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

[Will Cohen <wwcohen@gmail.com>]

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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.


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