Re: [Qemu-devel] [PATCH v6 04/24] util: add linux-only memfd fallback

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 29, 2015 at 06:34:34PM +0200, marcandre.lureau@redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> Implement memfd_create() fallback if not available in system libc.
> memfd_create() is still not included in glibc today, atlhough it's been
> available since Linux 3.17 in Oct 2014.
> 
> memfd has numerous advantages over traditional shm/mmap for ipc memory
> sharing with fd handler, which we are going to make use of for
> vhost-user logging memory in following patches.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  include/qemu/memfd.h | 20 +++++++++++++++++++
>  util/Makefile.objs   |  2 +-
>  util/memfd.c         | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 77 insertions(+), 1 deletion(-)
>  create mode 100644 include/qemu/memfd.h
>  create mode 100644 util/memfd.c
> 
> diff --git a/include/qemu/memfd.h b/include/qemu/memfd.h
> new file mode 100644
> index 0000000..8b1fe6a
> --- /dev/null
> +++ b/include/qemu/memfd.h
> @@ -0,0 +1,20 @@
> +#ifndef QEMU_MEMFD_H
> +#define QEMU_MEMFD_H
> +
> +#include "config-host.h"
> +
> +#ifndef F_LINUX_SPECIFIC_BASE
> +#define F_LINUX_SPECIFIC_BASE 1024
> +#endif
> +
> +#ifndef F_ADD_SEALS
> +#define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)
> +#define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)
> +
> +#define F_SEAL_SEAL     0x0001  /* prevent further seals from being set */
> +#define F_SEAL_SHRINK   0x0002  /* prevent file from shrinking */
> +#define F_SEAL_GROW     0x0004  /* prevent file from growing */
> +#define F_SEAL_WRITE    0x0008  /* prevent writes */
> +#endif
> +
> +#endif /* QEMU_MEMFD_H */
> diff --git a/util/Makefile.objs b/util/Makefile.objs
> index 114d657..84c5485 100644
> --- a/util/Makefile.objs
> +++ b/util/Makefile.objs
> @@ -1,6 +1,6 @@
>  util-obj-y = osdep.o cutils.o unicode.o qemu-timer-common.o
>  util-obj-$(CONFIG_WIN32) += oslib-win32.o qemu-thread-win32.o event_notifier-win32.o
> -util-obj-$(CONFIG_POSIX) += oslib-posix.o qemu-thread-posix.o event_notifier-posix.o qemu-openpty.o
> +util-obj-$(CONFIG_POSIX) += oslib-posix.o qemu-thread-posix.o event_notifier-posix.o qemu-openpty.o memfd.o
>  util-obj-y += envlist.o path.o module.o
>  util-obj-$(call lnot,$(CONFIG_INT128)) += host-utils.o
>  util-obj-y += bitmap.o bitops.o hbitmap.o
> diff --git a/util/memfd.c b/util/memfd.c
> new file mode 100644
> index 0000000..a98d57e
> --- /dev/null
> +++ b/util/memfd.c
> @@ -0,0 +1,56 @@
> +/*
> + * memfd.c
> + *
> + * Copyright (c) 2015 Red Hat, Inc.
> + *
> + * QEMU library functions on POSIX which are shared between QEMU and
> + * the QEMU tools.
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a copy
> + * of this software and associated documentation files (the "Software"), to deal
> + * in the Software without restriction, including without limitation the rights
> + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
> + * copies of the Software, and to permit persons to whom the Software is
> + * furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included in
> + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
> + * THE SOFTWARE.
> + */
> +
> +#include "config-host.h"
> +
> +#include "qemu/memfd.h"
> +
> +#ifdef CONFIG_MEMFD
> +#include <sys/memfd.h>

So the file is empty if CONFIG_MEMFD is set?
It would be more elegant to avoid linking it in.
Can be a patch on top.

> +#elif defined CONFIG_LINUX
> +#include <unistd.h>
> +#include <sys/syscall.h>
> +#include <asm/unistd.h>
> +
> +#ifndef MFD_CLOEXEC
> +#define MFD_CLOEXEC 0x0001U
> +#endif
> +
> +#ifndef MFD_ALLOW_SEALING
> +#define MFD_ALLOW_SEALING 0x0002U
> +#endif
> +
> +static inline int memfd_create(const char *name, unsigned int flags)
> +{
> +    return syscall(__NR_memfd_create, name, flags);
> +}
> +#else /* !LINUX */
> +static inline int memfd_create(const char *name, unsigned int flags)
> +{
> +    return -1;
> +}
> +#endif
> -- 
> 2.4.3