From: Pedro Alves <palves@redhat.com>
To: Josh Stone <jistone@redhat.com>, gdb-patches@sourceware.org
Cc: philippe.waroquiers@skynet.be, sergiodj@redhat.com, eliz@gnu.org,
xdje42@gmail.com
Subject: Re: [PATCH v2 1/2] gdbserver: Set Linux ptrace options ASAP
Date: Wed, 02 Dec 2015 14:01:00 -0000 [thread overview]
Message-ID: <565EF9C4.7070609@redhat.com> (raw)
In-Reply-To: <565E0060.3060104@redhat.com>
On 12/01/2015 08:17 PM, Josh Stone wrote:
> OK, I think I've got it, and it's a real regression from 7.10 even for
> other events like fork. I'm not using --wrapper, so I'm not sure of the
> interaction there, but even gdbserver's simple fork+exec can show the
> problem. Basically, on the very first stop we don't set flags yet, so
> the first resume from there continues without the right flags.
>
> The sequence I was running into with syscalls goes like this:
>
> - start_inferior calls create_inferior to fork, then calls mywait
> - the forked process calls ptrace(PTRACE_TRACEME), then execs
> - linux_low_filter_event sees a raw SIGTRAP for the child after exec
> - (we haven't set PTRACE_O_TRACEEXEC yet, so SIGTRAP is expected)
> - arch setup is needed, so it hits the early return (new since 7.10)
> ... thus child->must_set_ptrace_flags is not dealt with
> - start_inferior calls target_arch_setup
> - GDB sends QCatchSyscalls:1
> - linux_resume_one_lwp_throw calls ptrace(PTRACE_SYSCALL)
> - but we still haven't set any flags, especially PTRACE_O_TRACESYSGOOD
> - linux_low_filter_event sees a raw SIGTRAP for the first syscall entry
> - now we finally deal with child->must_set_ptrace_flags
> - linux_resume_one_lwp_throw calls ptrace(PTRACE_SYSCALL)
> - linux_low_filter_event sees SYSCALL_SIGTRAP for the return
> - entry/return logic is confused now, thinks this is an entry
> - (but if there's any other event, entry/return will get back in sync)
>
>
> But this problem isn't particular to my syscall patches. Consider this
> simple forking program and use 'catch fork':
>
> #include <unistd.h>
> int main() { fork(); return 0; }
>
> Compiled normally, with dynamically-linked libc et al, you get:
> - SIGTRAP after exec, ignores child->must_set_ptrace_flags.
> - SIGTRAP for a swbreak, I guess some gdb setup, then it sets the
> necessary flags, especially PTRACE_O_TRACEFORK.
> - SIGTRAP for PTRACE_EVENT_FORK, hooray!
>
> But compiled statically:
> - SIGTRAP after exec, ignores child->must_set_ptrace_flags.
> - CLD_EXITED, flags were never set!
> - if I add a breakpoint on main, flags will be set when that's reached,
> and then we do get the PTRACE_EVENT_FORK after all.
Ouch. Thanks, I'm clear now. It'd be super if one of these examples
got converted to a test case.
>
> So, we need some point to get the right flags set before the program
> starts running for real. If you don't like the way I moved the flags
> before that arch-setup early return, then when should we do it?
>
> - Perhaps before the ptrace call in linux_resume_one_lwp_throw? Then if
> any state changes while the thread is still stopped, triggering new
> must_set_ptrace_flags, we'll deal with it before resuming. But I don't
> know if this would interact well with your wrapper concerns.
>
Yeah, badly.
> - Perhaps at the end of linux_arch_setup? AIUI this will be after
> everything you're worried about wrappers.
Something like that, yes. gdb/linux-nat.c also does something like
that:
static void
linux_child_post_startup_inferior (struct target_ops *self, ptid_t ptid)
{
linux_init_ptrace (ptid_get_pid (ptid), 0);
}
I think we should rename gdbserver's target_ops:target_arch_setup method/hook
to target_post_create_inferior along the way, and then linux-low.c's
implementation can both call linux_arch_setup and set the ptrace options.
Only Linux implements that target_ops method currently, so it should
be trivial.
Thanks,
Pedro Alves
next prev parent reply other threads:[~2015-12-02 14:01 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-30 11:02 [PATCH] Implement 'catch syscall' for gdbserver Josh Stone
2015-10-30 13:26 ` Eli Zaretskii
2015-11-01 22:15 ` Doug Evans
2015-11-02 18:24 ` Josh Stone
2015-11-21 10:29 ` Philippe Waroquiers
2015-11-23 4:20 ` Doug Evans
2015-11-23 4:20 ` Doug Evans
2015-11-25 2:37 ` Josh Stone
2015-11-26 2:53 ` [PATCH v2 1/2] gdbserver: Set Linux ptrace options ASAP Josh Stone
2015-11-26 2:54 ` [PATCH v2 2/2] Implement 'catch syscall' for gdbserver Josh Stone
2015-11-26 10:34 ` [PATCH v2 1/2] gdbserver: Set Linux ptrace options ASAP Pedro Alves
2015-11-30 18:50 ` Josh Stone
2015-12-01 20:17 ` Josh Stone
2015-12-02 14:01 ` Pedro Alves [this message]
2015-12-04 2:26 ` [PATCH v3 1/2] gdbserver: set ptrace flags after creating inferiors Josh Stone
2015-12-04 2:27 ` [PATCH v3 2/2] Implement 'catch syscall' for gdbserver Josh Stone
2015-12-04 8:45 ` Eli Zaretskii
2015-12-05 2:14 ` Josh Stone
2015-12-05 8:02 ` Eli Zaretskii
2015-12-07 16:50 ` Josh Stone
2015-12-07 17:15 ` Eli Zaretskii
2015-12-04 13:18 ` Pedro Alves
2015-12-05 2:16 ` Josh Stone
2015-12-08 13:31 ` Pedro Alves
2015-12-08 19:02 ` Josh Stone
2015-12-08 13:37 ` Pedro Alves
2015-12-11 21:19 ` Josh Stone
2015-12-16 15:42 ` Pedro Alves
2016-01-09 3:09 ` [PATCH v4] " Josh Stone
2016-01-09 7:37 ` Eli Zaretskii
2016-01-11 17:44 ` Philippe Waroquiers
2016-01-12 12:05 ` Pedro Alves
2016-01-12 19:10 ` Josh Stone
2016-01-12 19:22 ` Pedro Alves
2016-01-12 20:01 ` Josh Stone
2016-03-29 14:27 ` Yao Qi
2016-03-29 18:12 ` Josh Stone
2016-03-29 23:49 ` Josh Stone
2016-03-30 12:23 ` Yao Qi
2016-03-31 1:10 ` Josh Stone
2016-04-01 13:05 ` Yao Qi
2016-04-01 16:38 ` Josh Stone
2016-05-29 16:47 ` [doc] NEWS: QCatchSyscalls: simplify Jan Kratochvil
2016-05-29 17:29 ` Eli Zaretskii
2016-05-29 17:50 ` Jan Kratochvil
2016-05-29 18:19 ` Eli Zaretskii
2016-05-29 18:47 ` [commit] " Jan Kratochvil
2015-12-04 12:16 ` [PATCH v3 1/2] gdbserver: set ptrace flags after creating inferiors Pedro Alves
2015-12-05 2:14 ` Josh Stone
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