Re: [PATCH v5 2/6] 9pfs: fix qemu_mknodat(S_IFSOCK) on macOS

[Greg Kurz]

On Fri, 29 Apr 2022 17:20:26 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Freitag, 29. April 2022 16:35:07 CEST Greg Kurz wrote:
> > On Fri, 29 Apr 2022 15:50:35 +0200
> > 
> > Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> > > On Freitag, 29. April 2022 14:56:50 CEST Greg Kurz wrote:
> > > > On Fri, 29 Apr 2022 12:25:11 +0200
> > > > 
> > > > Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> > > > > mknod() on macOS does not support creating sockets, so divert to
> > > > > call sequence socket(), bind() and fchmodat() respectively if S_IFSOCK
> > > > > was passed with mode argument.
> > > > > 
> > > > > Link: https://lore.kernel.org/qemu-devel/17933734.zYzKuhC07K@silver/
> > > > > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > > > > ---
> > > > > 
> > > > >  hw/9pfs/9p-util-darwin.c | 42
> > > > >  +++++++++++++++++++++++++++++++++++++++-
> > > > >  1 file changed, 41 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/hw/9pfs/9p-util-darwin.c b/hw/9pfs/9p-util-darwin.c
> > > > > index e24d09763a..619c403ba7 100644
> > > > > --- a/hw/9pfs/9p-util-darwin.c
> > > > > +++ b/hw/9pfs/9p-util-darwin.c
> > > > > @@ -74,6 +74,42 @@ int fsetxattrat_nofollow(int dirfd, const char
> > > > > *filename, const char *name,>
> > > > > 
> > > > >   */
> > > > >  
> > > > >  #if defined CONFIG_PTHREAD_FCHDIR_NP
> > > > > 
> > > > > +static int create_socket_file_at_cwd(const char *filename, mode_t
> > > > > mode) {
> > > > > +    int fd, err;
> > > > > +    struct sockaddr_un addr = {
> > > > > +        .sun_family = AF_UNIX
> > > > > +    };
> > > > > +
> > > > > +    err = snprintf(addr.sun_path, sizeof(addr.sun_path), "./%s",
> > > > > filename); +    if (err < 0 || err >= sizeof(addr.sun_path)) {
> > > > 
> > > > According to POSIX [1]:
> > > > 
> > > > The snprintf() function shall fail if:
> > > > 
> > > > [EOVERFLOW]
> > > > [CX] [Option Start] The value of n is greater than {INT_MAX}. [Option
> > > > End]
> > > > 
> > > > [1]
> > > > https://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.htm
> > > > l
> > > > 
> > > > Since we're passing sizeof(addr.sun_path), I'm pretty sure snprintf()
> > > > cannot fail. No big deal.
> > > 
> > > The question is whom you would want to trust on this? POSIX? ISO-C? Clang?
> > > BSD? Apple? And for how long into future? I mean in general yes, I would
> > > not
> > To improve overall portability across all possible hosts, I'd stick to
> > POSIX semantics but here this is macOS only code so you can assume
> > this is Apple's snprintf().
> > 
> > > expect it to fail with -1 here either, but there are various different API
> > > docs on snprintf() out there, and most of them don't even bother to
> > > enumarate which encoding errors may happen. And I'm pretty sure if I'd
> > > drop the negative err check here, then Akihiko would slap me for
> > > unforeseeable additional error cases on snprintf() that may be added in
> > > future.
> > 
> > /o\ ;-)
> > 
> > > Apple's documentation on snprintf() BTW just says:
> > >   "These functions return a negative value if an error occurs."
> > 
> > How valuable this is !!! ;-)
> > 
> > > So Apple does not even restrict the return value to -1 on errrors, you
> > > would also need to expect other negative values.
> > > 
> > > So on doubt, I leave this negative result check for now. ;-)
> > 
> > Fair enough.
> 
> Hey, don't shoot the servant! I'm just trying to find compromises that aim to 
> suit as many people as possible, as always. :)
> 

Oh I wasn't criticizing your work... rather feeling sorry for the
poor documentation of snprintf() on the Apple side. Please go
on with the great job you're doing on 9p ! :-)

Cheers,

--
Greg

> Best regards,
> Christian Schoenebeck
> 
>