Re: [PATCH v5 09/11] 9p: darwin: Implement compatibility for mknodat

[Will Cohen]

My inclination is to go with the __builtin_available(macOS 10.12, *) path, if acceptable, since it partially mirrors the API_AVAILABLE macro idea. I guess it's perhaps a tradeoff between predicting the future unknown availability of functions versus just ensuring a minimum macOS version and hoping for the best. With any luck, the distinction between the two approaches will be moot, if we try to assume that a future macOS version that removes this also provides mknodat.

On Tue, Feb 8, 2022 at 10:03 AM Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

On Dienstag, 8. Februar 2022 14:36:42 CET Will Cohen wrote:
> On Mon, Feb 7, 2022 at 5:56 PM Christian Schoenebeck
> <qemu_oss@crudebyte.com>
> wrote:
> > On Montag, 7. Februar 2022 23:40:22 CET Will Cohen wrote:
> > > From: Keno Fischer <keno@juliacomputing.com>
> > >
> > > Darwin does not support mknodat. However, to avoid race conditions
> > > with later setting the permissions, we must avoid using mknod on
> > > the full path instead. We could try to fchdir, but that would cause
> > > problems if multiple threads try to call mknodat at the same time.
> > > However, luckily there is a solution: Darwin includes a function
> > > that sets the cwd for the current thread only.
> > > This should suffice to use mknod safely.
> > >
> > > This function (pthread_fchdir_np) is protected by a check in
> > > meson in a patch later in tihs series.
> > >
> > > Signed-off-by: Keno Fischer <keno@juliacomputing.com>
> > > Signed-off-by: Michael Roitzsch <reactorcontrol@icloud.com>
> > > [Will Cohen: - Adjust coding style
> > >
> > >              - Replace clang references with gcc
> > >              - Note radar filed with Apple for missing syscall
> > >              - Replace direct syscall with pthread_fchdir_np and
> > >             
> > >                adjust patch notes accordingly
> > >             
> > >              - Move qemu_mknodat from 9p-util to osdep and os-posix]
> > >
> > > Signed-off-by: Will Cohen <wwcohen@gmail.com>
> > > ---
> >
> > Like already mentioned by me moments ago on previous v4 (just echoing) ...
> >
> > >  hw/9pfs/9p-local.c   |  4 ++--
> > >  include/qemu/osdep.h | 10 ++++++++++
> > >  os-posix.c           | 34 ++++++++++++++++++++++++++++++++++
> > >  3 files changed, 46 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
> > > index a0d08e5216..d42ce6d8b8 100644
> > > --- a/hw/9pfs/9p-local.c
> > > +++ b/hw/9pfs/9p-local.c
> > > @@ -682,7 +682,7 @@ static int local_mknod(FsContext *fs_ctx, V9fsPath
> > > *dir_path,
> > >
> > >      if (fs_ctx->export_flags & V9FS_SM_MAPPED ||
> > >     
> > >          fs_ctx->export_flags & V9FS_SM_MAPPED_FILE) {
> > >
> > > -        err = mknodat(dirfd, name, fs_ctx->fmode | S_IFREG, 0);
> > > +        err = qemu_mknodat(dirfd, name, fs_ctx->fmode | S_IFREG, 0);
> > >
> > >          if (err == -1) {
> > >         
> > >              goto out;
> > >         
> > >          }
> > >
> > > @@ -697,7 +697,7 @@ static int local_mknod(FsContext *fs_ctx, V9fsPath
> > > *dir_path, }
> > >
> > >      } else if (fs_ctx->export_flags & V9FS_SM_PASSTHROUGH ||
> > >     
> > >                 fs_ctx->export_flags & V9FS_SM_NONE) {
> > >
> > > -        err = mknodat(dirfd, name, credp->fc_mode, credp->fc_rdev);
> > > +        err = qemu_mknodat(dirfd, name, credp->fc_mode,
> > > credp->fc_rdev);
> > >
> > >          if (err == -1) {
> > >         
> > >              goto out;
> > >         
> > >          }
> > >
> > > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > > index d1660d67fa..f3a8367ece 100644
> > > --- a/include/qemu/osdep.h
> > > +++ b/include/qemu/osdep.h
> > > @@ -810,3 +810,13 @@ static inline int
> > > platform_does_not_support_system(const char *command) #endif
> > >
> > >  #endif
> > >
> > > +
> > > +/*
> > > + * As long as mknodat is not available on macOS, this workaround
> > > + * using pthread_fchdir_np is needed. qemu_mknodat is defined in
> > > + * os-posix.c
> > > + */
> > > +#ifdef CONFIG_DARWIN
> > > +int pthread_fchdir_np(int fd);
> > > +#endif
> >
> > I would make that:
> >
> > #ifdef CONFIG_DARWIN
> > int pthread_fchdir_np(int fd) API_AVAILABLE(macosx(10.12));
> > #endif
> >
> > here and ...
> >
> > > +int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t
> >
> > dev);
> >
> > > diff --git a/os-posix.c b/os-posix.c
> > > index ae6c9f2a5e..95c1607065 100644
> > > --- a/os-posix.c
> > > +++ b/os-posix.c
> > > @@ -24,6 +24,7 @@
> > >
> > >   */
> > > 
> > >  #include "qemu/osdep.h"
> > >
> > > +#include <os/availability.h>
> > >
> > >  #include <sys/wait.h>
> > >  #include <pwd.h>
> > >  #include <grp.h>
> > >
> > > @@ -332,3 +333,36 @@ int os_mlock(void)
> > >
> > >      return -ENOSYS;
> > > 
> > >  #endif
> > >  }
> > >
> > > +
> > > +/*
> > > + * As long as mknodat is not available on macOS, this workaround
> > > + * using pthread_fchdir_np is needed.
> > > + *
> > > + * Radar filed with Apple for implementing mknodat:
> > > + * rdar://FB9862426 (https://openradar.appspot.com/FB9862426)
> > > + */
> > > +#ifdef CONFIG_DARWIN
> > > +
> > > +int pthread_fchdir_np(int fd) API_AVAILABLE(macosx(10.12));
> >
> > ... drop the duplicate declaration of pthread_fchdir_np() here.
>
> Trying this out, it reminds me that this use of API_AVAILABLE in os-posix.c
> relies on the added #include <os/availability.h>.
>
> Leaving the include out leads to:
> .../include/qemu/osdep.h:820:31: error: expected function body after
> function declarator
> int pthread_fchdir_np(int fd) API_AVAILABLE(macosx(10.12));
>                               ^
> 1 error generated.
> ninja: build stopped: subcommand failed.
> make[1]: *** [run-ninja] Error 1
> make: *** [all] Error 2
>
> The admonition against modifying osdep.h's includes too much led me to
> steer away from putting it all in there. If there's no issue with adding
> +#include <os/availability.h> to osdep.h, then this change makes sense to
> me.

