GNU bug report logs - #21694
'clone' syscall binding unreliable

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: ludo <at> gnu.org (Ludovic Courtès)
Cc: tracker <at> debbugs.gnu.org
Subject: bug#21694: closed ('clone' syscall binding unreliable)
Date: Wed, 28 Oct 2015 14:40:03 +0000

[Message part 1 (text/plain, inline)]

Your message dated Wed, 28 Oct 2015 15:39:49 +0100
with message-id <87vb9r83be.fsf <at> gnu.org>
and subject line Re: bug#21694: 'clone' syscall binding unreliable
has caused the debbugs.gnu.org bug report #21694,
regarding 'clone' syscall binding unreliable
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
21694: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=21694
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Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: ludo <at> gnu.org (Ludovic Courtès)
To: David Thompson <davet <at> gnu.org>
Cc: bug-guix <at> gnu.org
Subject: 'clone' syscall binding unreliable
Date: Fri, 16 Oct 2015 22:39:59 +0200

[Message part 3 (text/plain, inline)]

I’m reporting the problem and (hopefully) the solution, but I think we’d
better double-check this.

The problem: Running the test below in a loop sometimes gets a SIGSEGV
in the child process (on x86_64, libc 2.22.)

--8<---------------cut here---------------start------------->8---
(use-modules (guix build syscalls) (ice-9 match))

(match (clone (logior CLONE_NEWUSER
                      CLONE_CHILD_SETTID
                      CLONE_CHILD_CLEARTID
                      SIGCHLD))
  (0
   (throw 'x))                                    ;XXX: sometimes segfaults
  (pid
   (match (waitpid pid)
     ((_ . status)
      (pk 'status status)
      (exit (not (status:term-sig status)))))))
--8<---------------cut here---------------end--------------->8---

Looking at (guix build syscalls) though, I see an ABI mismatch between
our definition and the actual ‘syscall’ C function, and between our
‘clone’ definition and the actual C function.

This leads to the attached patch, which also fixes the above problem for me.

[Message part 4 (text/x-patch, inline)]

diff --git a/guix/build/syscalls.scm b/guix/build/syscalls.scm
index 80b9d00..f931f8d 100644
--- a/guix/build/syscalls.scm
+++ b/guix/build/syscalls.scm
@@ -322,10 +322,16 @@ string TMPL and return its file name.  TMPL must end with 'XXXXXX'."
 (define CLONE_NEWNET         #x40000000)
 
 ;; The libc interface to sys_clone is not useful for Scheme programs, so the
-;; low-level system call is wrapped instead.
+;; low-level system call is wrapped instead.  The 'syscall' function is
+;; declared in <unistd.h> as a variadic function; in practice, it expects 6
+;; pointer-sized arguments, as shown in, e.g., x86_64/syscall.S.
 (define clone
   (let* ((ptr        (dynamic-func "syscall" (dynamic-link)))
-         (proc       (pointer->procedure int ptr (list int int '*)))
+         (proc       (pointer->procedure long ptr
+                                         (list long                   ;sysno
+                                               unsigned-long          ;flags
+                                               '* '* '*
+                                               '*)))
          ;; TODO: Don't do this.
          (syscall-id (match (utsname:machine (uname))
                        ("i686"   120)
@@ -336,7 +342,10 @@ string TMPL and return its file name.  TMPL must end with 'XXXXXX'."
       "Create a new child process by duplicating the current parent process.
 Unlike the fork system call, clone accepts FLAGS that specify which resources
 are shared between the parent and child processes."
-      (let ((ret (proc syscall-id flags %null-pointer))
+      (let ((ret (proc syscall-id flags
+                       %null-pointer               ;child stack
+                       %null-pointer %null-pointer ;ptid & ctid
+                       %null-pointer))             ;unused
             (err (errno)))
         (if (= ret -1)
             (throw 'system-error "clone" "~d: ~A"

[Message part 5 (text/plain, inline)]

Could you test this patch?

Now, there remains the question of CLONE_CHILD_SETTID and
CLONE_CHILD_CLEARTID.  Since we’re passing NULL for ‘ctid’, I expect
that these flags have no effect at all.

Conversely, libc uses these flags to update the thread ID in the child
process (x86_64/arch-fork.h):

--8<---------------cut here---------------start------------->8---
#define ARCH_FORK() \
  INLINE_SYSCALL (clone, 4,                                                   \
                  CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD, 0,     \
                  NULL, &THREAD_SELF->tid)
--8<---------------cut here---------------end--------------->8---

This is certainly useful, but we’d have troubles doing it from the FFI…
It may that this is fine if the process doesn’t use threads.

Ludo’.

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From: ludo <at> gnu.org (Ludovic Courtès)
To: "Thompson\, David" <dthompson2 <at> worcester.edu>
Cc: 21694-done <at> debbugs.gnu.org, David Thompson <davet <at> gnu.org>
Subject: Re: bug#21694: 'clone' syscall binding unreliable
Date: Wed, 28 Oct 2015 15:39:49 +0100

ludo <at> gnu.org (Ludovic Courtès) skribis:

> "Thompson, David" <dthompson2 <at> worcester.edu> skribis:
>
>> On Fri, Oct 16, 2015 at 4:39 PM, Ludovic Courtès <ludo <at> gnu.org> wrote:

[...]

>>> Now, there remains the question of CLONE_CHILD_SETTID and
>>> CLONE_CHILD_CLEARTID.  Since we’re passing NULL for ‘ctid’, I expect
>>> that these flags have no effect at all.
>>
>> I added those flags in commit ee78d02 because they solved a real issue
>> I ran into.  Adding those flags made 'clone' look like a
>> 'primitive-fork' call when examined with strace.
>
> Could you check whether removing these flags makes a difference now?

I removed them in commit after confirming that it affects neither the
test suite nor ‘guix system environment’ (on x86_64, with Linux-libre
4.2.3-gnu.)

Thanks,
Ludo’.

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