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.
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"
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’.
