Re: [Boost-users] [Interprocess] Shared Mutex - Permission Denied

Subject: Re: [Boost-users] [Interprocess] Shared Mutex - Permission Denied
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2010-07-06 16:40:20

On 06/07/2010 16:35, Matt Cupp wrote:
> From:
> http://groups.google.com/group/boost-list/browse_thread/thread/fc728b9d416c62e9/455055de23e177ad
>>  Interprocess has this embarrasing permission limitations, but I still
>  > don't know how to fix it.
>
> Does the permissions limitation apply to Linux (CentOS) and shared
> mutexes or segments created by other users or root?
>
> I have a mutex in the /dev/shm folder created by another user with these
> permissions (from ls -l):
> -rwxr-xr-x 1 webers users        160 Jul  1 15:24 Lookup_Mutex

In theory shm_open is called with these persissions:

www.boost.org/boost/interprocess/shared_memory_object.hpp

m_handle = shm_open(m_filename.c_str(), oflag, S_IRWXO | S_IRWXG | S_IRWXU);

But shm_open final permissions are dependent, just like open(), on 
umask(). Since by default umask() is 022, then you got that. Change 
umask() (a thread-unsafe and global function, by the way), so that you 
can get all the permissions. From posix:

"The permission bits of the shared memory object shall be set to the 
value of the mode argument except those set in the file mode creation 
mask of the process. When bits in mode other than the file permission 
bits are set, the effect is unspecified. The mode argument does not 
affect whether the shared memory object is opened for reading, for 
writing, or for both."

You can use fchmod after shm_open to change effectively permissions 
after the object is created:

#include <sys/stat.h>

int fchmod(int fildes, mode_t mode);

I will probably add this fchmod call in the future.

Hope this helps,

Ion