Re: [PATCH v8 09/11] 9p: darwin: Implement compatibility for mknodat

[Will Cohen <wwcohen@gmail.com>]

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On Tue, Feb 22, 2022 at 9:27 AM Christian Schoenebeck <
qemu_oss@crudebyte.com> wrote:

> On Sonntag, 20. Februar 2022 17:50:54 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 this 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
> >              - Move pthread_fchdir_np declaration only to osdep
> >              - Declare pthread_fchdir_np with
> >              - __attribute__((weak_import)) to allow checking for
> >                its presence before usage
> >              - Move declarations above cplusplus guard
> >              - Add CONFIG_PTHREAD_FCHDIR_NP to meson and check for
> >                presence in osdep.h and os-posix.c
> >              - Rebase to apply cleanly on top of the 2022-02-10
> >                changes to 9pfs]
> > Signed-off-by: Will Cohen <wwcohen@gmail.com>
> > ---
> >  hw/9pfs/9p-local.c   |  4 ++--
> >  include/qemu/osdep.h | 12 ++++++++++++
> >  meson.build          |  1 +
> >  os-posix.c           | 35 +++++++++++++++++++++++++++++++++++
> >  4 files changed, 50 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 ce12f64853..c0f442d791 100644
> > --- a/include/qemu/osdep.h
> > +++ b/include/qemu/osdep.h
> > @@ -818,6 +818,18 @@ static inline int
> > platform_does_not_support_system(const char *command) */
> >  struct dirent *qemu_dirent_dup(struct dirent *dent);
> >
> > +/*
> > + * 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. pthread_fchdir_np is weakly linked here as a guard
> > + * in case it disappears in future macOS versions, because it is
> > + * is a private API.
> > + */
> > +#if defined CONFIG_DARWIN && defined CONFIG_PTHREAD_FCHDIR_NP
> > +int pthread_fchdir_np(int fd) __attribute__((weak_import));
> > +#endif
> > +int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t
> dev);
> > +
> >  #ifdef __cplusplus
> >  }
> >  #endif
>
> Peter just informed me that osdep.h is not the right place to add something
> like this:
>
> https://lore.kernel.org/qemu-devel/CAFEAcA_=HAUNomKD2wurSVaAHa5mrk22A1oHKLWUDjk7v6Khmg@mail.gmail.com/
>
> So this should be moved into a different header file.
>

Does that logic apply here too? In some ways I'd say that mknodat falls
into the category of "things without which code would work on most
platforms but fail to compile or misbehave on a minority of host OSes." If
osdep.h is the wrong place, should it go back to 9p-util, where it lived
before it moved to osdep?


> > diff --git a/meson.build b/meson.build
> > index ae5f7eec6e..6fdc0281ad 100644
> > --- a/meson.build
> > +++ b/meson.build
> > @@ -1557,6 +1557,7 @@ config_host_data.set('CONFIG_POSIX_FALLOCATE',
> > cc.has_function('posix_fallocate'
> > config_host_data.set('CONFIG_POSIX_MEMALIGN',
> > cc.has_function('posix_memalign')) config_host_data.set('CONFIG_PPOLL',
> > cc.has_function('ppoll'))
> >  config_host_data.set('CONFIG_PREADV', cc.has_function('preadv', prefix:
> > '#include <sys/uio.h>'))
> +config_host_data.set('CONFIG_PTHREAD_FCHDIR_NP',
> > cc.has_function('pthread_fchdir_np'))
> > config_host_data.set('CONFIG_SEM_TIMEDWAIT',
> > cc.has_function('sem_timedwait', dependencies: threads))
> > config_host_data.set('CONFIG_SENDFILE', cc.has_function('sendfile'))
> > config_host_data.set('CONFIG_SETNS', cc.has_function('setns') and
> > cc.has_function('unshare')) diff --git a/os-posix.c b/os-posix.c
> > index ae6c9f2a5e..ccc3d1e9d3 100644
> > --- a/os-posix.c
> > +++ b/os-posix.c
> > @@ -332,3 +332,38 @@ 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)
> > + */
> > +#if defined CONFIG_DARWIN && defined CONFIG_PTHREAD_FCHDIR_NP
> > +
> > +int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t
> dev)
> > +{
> > +    int preserved_errno, err;
> > +    if (!pthread_fchdir_np) {
> > +        error_report_once("pthread_fchdir_np() is not available on this
> > version of macOS"); +        return -ENOTSUP;
> > +    }
> > +    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
>
>
>

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