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_...@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_...@crudebyte.com>
> > wrote:
> > > On Montag, 7. Februar 2022 23:40:22 CET Will Cohen wrote:
> > > > From: Keno Fischer <k...@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 <k...@juliacomputing.com>
> > > > Signed-off-by: Michael Roitzsch <reactorcont...@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 <wwco...@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
>
>
>
