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

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.

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.

That makes sense.

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

That makes sense too.

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().

if (flags & O_NONBLOCK)
    use fcntl(F_GETFL/F_SETFL) to set O_NONBLOCK
    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?

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



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