Re: [Qemu-devel] [PATCH v3 03/16] util: add memfd helpers

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

On Thu, Aug 06, 2015 at 02:40:39PM +0200, marcandre.lureau@redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> Add qemu_memfd_alloc/free() helpers.
> 
> The function helps to allocate and seal a memfd, and implements an
> open/unlink/mmap fallback for system that do not support memfd.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  include/qemu/memfd.h |  4 +++
>  util/memfd.c         | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 76 insertions(+), 2 deletions(-)
> 
> diff --git a/include/qemu/memfd.h b/include/qemu/memfd.h
> index 8b1fe6a..950fb88 100644
> --- a/include/qemu/memfd.h
> +++ b/include/qemu/memfd.h
> @@ -17,4 +17,8 @@
>  #define F_SEAL_WRITE    0x0008  /* prevent writes */
>  #endif
>  
> +void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
> +                       int *fd);
> +void qemu_memfd_free(void *ptr, size_t size, int fd);
> +
>  #endif /* QEMU_MEMFD_H */
> diff --git a/util/memfd.c b/util/memfd.c
> index a98d57e..8b2b785 100644
> --- a/util/memfd.c
> +++ b/util/memfd.c
> @@ -27,6 +27,14 @@
>  
>  #include "config-host.h"
>  
> +#include <glib.h>
> +#include <glib/gprintf.h>
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <fcntl.h>
> +#include <sys/mman.h>
> +
>  #include "qemu/memfd.h"
>  
>  #ifdef CONFIG_MEMFD
> @@ -44,13 +52,75 @@
>  #define MFD_ALLOW_SEALING 0x0002U
>  #endif
>  
> -static inline int memfd_create(const char *name, unsigned int flags)
> +static 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)
> +static int memfd_create(const char *name, unsigned int flags)
>  {
>      return -1;
>  }
>  #endif
> +
> +void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
> +                       int *fd)
> +{
> +    void *ptr;
> +    int mfd;
> +
> +    mfd = memfd_create(name, MFD_ALLOW_SEALING|MFD_CLOEXEC);


Hmm. Does this interact correctly with the -mem-prealloc flag?

> +    if (mfd != -1) {
> +        if (ftruncate(mfd, size) == -1) {

Any limitations on size?

> +            perror("ftruncate");
> +            close(mfd);
> +            return NULL;
> +        }
> +
> +        if (fcntl(mfd, F_ADD_SEALS, seals) == -1) {
> +            perror("fcntl");
> +            close(mfd);
> +            return NULL;
> +        }
> +    } else {
> +        const char *tmpdir = getenv("TMPDIR");
> +        gchar *fname;
> +
> +        tmpdir = tmpdir ? tmpdir : "/tmp";
> +
> +        fname = g_strdup_printf("%s/memfd-XXXXXX", tmpdir);

This means there's now work to be done to set up selinux
to allow QEMU creating memfd under /tmp.

Maybe it's better to just fail gracefully for now.

> +        mfd = mkstemp(fname);
> +        unlink(fname);
> +        g_free(fname);
> +
> +        if (mfd == -1) {
> +            perror("mkstemp");
> +            return NULL;
> +        }
> +
> +        if (ftruncate(mfd, size) == -1) {
> +            perror("ftruncate");
> +            close(mfd);
> +            return NULL;
> +        }
> +    }
> +
> +    ptr = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, mfd, 0);

Pls add space around | here and elsewhere.


> +    if (ptr == MAP_FAILED) {
> +        perror("mmap");
> +        close(mfd);
> +        return NULL;
> +    }
> +
> +    *fd = mfd;
> +    return ptr;
> +}
> +
> +void qemu_memfd_free(void *ptr, size_t size, int fd)
> +{
> +    if (ptr) {
> +        munmap(ptr, size);
> +    }
> +
> +    close(fd);

I notice you close fd unconditionally, but it's only returned
on success above. So this will close an uninitialized one?

> +}
> -- 
> 2.4.3