Re: [libvirt] [PATCH v4 4/4] qemu-config: Add new -add-fd command line option

Am 18.10.2012 23:37, schrieb Corey Bryant: > > > On 10/18/2012 04:43 PM, Eric Blake wrote: >> On 10/18/2012 01:19 PM, Corey Bryant wrote: >>> This option can be used for passing file descriptors on the >>> command line. It mirrors the existing add-fd QMP command which >>> allows an fd to be passed to QEMU via SCM_RIGHTS and added to an >>> fd set. >>> >>> This can be combined with commands such as -drive to link file >>> descriptors in an fd set to a drive: >>> >>> qemu-kvm -add-fd fd=3,set=2,opaque="rdwr:/path/to/file" >>> -add-fd fd=4,set=2,opaque="rdonly:/path/to/file" >>> -drive file=/dev/fdset/2,index=0,media=disk >>> >>> This example adds dups of fds 3 and 4, and the accompanying opaque >>> strings to the fd set with ID=2. qemu_open() already knows how >>> to handle a filename of this format. qemu_open() searches the >>> corresponding fd set for an fd and when it finds a match, QEMU >>> goes on to use a dup of that fd just like it would have used an >>> fd that it opened itself. >>> >>> Signed-off-by: Corey Bryant <coreyb(a)linux.vnet.ibm.com&gt; >> >>> + >>> + if (fcntl(fd, F_GETFD) & FD_CLOEXEC) { >>> + qerror_report(ERROR_CLASS_GENERIC_ERROR, >>> + "fd is not valid or already in use"); >>> + return -1; >>> + } >> >> Hmm, I was about to call you on the fact that you didn't check whether >> fcntl() succeeded; but then realized that in the failure case it is >> required by POSIX to return -1 which happens to include the FD_CLOEXEC >> bit, so you actually ended up with a sneaky optimization that does the >> right thing for both open and closed fds. > > Yep it works for both cases. I have to admit I stumbled into this at > first and then decided to leave it this way since it worked. :) I wouldn't be surprised to find such subtleties in Fabrice's code, but I'm not sure if adding new instances is the best idea ever. :-) >> Perhaps a comment in the code is warranted (after all, it is not >> immediately apparent from reading just this if statement why it works); >> maybe "/* All fds inherited across exec() necessarily have FD_CLOEXEC >> clear, while qemu sets FD_CLOEXEC on all other fds opened from command >> line arguments */". But I'm not going to require a v5 just for a >> comment addition. > > I agree, a comment would be useful. Maybe Kevin can add if this series > gets pushed? I'll amend the following to this patch, hope you both agree with the change: diff --git a/vl.c b/vl.c index 47095a2..5fb40da 100644 --- a/vl.c +++ b/vl.c @@ -792,7 +792,7 @@ static int parse_sandbox(QemuOpts *opts, void *opaque) static int parse_add_fd(QemuOpts *opts, void *opaque) { - int fd, dupfd; + int fd, dupfd, flags; int64_t fdset_id; const char *fd_opaque = NULL; @@ -812,7 +812,12 @@ static int parse_add_fd(QemuOpts *opts, void *opaque) return -1; } - if (fcntl(fd, F_GETFD) & FD_CLOEXEC) { + /* + * All fds inherited across exec() necessarily have FD_CLOEXEC + * clear, while qemu sets FD_CLOEXEC on all other fds used internally. + */ + flags = fcntl(fd, F_GETFD); + if (flags == -1 || (flags & FD_CLOEXEC)) { qerror_report(ERROR_CLASS_GENERIC_ERROR, "fd is not valid or already in use"); return -1;