Calling functions through a pointer of incorrect type

Calling functions through a pointer of incorrect type

[Florian Weimer]

Just a sanity check: It is no longer acceptable glibc coding style to
call functions through mismatching pointer types.  Correct?

Typical mismatches are: The function has an argument of type struct foo
**, but the function pointer has type void ** for the same argument.  Or
the function pointer has additional trailing arguments not declared by
the function.

These calls result in undefined behavior in standard C, and I don't
think it's a support GCC extension.

Florian

Re: Calling functions through a pointer of incorrect type

[Joseph Myers]

On Tue, 22 Mar 2016, Florian Weimer wrote:

> Just a sanity check: It is no longer acceptable glibc coding style to
> call functions through mismatching pointer types.  Correct?

I think more specific examples would help so we can see if there are 
cleaner approaches that generate the same code.  (For example, it's 
deliberate that in some places functions with incompatible C types but 
compatible ABIs are aliased to each other.)

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: Calling functions through a pointer of incorrect type

[Florian Weimer]

On 03/22/2016 08:14 PM, Joseph Myers wrote:
> On Tue, 22 Mar 2016, Florian Weimer wrote:
> 
>> Just a sanity check: It is no longer acceptable glibc coding style to
>> call functions through mismatching pointer types.  Correct?
> 
> I think more specific examples would help so we can see if there are 
> cleaner approaches that generate the same code.  (For example, it's 
> deliberate that in some places functions with incompatible C types but 
> compatible ABIs are aliased to each other.)

I saw this in getaddrinfo:

  if (fct == NULL)
    /* We are cheating here.  The gethostbyname2_r
       function does not have the same interface as
       gethostbyname3_r but the extra arguments the
       latter takes are added at the end.  So the
       gethostbyname2_r code will just ignore them.  */
    fct = __nss_lookup_function (nip, "gethostbyname2_r");

The void ** vs concrete pointer example is __nss_getent.

For __nss_getent, it wouldn't be too hard to split the buffer management
from the call, and move the call to the caller of __nss_getent, where
the correct type is known.

getaddrinfo may be harder and involve splitting the code around the caller.

Florian

Re: Calling functions through a pointer of incorrect type

[Joseph Myers]

On Tue, 22 Mar 2016, Florian Weimer wrote:

> On 03/22/2016 08:14 PM, Joseph Myers wrote:
> > On Tue, 22 Mar 2016, Florian Weimer wrote:
> > 
> >> Just a sanity check: It is no longer acceptable glibc coding style to
> >> call functions through mismatching pointer types.  Correct?
> > 
> > I think more specific examples would help so we can see if there are 
> > cleaner approaches that generate the same code.  (For example, it's 
> > deliberate that in some places functions with incompatible C types but 
> > compatible ABIs are aliased to each other.)
> 
> I saw this in getaddrinfo:
> 
>   if (fct == NULL)
>     /* We are cheating here.  The gethostbyname2_r
>        function does not have the same interface as
>        gethostbyname3_r but the extra arguments the
>        latter takes are added at the end.  So the
>        gethostbyname2_r code will just ignore them.  */
>     fct = __nss_lookup_function (nip, "gethostbyname2_r");

Well, glibc is written with knowledge of ABI properties - that is, objects 
can assume that other objects follow the ABI and so interact in ways that 
are not valid whole-program C but are valid given that each object is an 
ABI-conforming compilation of its own sources on their own.  So while 
cleaning up cases like this would seem desirable unless particularly 
performance-critical, and I wouldn't like to see new code like this 
without strong justification, they aren't inherently bugs if all supported 
ABIs have the required property.

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: Calling functions through a pointer of incorrect type

[Richard Henderson]

On 03/22/2016 02:33 PM, Joseph Myers wrote:
> On Tue, 22 Mar 2016, Florian Weimer wrote:
>>   if (fct == NULL)
>>     /* We are cheating here.  The gethostbyname2_r
>>        function does not have the same interface as
>>        gethostbyname3_r but the extra arguments the
>>        latter takes are added at the end.  So the
>>        gethostbyname2_r code will just ignore them.  */
>>     fct = __nss_lookup_function (nip, "gethostbyname2_r");
> 
> ... they aren't inherently bugs if all supported
> ABIs have the required property.

The only ABI I can think of that doesn't have this property is Windows
__stdcall.  Which obviously isn't used in any glibc configuration.


r~