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From: | Corey Bryant |
Subject: | Re: [Qemu-devel] [PATCH v3 3/5] osdep: Enable qemu_open to dup pre-opened fd |
Date: | Fri, 15 Jun 2012 14:16:59 -0400 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0 |
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 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?
Treat O_APPEND similarly to O_NONBLOCK (with fcntl(F_GETFL/F_SETFL) to match the desired open() setting).
Ok -- Regards, Corey
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