Re: Shared Memory Access Problem

[Bob Furber <burnsmicro at gmail dot com>]

On 12-09-07 04:08 PM, Jonathan Wakely wrote:
> On 8 September 2012 00:03, Bob Furber wrote:
> > Why is the client capable of accessing shared memory when executed by root
> > and not able to, when executed by a user?
>
> This doesn't sound like it has anything to do with GCC.

After a lot of experimentation, I think the problem lies in shm_open().

The following code generates a /dev/shm/shared with -rw-r--r-- access 
privileges, making it inaccessible to others, even though the mode is 
set to rw-rw-rw-:

shmfd = shm_open( shm_name,	// shared memory block with this "/name"
		  O_CREAT,	// Create if not already there
		  0666 );	// with these access privileges

Same for:

shmfd = shm_open( shm_name,	// shared memory block with this "/name"
		  O_CREAT,	// Create if not already there
		| O_RDWR,	// allow read and write access
		  S_IRWXU	// 00700 user has rwx privileges
		| S_IRWXG	// 00070 group has rwx privileges
		| S_IRWXO );	// 00007 others has rwx privileges


However, if this is followed by:

fchmod( shmfd,
	  O_CREAT	// Create if not already there
	| O_RDWR	// allow read and write access
	| S_IRWXU	// 00700 user (file owner) has rwx privileges
	| S_IRWXG	// 00070 group (file owner) has rwx privileges
	| S_IRWXO );	// 00007 others has rwx privileges

/dev/shm/shared ends up with rw-rw-rw- privileges (since it is not 
executable).

smget() was more difficult to troubleshoot because it does not produce a 
file descriptor and I could not find the corresponding file to check its 
access privileges.

RF