Re: libselinux APIs should take "const" qualifier?

[KaiGai Kohei <kaigai@ak.jp.nec.com>]

[-- Attachment #1: Type: text/plain, Size: 3067 bytes --]

(2010/03/19 22:32), Stephen Smalley wrote:
> On Fri, 2010-03-19 at 16:52 +0900, KaiGai Kohei wrote:
>> Right now, security_context_t is an alias of char *, declared in selinux.h.
>>
>> Various kind of libselinux API takes security_context_t arguments,
>> however, it is inconvenience in several situations.
>>
>> For example, the following query is parsed, then delivered to access
>> control subsystem with the security context as "const char *" cstring.
>>
>>    ALTER TABLE my_tbl SECURITY LABEL TO 'system_u:object_r:sepgsql_table_t:SystemHigh';
>>                  const char *<----    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>> In this case, we want to call selinux_trans_to_raw_context() to translate
>> the given security context into raw format. But it takes security_context_t
>> argument for the source context, although this pointer is read-only.
>> In the result, compiler raises warnings because we gave "const char *" pointer
>> into functions which take security_context_t (= char *).
>>
>> Any comments?
>>
>> It seems to me the following functions' prototype should be qualified by
>> "const".
> 
> That seems reasonable and should have no impact on library ABI.
> On the other hand, others have pointed out that security_context_t is
> not a properly encapsulated data type at all, and perhaps should be
> deprecated and replaced with direct use of char*/const char* throughout.
> 
> There are other library API issues as well that have come up in the
> past, such as lack of adequate namespacing (with approaches put forth),
> but we don't ever seem to get a round tuit.

At first, I tried to add const qualifiers read-only security_context_t
pointers, but didn't replace them by char */const char * yet, right now.

BTW, I could find out the following code:

  int security_compute_create(security_context_t scon,
                              security_context_t tcon,
                              security_class_t tclass,
                              security_context_t * newcon)
  {
          int ret;
          security_context_t rscon = scon;
          security_context_t rtcon = tcon;
          security_context_t rnewcon;

          if (selinux_trans_to_raw_context(scon, &rscon))
                  return -1;
          if (selinux_trans_to_raw_context(tcon, &rtcon)) {
                  freecon(rscon);
                  return -1;
          }
      :

In this case, scon and tcon can be qualified by const, and the first
argument of selinux_trans_to_raw_context() can take const pointer.
But it tries to initialize rscon and tscon by const pointer, although
these are used to store raw security contexts.
The selinux_trans_to_raw_context() always set dynamically allocated
text string on the second argument, so we don't need to initialize it
anyway. I also removed these initializations in this patch.

Does the older mcstrans code could return without allocation of raw
format when the given scon is already raw format? I don't know why
these are initialized in this manner.

Thanks.
-- 
KaiGai Kohei <kaigai@ak.jp.nec.com>

[-- Attachment #2: libselinux-const-qualify.1.patch --]
[-- Type: application/octect-stream, Size: 28020 bytes --]