If you embed that include into ifdefs, sure!

#ifdef CONFIG_DARWIN
/* defines API_AVAILABLE(...) */
#include <os/availability.h>
#endif

One more thing though ...

> > > +
> > > +int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t
> >
> > dev)
> >
> > > +{
> > > +    int preserved_errno, err;

pthread_fchdir_np() is weakly linked. So I guess here should be a check like:

        if (!pthread_fchdir_np) {
                return -ENOTSUPP;
        }

Before trying to call pthread_fchdir_np() below. As already discussed with the
Chromium [1] example, some do that a bit differently by using
__builtin_available():

        if (__builtin_available(macOS 10.12, *)) {
                return -ENOTSUPP;
        }

Which makes me wonder why they are not doing a simple NULL check?

[1] https://chromium.googlesource.com/chromium/src/+/lkgr/base/process/launch_mac.cc#110

> > > +    if (pthread_fchdir_np(dirfd) < 0) {
> > > +        return -1;
> > > +    }
> > > +    err = mknod(filename, mode, dev);
> > > +    preserved_errno = errno;
> > > +    /* Stop using the thread-local cwd */
> > > +    pthread_fchdir_np(-1);
> > > +    if (err < 0) {
> > > +        errno = preserved_errno;
> > > +    }
> > > +    return err;
> > > +}
> > > +#else
> > > +int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t
> >
> > dev)
> >
> > > +{
> > > +    return mknodat(dirfd, filename, mode, dev);
> > > +}
> > > +#endif