Message ID | 50813413.3010705@redhat.com |
---|---|
State | New |
Headers | show |
On 10/19/2012 07:05 AM, Kevin Wolf wrote: > 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@linux.vnet.ibm.com> >>> >>>> + >>>> + 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; > That works for me. Thanks Kevin!
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;