[uml-devel] should there be os_clone() analogous to os_getpid() ?

[John Reiser <jreiser@BitWagon.com>]

In source file arch/um/os-Linux/process.c there is a warning:
-----
/* Don't use the glibc version, which caches the result in TLS. It misses some
 * syscalls, and also breaks with clone(), which does not unshare the TLS.
 */

int os_getpid(void)
-----

I see no os_clone(), yet the glibc clone() does the same caching of pid in
ThreadLocalStorage [TLS], and the TLS still may be shared.  If nobody reads
glibc's shared TLS slot for PID then an actual bug will be avoided.  However,
it is unsafe to leave such a tempting pitfall.  Also, if you are ptrace()ing
through a glibc clone(), then in many cases you will see syscall(__NR_getpid)
*from glibc* immediately following!  There is an "extra" getpid()
that the tracking logic might not expect.  So it seems to me that
there should be an os_clone() that refrains from fiddling with getpid.
[Unfortunately os_clone() is not so simple as os_getpid().]


The clone() we're talking about here is _not_ the bare syscall:
-----
	_syscall5(int, clone, int, flags, void *, child_stack,
            int *, parent_tidptr, struct user_desc *, newtls,
            int *, child_tidptr)
-----
but rather the C-language interface:
-----
       int clone(int (*fn)(void *), void *child_stack,
                 int flags, void *arg, ...
                 /* pid_t *pid, struct user_desc *tls, pid_t *ctid */ );
-----
which is declared in arch/um/include/kern.h and referenced in
   arch/um/drivers/ubd_user.c
   arch/um/kernel/tt/tracer.c
   arch/um/os/tt.c
   arch/um/os/start_up.c
   arch/um/os/skas/process.c
This clone() is implemented by glibc, and at runtime lives in the shared
library /lib/libc.so.6.

Not only that, but some versions of glibc for x86 use "int $0x80" directly
only for the __NR_clone call.  They use "ENTER_KERNEL" for the getpid(),
which in some cases (such as Fedora 7 and 8, but not Ubuntu 7.04) expands to
"call *%gs:nnnnn" which points at "sysenter; ret".

-- 
John Reiser, jreiser@BitWagon.com

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