[PATCH 21/46] vfork: Use clone if arch does not have the vfork syscall

[Rich Felker]

On Tue, Nov 13, 2012 at 11:31:30AM +0000, Markos Chandras wrote:
> From: Markos Chandras <markos.chandras at imgtec.com>
> 
> Signed-off-by: Markos Chandras <markos.chandras at imgtec.com>
> ---
>  libc/sysdeps/linux/common/vfork.c |   20 +++++++++++++++++++-
>  1 files changed, 19 insertions(+), 1 deletions(-)
> 
> diff --git a/libc/sysdeps/linux/common/vfork.c b/libc/sysdeps/linux/common/vfork.c
> index e7c9208..16f5f79 100644
> --- a/libc/sysdeps/linux/common/vfork.c
> +++ b/libc/sysdeps/linux/common/vfork.c
> @@ -4,13 +4,31 @@
>   * Licensed under the LGPL v2.1, see the file COPYING.LIB in this tarball.
>   */
>  
> +#include <signal.h>
>  #include <unistd.h>
>  #include <sys/types.h>
>  #include <sys/syscall.h>
>  
>  extern __typeof(vfork) __vfork attribute_hidden;
>  
> -#ifdef __NR_vfork
> +#if defined(__NR_clone) && ! defined(__NR_vfork)
> +pid_t __vfork(void)
> +{
> +	pid_t pid;
> +	pid = INLINE_SYSCALL(clone, 4, CLONE_VFORK | CLONE_VM |
> +		SIGCHLD, NULL, NULL, NULL);
> +
> +	if (pid<0) {
> +		__set_errno(-pid);
> +		return -1
> +	}
> +
> +	return pid;
> +}
> +weak_alias(__vfork,vfork)
> +libc_hidden_weak(vfork)

This is not valid. clone will return twice, both in the parent and the
child, but the child will already have clobbered the return address on
the parent's stack due to sharing memory. Whether you use the clone
syscall or the vfork syscall, the only way to implement vfork is from
asm. If you want a generic C fallback, it should just behave as fork
rather than as vfork.

Rich