Re: [Qemu-devel] [PATCH v3 3/5] osdep: Enable qemu_open to dup pre-opened fd

[Corey Bryant]

On 06/15/2012 02:46 PM, Kevin Wolf wrote:
> Am 15.06.2012 20:16, schrieb Corey Bryant:
> > On 06/15/2012 11:16 AM, Eric Blake wrote:
> > > On 06/14/2012 09:55 AM, Corey Bryant wrote:
> > > > This patch adds support to qemu_open to dup(fd) a pre-opened file
> > > > descriptor if the filename is of the format /dev/fd/X.
> > >
> > > > +++ b/osdep.c
> > > > @@ -82,6 +82,19 @@ int qemu_open(const char *name, int flags, ...)
> > > >        int ret;
> > > >        int mode = 0;
> > > >
> > > > +#ifndef _WIN32
> > > > +    const char *p;
> > > > +
> > > > +    /* Attempt dup of fd for pre-opened file */
> > > > +    if (strstart(name, "/dev/fd/", &p)) {
> > > > +        ret = qemu_parse_fd(p);
> > > > +        if (ret == -1) {
> > > > +            return -1;
> > > > +        }
> > > > +        return dup(ret);
> > >
> > > I think you need to honor flags so that the end use of the fd will be as
> > > if qemu had directly opened the file, rather than just doing a blind dup
> > > with a resulting fd that is in a different state than the caller
> > > expected.  I can think of at least the following cases (there may be more):
> >
> > I was thinking libvirt would handle all the flag settings on open
> > (obviously since that's how I coded it).  I think you're right with this
> > approach though as QEMU will re-open the same file various times with
> > different flags.
> >
> > There are some flags that I don't think we'll be able to change.  For
> > example: O_RDONLY, O_WRONLY, O_RDWR.  I assume libvirt would open all
> > files O_RDWR.
>
> I think we need to check all of them and fail qemu_open() if they don't
> match. Those that qemu can change, should be just changed, of course.

Ok.  I remember a scenario where QEMU opens a file read-only (perhaps to 
check headers and determine the file format) before re-opening it 
read-write.  Perhaps this is only when format= isn't specified with 
-drive.  I'm thinking we may need to change flags to read-write where 
they used to be read-only, in some circumstances.

> > > Oh, and are we using MSG_CMSG_CLOEXEC where possible (and where not
> > > possible, falling back to fcntl(F_GETFD/F_SETFD) to set FD_CLOEXEC) on
> > > all fds received by 'getfd' and 'pass-fd'?  I can't think of any reason
> > > why 'migrate fd:name' would need to be inheritable, and in the case of
> > > /dev/fd/ parsing, while the dup() result may need to be inheritable, the
> > > original that we are dup'ing from should most certainly be cloexec.
> >
> > It doesn't look like we use MSG_CMSG_CLOEXEC anywhere in QEMU.  I don't
> > think we can modify getfd at this point (compatibility) but we could
> > update pass-fd to set it.  It may make more sense to set it with
> > fcntl(FD_CLOEXEC) in qmp_pass_fd().
>
> In which scenario would any client break if we set FD_CLOEXEC? I don't
> think compatibility means we can't fix any bugs.

I don't know if it breaks any client.  Maybe it's not a compatibility 
error.  It dopes change behavior down the line though.  If you think 
it's ok to set FD_CLOEXEC for getfd too, then I'm happy to do it.

> > > if (flags & O_NONBLOCK)
> > >      use fcntl(F_GETFL/F_SETFL) to set O_NONBLOCK
> > > else
> > >      use fcntl(F_GETFL/F_SETFL) to clear O_NONBLOCK
> > >
> > > or maybe we document that callers of pass-fd must always pass fds with
> > > O_NONBLOCK clear instead of clearing it ourselves.  Or maybe we make
> > > sure part of the process of tying name with fd in the lookup list of
> > > named fds is determining the current O_NONBLOCK state in case future
> > > qemu_open() need it in the opposite state.
> >
> > Just documenting it seems error-prone.  Why not just set/clear it based
> > on the flag passed to qemu_open?
>
> I agree. We could just check and return an error if they aren't set
> correctly, but I think adjusting the flags is nicer.
>
> Kevin

Ok thanks for the input!

--
Regards,
Corey