Re: [PATCH] mlock2 and memfd_create

[Andrei Vagin <avagin@gmail.com>]

On Tue, Jun 19, 2018 at 10:43:14PM +0200, Szabolcs Nagy wrote:
> separate patches for mlock2 and memfd_create.

> From ee3fa6fea375a941f0f11da9a5767f35dd53c6f0 Mon Sep 17 00:00:00 2001
> From: Szabolcs Nagy <nsz@port70.net>
> Date: Sat, 28 Apr 2018 17:25:41 +0000
> Subject: [PATCH 1/2] Add mlock2 linux syscall wrapper
> 
> mlock2 syscall was added in linux v4.4 and glibc has api for it.
> It falls back to mlock in case of flags==0, so that case works
> even on older kernels.
> 
> MLOCK_ONFAULT is moved under _GNU_SOURCE following glibc.
> ---
>  include/sys/mman.h | 11 ++++++++---
>  src/linux/mlock2.c | 10 ++++++++++
>  2 files changed, 18 insertions(+), 3 deletions(-)
>  create mode 100644 src/linux/mlock2.c
> 
> diff --git a/include/sys/mman.h b/include/sys/mman.h
> index 19dd844e..80e1da75 100644
> --- a/include/sys/mman.h
> +++ b/include/sys/mman.h
> @@ -94,6 +94,13 @@ extern "C" {
>  #define MADV_SOFT_OFFLINE 101
>  #endif
>  
> +#ifdef _GNU_SOURCE
> +#define MREMAP_MAYMOVE 1
> +#define MREMAP_FIXED 2
> +
> +#define MLOCK_ONFAULT 0x01
> +#endif
> +
>  #include <bits/mman.h>
>  
>  void *mmap (void *, size_t, int, int, int, off_t);
> @@ -110,14 +117,12 @@ int mlockall (int);
>  int munlockall (void);
>  
>  #ifdef _GNU_SOURCE
> -#define MREMAP_MAYMOVE 1
> -#define MREMAP_FIXED 2
>  void *mremap (void *, size_t, size_t, int, ...);
>  int remap_file_pages (void *, size_t, int, size_t, int);
> +int mlock2 (const void *, size_t, unsigned);
>  #endif
>  
>  #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
> -#define MLOCK_ONFAULT   0x01
>  int madvise (void *, size_t, int);
>  int mincore (void *, size_t, unsigned char *);
>  #endif
> diff --git a/src/linux/mlock2.c b/src/linux/mlock2.c
> new file mode 100644
> index 00000000..10132742
> --- /dev/null
> +++ b/src/linux/mlock2.c
> @@ -0,0 +1,10 @@
> +#define _GNU_SOURCE 1
> +#include <sys/mman.h>
> +#include "syscall.h"
> +
> +int mlock2(const void *addr, size_t len, unsigned flags)
> +{
> +	if (flags == 0)
> +		return mlock(addr, len);
> +	return syscall(SYS_mlock2, addr, len, flags);

I would prefer another way to support old kernels:

	int ret;

	ret = syscall(SYS_mlock2, addr, len, flags);
	if (ret == -1 && errno == ENOSYS && flags == 0)
		return mlock(addr, len);
	return ret;

This way works a bit slower on old kernels, but it doesn't have side
effects if mlock2 is supported.

For example, the user can set seccomp rules, and he will not expect that
the mlock syscall will be executed, when he calls mlock2() in a code.

Thanks,
Andrei

> +}
> -- 
> 2.16.3
> 

> From ed7309bd4156c77d813390ae5b2507719b43face Mon Sep 17 00:00:00 2001
> From: Szabolcs Nagy <nsz@port70.net>
> Date: Tue, 19 Jun 2018 20:28:03 +0000
> Subject: [PATCH 2/2] Add memfd_create syscall wrapper
> 
> memfd_create was added in linux v3.17 and glibc has api for it.
> ---
>  include/sys/mman.h       | 5 +++++
>  src/linux/memfd_create.c | 8 ++++++++
>  2 files changed, 13 insertions(+)
>  create mode 100644 src/linux/memfd_create.c
> 
> diff --git a/include/sys/mman.h b/include/sys/mman.h
> index 80e1da75..99d02a2e 100644
> --- a/include/sys/mman.h
> +++ b/include/sys/mman.h
> @@ -99,6 +99,10 @@ extern "C" {
>  #define MREMAP_FIXED 2
>  
>  #define MLOCK_ONFAULT 0x01
> +
> +#define MFD_CLOEXEC 0x0001U
> +#define MFD_ALLOW_SEALING 0x0002U
> +#define MFD_HUGETLB 0x0004U
>  #endif
>  
>  #include <bits/mman.h>
> @@ -119,6 +123,7 @@ int munlockall (void);
>  #ifdef _GNU_SOURCE
>  void *mremap (void *, size_t, size_t, int, ...);
>  int remap_file_pages (void *, size_t, int, size_t, int);
> +int memfd_create (const char *, unsigned);
>  int mlock2 (const void *, size_t, unsigned);
>  #endif
>  
> diff --git a/src/linux/memfd_create.c b/src/linux/memfd_create.c
> new file mode 100644
> index 00000000..1649fe55
> --- /dev/null
> +++ b/src/linux/memfd_create.c
> @@ -0,0 +1,8 @@
> +#define _GNU_SOURCE 1
> +#include <sys/mman.h>
> +#include "syscall.h"
> +
> +int memfd_create(const char *name, unsigned flags)
> +{
> +	return syscall(SYS_memfd_create, name, flags);
> +}
> -- 
> 2.16.3
>