cvs commit: src/lib/libc/gen Makefile.inc posixshm.c shm_open.3 src/lib/libc/sys Makefile.inc shm_open.2 src/sys/compat/freebsd32 syscalls.master src/sys/conf files src/sys/kern kern_descrip.c syscalls.master uipc_shm.c src/sys/security/mac mac_framework.h ...

[John Baldwin]

On Tuesday 08 January 2008 04:58:16 pm John Baldwin wrote:
> jhb         2008-01-08 21:58:16 UTC
> 
>   FreeBSD src repository
> 
>   Modified files:
>     lib/libc/gen         Makefile.inc 
>     lib/libc/sys         Makefile.inc shm_open.2 
>     sys/compat/freebsd32 syscalls.master 
>     sys/conf             files 
>     sys/kern             kern_descrip.c syscalls.master 
>     sys/security/mac     mac_framework.h mac_policy.h 
>     sys/security/mac_stub mac_stub.c 
>     sys/security/mac_test mac_test.c 
>     sys/sys              fcntl.h file.h mman.h 
>     sys/vm               vm_mmap.c 
>   Added files:
>     sys/kern             uipc_shm.c 
>     sys/security/mac     mac_posix_shm.c 
>   Removed files:
>     lib/libc/gen         posixshm.c shm_open.3 
>   Log:
>   Add a new file descriptor type for IPC shared memory objects and use it to
>   implement shm_open(2) and shm_unlink(2) in the kernel:
>   - Each shared memory file descriptor is associated with a swap-backed vm
>     object which provides the backing store.  Each descriptor starts off with
>     a size of zero, but the size can be altered via ftruncate(2).  The shared
>     memory file descriptors also support fstat(2).  read(2), write(2),
>     ioctl(2), select(2), poll(2), and kevent(2) are not supported on shared
>     memory file descriptors.
>   - shm_open(2) and shm_unlink(2) are now implemented as system calls that
>     manage shared memory file descriptors.  The virtual namespace that maps
>     pathnames to shared memory file descriptors is implemented as a hash
>     table where the hash key is generated via the 32-bit Fowler/Noll/Vo hash
>     of the pathname.
>   - As an extension, the constant 'SHM_ANON' may be specified in place of the
>     path argument to shm_open(2).  In this case, an unnamed shared memory
>     file descriptor will be created similar to the IPC_PRIVATE key for
>     shmget(2).  Note that the shared memory object can still be shared among
>     processes by sharing the file descriptor via fork(2) or sendmsg(2), but
>     it is unnamed.  This effectively serves to implement the getmemfd() idea
>     bandied about the lists several times over the years.
>   - The backing store for shared memory file descriptors are garbage
>     collected when they are not referenced by any open file descriptors or
>     the shm_open(2) virtual namespace.
>   
>   Submitted by:   dillon, peter (previous versions)
>   Submitted by:   rwatson (I based this on his version)
>   Reviewed by:    alc (suggested converting getmemfd() to shm_open())

Some ABI compat stuff I forgot to mention:

1) Even though shm_open() and shm_unlink() are now syscalls instead of
   functions in libc, the ABI actually should not change since syscalls
   also show up as functions in libc and they take the same arguments
   now that they did before.
2) Programs compiled with older libcs are still going to use files to
   back shm_open() instead of the new system calls.  To that end, the
   flag shm_open() uses to flag a file as a shm fd for special mmap()
   treatment (FPOSIXSHM) is still supported if any of COMPAT_FREEBSD[4567].
3) As a better fix for the above I plan to teach the shm_open() and
   shm_unlink() functions in libc in RELENG_[4567] to use these new
   system calls if kern.feature.posix_shm is present and enabled so that
   older dynamically linked binaries will have the same behavior for
   shm_open() and shm_unlink().  This will leave only old static binaries
   that using shm_open() and shm_unlink() in the situation that they
   use different backing store for the same shared memory object.

-- 
John Baldwin