Re: [Qemu-devel] [PATCH] linux-user: add memfd_create

[Peter Maydell <peter.maydell@linaro.org>]

On Fri, 16 Aug 2019 at 22:28, Shu-Chun Weng via Qemu-devel
<qemu-devel@nongnu.org> wrote:
>
> Add support for the memfd_create syscall. If the host does not have the
> libc wrapper, translate to a direct syscall with NC-macro.
>
> Buglink: https://bugs.launchpad.net/qemu/+bug/1734792
> Signed-off-by: Shu-Chun Weng <scw@google.com>
> ---
>  include/qemu/memfd.h |  4 ++++
>  linux-user/syscall.c | 11 +++++++++++
>  util/memfd.c         |  2 +-
>  3 files changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/include/qemu/memfd.h b/include/qemu/memfd.h
> index d551c28b68..975b6bdb77 100644
> --- a/include/qemu/memfd.h
> +++ b/include/qemu/memfd.h
> @@ -32,6 +32,10 @@
>  #define MFD_HUGE_SHIFT 26
>  #endif
>
> +#if defined CONFIG_LINUX && !defined CONFIG_MEMFD
> +int memfd_create(const char *name, unsigned int flags);
> +#endif
> +
>  int qemu_memfd_create(const char *name, size_t size, bool hugetlb,
>                        uint64_t hugetlbsize, unsigned int seals, Error **errp);
>  bool qemu_memfd_alloc_check(void);
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 8367cb138d..b506c1f40e 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -20,6 +20,7 @@
>  #include "qemu/osdep.h"
>  #include "qemu/cutils.h"
>  #include "qemu/path.h"
> +#include "qemu/memfd.h"
>  #include <elf.h>
>  #include <endian.h>
>  #include <grp.h>
> @@ -11938,6 +11939,16 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
>          /* PowerPC specific.  */
>          return do_swapcontext(cpu_env, arg1, arg2, arg3);
>  #endif
> +#ifdef TARGET_NR_memfd_create
> +    case TARGET_NR_memfd_create:
> +        p = lock_user_string(arg1);
> +        if (!p) {
> +            return -TARGET_EFAULT;
> +        }
> +        ret = get_errno(memfd_create(p, arg2));

I think here you may need to call
           fd_trans_unregister(ret);

since memfd_create() returns a file descriptor.
(This call unregisters any pre-existing hooks for "we need
to mangle the data for reads/writes of this file descriptor",
in case the fd was previously being used for a file descriptor
type that needed that.) This is what eg NR_open does.

Laurent -- do I have this right? It's not entirely clear
to me that we could ever be in a situation where a syscall
like open or memfd_create returns an fd that's got an fd_trans
hook active, because that would imply we'd failed to delete
the hook on fd-close somehow. Is this just a belt-and-braces
bit of coding ?

thanks
-- PMM