inferior): Don't call
solib_create_inferior_hook, call post_create_inferior if a new
inferior was created.
* jit.c (maint_info_jit_cmd): New.
(_initialize_jit): Register new command.
* linux-nat.c (linux_nat_target::follow_fork): Don't call
check_for_thread_db.
* linux-nat.h (check_for_thread_db): Remove declaration.
* linux-thread-db.c (check_thread_signals): Make static.
gdb/doc/ChangeLog:
* gdb.texinfo (Maintenance Commands): Mention "maint info jit".
gdb/testsuite/ChangeLog:
* gdb.base/jit-elf-fork-main.c: New test.
* gdb.base/jit-elf-fork-solib.c: New test.
* gdb.base/jit-elf-fork.exp: New test.
Change-Id: I9a192e55b8a451c00e88100669283fc9ca60de5c
2021-04-07 02:31:50 +08:00
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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/* This simulates a JIT library. The function name is supplied during
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compilation as a macro. */
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#ifndef FUNCTION_NAME
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#error "Must define the FUNCTION_NAME macro to set a jited function name"
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#endif
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int FUNCTION_NAME() { return 42; }
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