Re: [Qemu-devel] [PATCH] Fix vfork() syscall emulation

[andrzej zaborowski]

2008/9/20 Kirill A. Shutemov <address@hidden>:
> On Sat, Sep 20, 2008 at 03:52:55PM +0200, andrzej zaborowski wrote:
>> 2008/9/20 Kirill A. Shutemov <address@hidden>:
>> > So, implementation vfork() using pthread is wrong.
>>
>> Agreed, but implementation of vfork() using fork() is wrong, too.
>
> Why?
>
> man 2 vfork():
>
> BUGS
>       It  is  rather  unfortunate  that Linux revived this specter from the
>       past.  The BSD man page states: "This system call will be  eliminated
>       when  proper system sharing mechanisms are implemented.  Users should
>       not depend on the memory sharing semantics of vfork() as it will,  in
>       that case, be made synonymous to fork(2)."
>
> If any program doesn't work with vfork() implemented using fork(). it's
> program bug.
>
>
>> If
>> we allow a hack, it should be commented, the second thing that needs
>> to be commented is why the value of CLONE_VM flag is ignored if
>> CLONE_VFORK is set -- on Linux it's not ignored.
>
> vfork() is a hack itself. It was introduced when fork() was very expensive.

Ok, perhaps I'm nit picking.  clone(2) specifies some semantics for
CLONE_VFORK regardless of the purpose and this implementation is
nowhere near these semantics.  I'll just add the same comment that
klibc has and push the patch.

Cheers