Re: [Qemu-devel] [PULL 4/5] usb-mtp: Introduce write support for MTP objects

[Eric Blake <eblake@redhat.com>]

On 02/20/2018 09:28 AM, Gerd Hoffmann wrote:
> From: Bandan Das <bsd@redhat.com>
> 
> Allow write operations on behalf of the initiator. The
> precursor to write is the sending of the write metadata
> that consists of the ObjectInfo dataset. This patch introduces
> a flag that is set when the responder is ready to receive
> write data based on a previous SendObjectInfo operation by
> the initiator (The SendObjectInfo implementation is in a
> later patch)
> 
> Signed-off-by: Bandan Das <bsd@redhat.com>
> Message-id: 20180215231129.14710-5-bsd@redhat.com
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>   hw/usb/dev-mtp.c | 159 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 157 insertions(+), 2 deletions(-)
> 

> @@ -1472,12 +1492,133 @@ static void usb_mtp_cancel_packet(USBDevice *dev, USBPacket *p)
>       fprintf(stderr, "%s\n", __func__);
>   }
>   
> +mode_t getumask(void)
> +{
> +    mode_t mask = umask(0);
> +    umask(mask);
> +    return mask;
> +}

This is dangerous.  'man getumask' on my Fedora machine states:

CONFORMING TO
        This is a vaporware GNU extension.

NOTES
        This  function is documented in the glibc manual, but, as at 
glibc ver‐
        sion 2.24, it is not implemented on Linux.  (See umask(2) for a 
thread-
        safe method of discovering a process's umask.)


and 'man 2 umask' concurs:

        It  is  impossible to use umask() to fetch a process's umask 
without at
        the same time changing it.  A second call  to  umask()  would 
then  be
        needed  to restore the umask.  The nonatomicity of these two 
steps pro‐
        vides the potential for races in multithreaded programs.

It is ONLY safe to grab umask() prior to spawning threads, cache that 
value, and refer to the cache at all later points.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org