Re: [PATCH v4 09/11] 9p: darwin: Implement compatibility for mknodat

[Will Cohen <wwcohen@gmail.com>]

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On Mon, Feb 7, 2022 at 4:07 PM Will Cohen <wwcohen@gmail.com> wrote:

> On Mon, Feb 7, 2022 at 9:21 AM Christian Schoenebeck <
> qemu_oss@crudebyte.com> wrote:
>
>> On Montag, 7. Februar 2022 11:57:25 CET Dr. David Alan Gilbert wrote:
>> > * Greg Kurz (groug@kaod.org) wrote:
>> > > On Mon, 7 Feb 2022 11:30:18 +0100
>> > >
>> > > Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>> > > > On 7/2/22 09:47, Greg Kurz wrote:
>> > > > > On Sun, 6 Feb 2022 20:10:23 -0500
>> > > > >
>> > > > > Will Cohen <wwcohen@gmail.com> wrote:
>> > > > >> This patch set currently places it in 9p-util only because 9p is
>> the
>> > > > >> only
>> > > > >> place where this issue seems to have come up so far and we were
>> wary
>> > > > >> of
>> > > > >> editing files too far afield, but I have no attachment to its
>> > > > >> specific
>> > > > >> location!
>> > > > >
>> > > > > Inline comments are preferred on qemu-devel. Please don't top
>> post !
>> > > > > This complicates the review a lot.
>> > > > >
>> > > > > This is indeed a good candidate for osdep. This being said, unless
>> > > > > there's
>> > > > > some other user in the QEMU code base, it is acceptable to leave
>> it
>> > > > > under
>> > > > > 9pfs.
>> > > >
>> > > > virtiofsd could eventually use it.
>> > >
>> > > Indeed but virtiofsd is for linux hosts only AFAICT and I'm not aware
>> of
>> > > any work to support any other host OS.
>> > >
>> > > Cc'ing virtio-fs people for inputs on this topic.
>> >
>> > Indeeed, there's a lot of Linux specific code in the virtiofsd - I know
>> > people are interested in other platforms, but I'm not sure that's the
>> > right starting point.
>> >
>> > Dave
>>
>> Agreeing with Greg here: i.e. I would have placed this into osdep, but I
>> would
>> not insist on it either.
>>
>> Best regards,
>> Christian Schoenebeck
>>
>>
> This makes sense. A revised version of this patch, moving qemu_mknodat
> from 9p-util to osdep and os-posix, is attached below. I'd appreciate any
> feedback from those looped in here, so that the context isn't lost before
> resubmitting as a v5 patch, especially since this is starting to touch
> files outside of 9p.
>
> From c9713c87163da7c96b5357d0d85ac318ae3d3051 Mon Sep 17 00:00:00 2001
> From: Keno Fischer <keno@juliacomputing.com>
> Date: Sat, 16 Jun 2018 20:56:55 -0400
> Subject: [PATCH] 9p: darwin: Implement compatibility for mknodat
>
> 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>
> ---
>  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
> +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));
> +
> +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
> --
> 2.34.1
>

Noting on
https://www.qemu.org/docs/master/devel/submitting-a-patch.html#when-resending-patches-add-a-version-tag
that resending a single patch outside of a new entire patch series is not
helpful, so I'm resubmitting with this patch substituted in as v5 shortly.

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