[igt-dev] [PATCH i-g-t v2 2/2] build: provide stub implementation for memfd_create

[De Marchi, Lucas]

On Wed, 2018-07-25 at 09:46 -0700, Antonio Argenziano wrote:
> 
> On 24/07/18 15:20, Lucas De Marchi wrote:
> > When libc misses memfd_create(), provide a stub implementation to go
> > through the syscall() route. Syscall numbers are provided for platforms
> > currently supported by i-g-t only.
> > 
> > v2: add support to autotools
> > 
> > Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
> > ---
> >   configure.ac                  |  3 +++
> >   lib/stubs/syscalls/sys/mman.h | 37 +++++++++++++++++++++++++++++++++++
> >   meson.build                   |  3 +++
> 
> I thought you were going to remove "#include <linux/memfd.h>" from 
> gem_userptr_blits and include the stub instead.

I decided not to. To summarize:

In order to get the memfd_create() function definition one needs to include
sys/mman.h. The man page is wrong and I sent a patch.

linux/memfd.h is the kernel header (i.e. not from libc). It's useful to have
it if we are using or going to use new flags not present in the libc headers.
Or harmless. This is their content for me:

kernel header (linux/memfd.h):
/* flags for memfd_create(2) (unsigned int) */
#define MFD_CLOEXEC             0x0001U
#define MFD_ALLOW_SEALING       0x0002U
#define MFD_HUGETLB             0x0004U

libc header (bits/mman-shared.h which is down the include chain for
sys/mman.h):

#ifdef __USE_GNU
/* Flags for memfd_create.  */
# ifndef MFD_CLOEXEC
#  define MFD_CLOEXEC 1U
#  define MFD_ALLOW_SEALING 2U
#  define MFD_HUGETLB 4U
# endif
...

We could also check if linux/memfd.h is available or just remove the include
since we are not using any flag not available on libc, but IMO it's not
necessary or at least not for this patch.


Lucas De Marchi

> 
> Antonio.
> 
> >   3 files changed, 43 insertions(+)
> >   create mode 100644 lib/stubs/syscalls/sys/mman.h
> > 
> > diff --git a/configure.ac b/configure.ac
> > index 0a5b0425..416a3240 100644
> > --- a/configure.ac
> > +++ b/configure.ac
> > @@ -81,6 +81,9 @@ AC_CHECK_FUNCS(timer_create, [], [
> >   ])
> >   AC_SUBST(TIMER_LIBS)
> >   
> > +dnl Check for memfd_create
> > +AC_CHECK_FUNCS(memfd_create)
> > +
> >   dnl Check for CPUID
> >   cpuid="yes"
> >   AC_TRY_LINK([
> > diff --git a/lib/stubs/syscalls/sys/mman.h b/lib/stubs/syscalls/sys/mman.h
> > new file mode 100644
> > index 00000000..2ac2da6a
> > --- /dev/null
> > +++ b/lib/stubs/syscalls/sys/mman.h
> > @@ -0,0 +1,37 @@
> > +/* SPDX-License-Identifier: MIT */
> > +
> > +#pragma once
> > +
> > +#include_next <sys/mman.h>
> > +
> > +#if !defined(HAVE_MEMFD_CREATE) || !HAVE_MEMFD_CREATE
> > +#include <errno.h>
> > +#include <sys/syscall.h>
> > +#include <sys/types.h>
> > +#include <unistd.h>
> > +
> > +#ifndef __NR_memfd_create
> > +#if defined __x86_64__
> > +#define __NR_memfd_create 319
> > +#elif defined __i386__
> > +#define __NR_memfd_create 356
> > +#elif defined __arm__
> > +#define __NR_memfd_create 385
> > +#else
> > +#warning "__NR_memfd_create unknown for your architecture"
> > +#endif
> > +#endif
> > +
> > +static inline int missing_memfd_create(const char *name, unsigned int
> > flags)
> > +{
> > +#ifdef __NR_memfd_create
> > +	return syscall(__NR_memfd_create, name, flags);
> > +#else
> > +	errno = ENOSYS;
> > +	return -1;
> > +#endif
> > +}
> > +
> > +#define memfd_create missing_memfd_create
> > +
> > +#endif
> > diff --git a/meson.build b/meson.build
> > index 22118828..63ac6292 100644
> > --- a/meson.build
> > +++ b/meson.build
> > @@ -196,6 +196,9 @@ if cc.has_member('struct sysinfo', 'totalram',
> >   	config.set('HAVE_STRUCT_SYSINFO_TOTALRAM', 1)
> >   endif
> >   
> > +have = cc.has_function('memfd_create', prefix : '''#include
> > <sys/mman.h>''', args : '-D_GNU_SOURCE')
> > +config.set10('HAVE_MEMFD_CREATE', have)
> > +
> >   add_project_arguments('-D_GNU_SOURCE', language : 'c')
> >   add_project_arguments('-include', 'config.h', language : 'c')
> >   
> >