Re: [PATCH v1 17/22] util/char_dev: Add open_cdev()

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Aug 30, 2023 at 06:37:49PM +0800, Zhenzhong Duan wrote:
> From: Yi Liu <yi.l.liu@intel.com>
> 
> /dev/vfio/devices/vfioX may not exist. In that case it is still possible
> to open /dev/char/$major:$minor instead. Add helper function to abstract
> the cdev open.
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
> ---
>  MAINTAINERS             |  6 ++++
>  include/qemu/char_dev.h | 16 +++++++++++
>  util/chardev_open.c     | 61 +++++++++++++++++++++++++++++++++++++++++

Using the same naming scheme for the .c and .h is strongly desired.

>  util/meson.build        |  1 +
>  4 files changed, 84 insertions(+)
>  create mode 100644 include/qemu/char_dev.h
>  create mode 100644 util/chardev_open.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 04663fbb6f..74d18593fe 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3372,6 +3372,12 @@ S: Maintained
>  F: include/qemu/iova-tree.h
>  F: util/iova-tree.c
>  
> +cdev Open
> +M: Yi Liu <yi.l.liu@intel.com>
> +S: Maintained
> +F: include/qemu/char_dev.h
> +F: util/chardev_open.c
> +


> diff --git a/util/chardev_open.c b/util/chardev_open.c
> new file mode 100644
> index 0000000000..d03e415131
> --- /dev/null
> +++ b/util/chardev_open.c
> @@ -0,0 +1,61 @@
> +/*
> + * Copyright (C) 2023 Intel Corporation.
> + * Copyright (c) 2019, Mellanox Technologies. All rights reserved.
> + *
> + * Authors: Yi Liu <yi.l.liu@intel.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2.  See
> + * the COPYING file in the top-level directory.
> + *
> + * Copied from
> + * https://github.com/linux-rdma/rdma-core/blob/master/util/open_cdev.c
> + *
> + */

Since this is GPL-2.0-only, IMHO it would be preferrable to keep it
out of the util/ directory, as we're aiming to not add further 2.0
only code, except for specific subdirs. This only appears to be used
by code under hw/vfio/, whcih is one of the dirs still permitting
2.0-only code. So I think better to keep this file where it is used.

> +#ifndef _GNU_SOURCE
> +#define _GNU_SOURCE
> +#endif

This is set globally for building all files in QEMU

> +#include "qemu/osdep.h"
> +#include "qemu/char_dev.h"
> +
> +static int open_cdev_internal(const char *path, dev_t cdev)
> +{
> +    struct stat st;
> +    int fd;
> +
> +    fd = qemu_open_old(path, O_RDWR);
> +    if (fd == -1) {
> +        return -1;
> +    }
> +    if (fstat(fd, &st) || !S_ISCHR(st.st_mode) ||
> +        (cdev != 0 && st.st_rdev != cdev)) {
> +        close(fd);
> +        return -1;
> +    }
> +    return fd;
> +}
> +
> +static int open_cdev_robust(dev_t cdev)
> +{
> +    char *devpath;

g_autofree for this...

> +    int ret;
> +
> +    /*
> +     * This assumes that udev is being used and is creating the /dev/char/
> +     * symlinks.
> +     */
> +    devpath = g_strdup_printf("/dev/char/%u:%u", major(cdev), minor(cdev));
> +    ret = open_cdev_internal(devpath, cdev);
> +    g_free(devpath);

...avoids the need for g_free, and also avoids the need for
the intermediate 'ret' variable.

> +    return ret;
> +}
> +
> +int open_cdev(const char *devpath, dev_t cdev)
> +{
> +    int fd;
> +
> +    fd = open_cdev_internal(devpath, cdev);
> +    if (fd == -1 && cdev != 0) {
> +        return open_cdev_robust(cdev);
> +    }
> +    return fd;
> +}
> diff --git a/util/meson.build b/util/meson.build
> index a375160286..d5313d858f 100644
> --- a/util/meson.build
> +++ b/util/meson.build
> @@ -107,6 +107,7 @@ if have_block
>      util_ss.add(files('filemonitor-stub.c'))
>    endif
>    util_ss.add(when: 'CONFIG_LINUX', if_true: files('vfio-helpers.c'))
> +  util_ss.add(when: 'CONFIG_LINUX', if_true: files('chardev_open.c'))
>  endif
>  
>  if cpu == 'aarch64'
> -- 
> 2.34.1
> 
> 

With regards,
Daniel
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