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

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

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.

> 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