Re: [PATCH 07/16] hw/9pfs: Implement Windows specific utilities functions for 9pfs

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Wednesday, November 2, 2022 4:07:35 AM CET Shi, Guohuai wrote:
> 
> > -----Original Message-----
> > From: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > Sent: Wednesday, November 2, 2022 02:22
> > To: qemu-devel@nongnu.org
> > Cc: Greg Kurz <groug@kaod.org>; Meng, Bin <Bin.Meng@windriver.com>; Shi,
> > Guohuai <Guohuai.Shi@windriver.com>
> > Subject: Re: [PATCH 07/16] hw/9pfs: Implement Windows specific utilities
> > functions for 9pfs
> > 
> > [Please note: This e-mail is from an EXTERNAL e-mail address]
> > 
> > On Tuesday, November 1, 2022 4:20:53 PM CET Shi, Guohuai wrote:
> > >
> > [...]
> > > > > > Windows POSIX API and MinGW library do not provide the NO_FOLLOW
> > > > > > flag, and do not allow opening a directory by POSIX open(). This
> > > > > > causes all
> > > > > > xxx_at() functions cannot work directly. However, we can provide
> > > > > > Windows handle based functions to emulate xxx_at() functions (e.g.:
> > > > > > openat_win32, utimensat_win32, etc.).
> > > > > >
> > > > > > Windows does not support extended attributes. 9pfs for Windows
> > > > > > uses NTFS ADS (Alternate Data Streams) to emulate extended
> > attributes.
> > > > > >
> > > > > > Windows does not provide POSIX compatible readlink(), and
> > > > > > symbolic link feature in 9pfs will be disabled on Windows.
> > > > >
> > > > > Wouldn't it be more user friendly if the relevant error locations
> > > > > would use something like error_report_once() and suggesting to
> > > > > enable
> > > > > mapped(-xattr) to make 9p symlinks on guest working if desired by the
> > user?
> > > > >
> > > > > Probably this error case would need to wrapped into a dedicated
> > > > > function, otherwise I guess error_report_once() would fire several
> > > > > times by different callers.
> > > > >
> > > >
> > > > Windows (MinGW) does not only support symlink, but also does not
> > > > have symlink definitions.
> > > > Windows does not support symlink flags S_IFLNK.
> > > >
> > > > So even I add symlink support by mapped-xattr, the MinGW library
> > > > does not have symlink flags and get a build error.
> > > > And this flags is defined by Windows header files.
> > > > The impact of adding a new flags to an pre-defined structure (struct
> > > > stat) is unknown.
> > > >
> > > > So I think it is not a good idea to do that.
> > >
> > > Because Windows does not support symlink, so error_report_once() and report
> > it to user will be OK.
> > > But mapped-xattr could not work.
> > 
> > Showing an error makes sense for "passthrough" security model, but not for
> > the "mapped" one.
> > 
> > Just to avoid misapprehensions: are you aware that there is already a system-
> > agnostic implementation for symlinks if "mapped" is used?
> > 
> > When mapped security model is enabled, then creating symlinks on guest will
> > simply create a corresponding *regular* file on host and the content of that
> > regular file on host is the pointing path as raw string. Additionally the
> > symlink flag is added to "virtfs.mode" xattr to mark that regular file as a
> > symlink, a virtual one that is. So this does not require any support for
> > symlinks by either the underlying host file system, nor by host OS.
> > 
> > Likewise interpreting and walking those virtual symlinks in "mapped" mode is
> > also implemented in the local fs driver already.
> 
> Yes, symlink can be supported by "mapped" mode.
> I mean that MinGW does not provide symlink mode flags "S_IFLNK" and some other related functions and defines.
> You can check with "9p.c": S_ISLNK, S_ISSOCK and S_ISFIFO are not valid on Windows (MinGW) host.
> So even I enabled symlink supported by "mapped" mode on local-agent code, "9p.c" can not be built.
> 
> So I disabled symlink totally, because MinGW interface does not support it.
> 
> To resolve this issue, MinGW should add the missing defines at first.

And what's wrong with something like the following?

#ifdef CONFIG_WIN32
...
#ifndef S_ISLNK
#define S_ISLNK(x) ...
#endif
...
#endif

Best regards,
Christian Schoenebeck