Re: [Qemu-devel] [PATCH v6 05/22] osdep: Add qemu_lock_fd and qemu_unlock_fd

[Kevin Wolf]

Am 22.06.2016 um 09:34 hat Fam Zheng geschrieben:
> On Fri, 06/17 14:12, Kevin Wolf wrote:
> > Am 03.06.2016 um 10:48 hat Fam Zheng geschrieben:
> > > They are wrappers of POSIX fcntl "file private locking".
> > > 
> > > Signed-off-by: Fam Zheng <address@hidden>
> > > ---
> > >  include/qemu/osdep.h |  2 ++
> > >  util/osdep.c         | 29 +++++++++++++++++++++++++++++
> > >  2 files changed, 31 insertions(+)
> > > 
> > > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > > index 6937694..749214a 100644
> > > --- a/include/qemu/osdep.h
> > > +++ b/include/qemu/osdep.h
> > > @@ -280,6 +280,8 @@ int qemu_madvise(void *addr, size_t len, int advice);
> > >  
> > >  int qemu_open(const char *name, int flags, ...);
> > >  int qemu_close(int fd);
> > > +int qemu_lock_fd(int fd, int64_t start, int64_t len, bool exclusive);
> > > +int qemu_unlock_fd(int fd, int64_t start, int64_t len);
> > >  
> > >  #if defined(__HAIKU__) && defined(__i386__)
> > >  #define FMT_pid "%ld"
> > > diff --git a/util/osdep.c b/util/osdep.c
> > > index 9a7a439..085ed52 100644
> > > --- a/util/osdep.c
> > > +++ b/util/osdep.c
> > > @@ -137,6 +137,35 @@ static int qemu_parse_fdset(const char *param)
> > >  {
> > >      return qemu_parse_fd(param);
> > >  }
> > > +
> > > +static int qemu_lock_fcntl(int fd, int64_t start, int64_t len, int 
> > > fl_type)
> > > +{
> > > +#ifdef F_OFD_SETLK
> > > +    int ret;
> > > +    struct flock fl = {
> > > +        .l_whence = SEEK_SET,
> > > +        .l_start  = start,
> > > +        .l_len    = len,
> > > +        .l_type   = fl_type,
> > > +    };
> > > +    do {
> > > +        ret = fcntl(fd, F_OFD_SETLK, &fl);
> > > +    } while (ret == -1 && errno == EINTR);
> > > +    return ret == -1 ? -errno : 0;
> > > +#else
> > > +    return -ENOTSUP;
> > > +#endif
> > > +}
> > 
> > This will return -ENOTSUP in the case that the function wasn't available
> > at build time, but -EINVAL if it was available at build time but the
> > kernel doesn't support it at runtime. Should we unify this?
> 
> I'm not sure we can reliably distinguish "fcntl cmd not supported" and "fcntl
> cmd supported but parameters have invalid value" out of -EINVAL.

Well, the other option is returning -EINVAL instead of -ENOTSUP in the
#else branch.

> Quoting the manual:
> 
> > https://www.gnu.org/software/libc/manual/html_node/Open-File-Description-Locks.html
> > 
> > Macro: int F_OFD_SETL
> > 
> >     EINVAL
> > 
> >     Either the lockp argument doesn’t specify valid lock information, the
> >     operating system kernel doesn’t support open file description locks, or 
> > the
> >     file associated with filedes doesn’t support locks.
> > 
> 
> I'd expect the user to know what he's doing if he is using a kernel that is
> much older than the one built QEMU, since this is relatively a very uncommon
> thing to do.

I'm talking about possible bugs if a caller of this function is checking
for -ENOTSUP, it's easy to forget -EINVAL there. Fixing qemu isn't one
of the things that we should expect even from a "user who knows what
he's doing".

Kevin