selftests/sgx: Improve error detection and messages

The SGX device file (/dev/sgx_enclave) is unusual in that it requires
execute permissions.  It has to be both "chmod +x" *and* be on a
filesystem without 'noexec'.

In the future, udev and systemd should get updates to set up systems
automatically.  But, for now, nobody's systems do this automatically,
and everybody gets error messages like this when running ./test_sgx:

	0x0000000000000000 0x0000000000002000 0x03
	0x0000000000002000 0x0000000000001000 0x05
	0x0000000000003000 0x0000000000003000 0x03
	mmap() failed, errno=1.

That isn't very user friendly, even for forgetful kernel developers.

Further, the test case is rather haphazard about its use of fprintf()
versus perror().

Improve the error messages.  Use perror() where possible.  Lastly,
do some sanity checks on opening and mmap()ing the device file so
that we can get a decent error message out to the user.

Now, if your user doesn't have permission, you'll get the following:

	$ ls -l /dev/sgx_enclave
	crw------- 1 root root 10, 126 Mar 18 11:29 /dev/sgx_enclave
	$ ./test_sgx
	Unable to open /dev/sgx_enclave: Permission denied

If you then 'chown dave:dave /dev/sgx_enclave' (or whatever), but
you leave execute permissions off, you'll get:

	$ ls -l /dev/sgx_enclave
	crw------- 1 dave dave 10, 126 Mar 18 11:29 /dev/sgx_enclave
	$ ./test_sgx
	no execute permissions on device file

If you fix that with "chmod ug+x /dev/sgx" but you leave /dev as
noexec, you'll get this:

	$ mount | grep "/dev .*noexec"
	udev on /dev type devtmpfs (rw,nosuid,noexec,...)
	$ ./test_sgx
	ERROR: mmap for exec: Operation not permitted
	mmap() succeeded for PROT_READ, but failed for PROT_EXEC
	check that user has execute permissions on /dev/sgx_enclave and
	that /dev does not have noexec set: 'mount | grep "/dev .*noexec"'

That can be fixed with:

	mount -o remount,noexec /devESC

Hopefully, the combination of better error messages and the search
engines indexing this message will help people fix their systems
until we do this properly.

 [ bp: Improve error messages more. ]

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20210318194301.11D9A984@viggo.jf.intel.com