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

[Eric Blake]

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):

if (flags & O_TRUNCATE ||
      ((flags & (O_CREAT|O_EXCL)) == (O_CREAT|O_EXCL))
    ftruncate(ret, 0);

Why do I truncate on O_CREAT|O_EXCL?  Because that's a case where open()
guarantees that the file will have just been created empty.

if (flags & O_CLOEXEC)
   use fcntl(F_DUPFD_CLOEXEC) instead of dup(), if possible
   else fallback to fcntl(F_GETFD/F_SETFD) to set FD_CLOEXEC non-atomically

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.

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.

Treat O_APPEND similarly to O_NONBLOCK (with fcntl(F_GETFL/F_SETFL) to
match the desired open() setting).

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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