Re: capable open_port() check wrong for kmem

[Chris Wright]

* carbonated beverage ([e-mail]) wrote:
> 
> It's rather annoying and counter-intuitive to have:
> 
> crw-r-----    1 root     kmem       1,   2 Sep  8 21:56 /dev/kmem
> 
> but to have the following code fragment give:
> 
> int fd;
> 	fd = open("/dev/kmem", O_RDONLY);
> 	if(fd == -1) {
> 		fprintf(stderr, "Can't open /dev/kmem: %s\n", strerror(errno));
> 		exit(EXIT_FAILURE);
> 	}
> 
> Can't open /dev/kmem: Operation not permitted
> 
> with a user in the kmem group.
> 
> Also, the utility I'm writing doesn't need write access, so why give it to
> the process in the first place?

Then open O_RDONLY (with CAP_SYS_RAWIO).  Then the utility won't have
write access.  If that's all you are worried about.

or.

If you have only one capability (CAP_SYS_RAWIO), you are not owner of
/dev/kmem, you are in group kmem, and /dev/kmem is 0640, then all you will
get is read-only access to /dev/kmem.  This does not require kernel changes.

thanks,
-chris
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