[krbdev.mit.edu #6308] Alignment problem in resolver test

[Ezra Peisach via RT]

 Great - thanks for the insight... I will try to code around it next week...

Ezra
Jorgen Wahlsten via RT wrote:
> Hello Ezra.
>
> Yes, I used a 32-bit compiled gcc 4.1.1.
>
> I do not have Sun's C compiler and can not verify that it is working with
> it, but if it's important for you to verify that it is working with Sun's C
> compiler, I can try to get it. Let me know.
>
> If I turn on optimization (-O2 or -O3) the program works, while using
> -O0/-O1 the program still core dumps.
>
> Compiling resolve.c as a 32-bit binary or a 64-bit binary does not seem to
> matter. It core dumps no matter what.
>
> I was thinking this had something to do with automatic variable alignment,
> and therefore changed:
>
> char myname[MAXHOSTNAMELEN+1]
>
> to
>
> char myname[MAXHOSTNAMELEN+8]
>
> (MAXHOSTNAMELEN is defined as 256 in netdb.h),
>
> which makes the program work again (not core dump).
>
> For what it's worth, I think Sun's C compiler *always* aligns the automatic
> variables on 8-byte boundaries, while GCC tries to fit the addrcopy[4] into
> the "padding" of myname[256+1] (7 bytes padding?), as an optimization. It
> does seem strange that this is done *without* optimization though.
>
> Regardless, of the differences between Sun's C compiler and GCC, I would be
> inclined to agree with you that this is a solaris C library bug, that is
> being triggered by using GCC in this particular case.
>
> As for my comment about using an in_addr pointer, I think I merely looked at
> the EXAMPLE section in the solaris 2.10 gethostbyaddr man page. There is
> also a NOTE in that man page about INET_ADDR.
>
> Let me know if you want to any additional information.
>
> Thanks,
> Jorgen
>
>
>
>
> On Sat, Dec 27, 2008 at 3:17 PM, Ezra Peisach via RT <
> rt-comment at krbdev.mit.edu> wrote:
>
>   
>> Okay - I have been able to reproduce the problem on one MIT's solaris
>> machines - but only when using gcc....
>>
>> This is gcc version 3.4.3 on a machine running  Solaris 10 3/05
>> s10_74L2a SPARC.
>>
>> I modified the code to allow me to vary the alignment of the memory.
>> Sure enough, gethostbyaddr segfaults in the nss library.
>>
>> Now - why is this only w/ gcc and not sunsoft's compiler - I could not
>> tell you.
>>
>> Looking at the opensolaris library source for libnss - I do see that
>> there is a path where the pointer to the char * is assigned a struct
>> in_addr *.
>>
>> See IN6_INADDR_TO_V4MAPPED macro in /usr/include/netinet/in.h.
>>
>> The error does not appear to be an optimization issue...
>>
>> Now - why is gcc screwing up? I will need to investigate... I will have
>> to look at the assembly code to see what is going on....
>>
>> So - can you please confirm that your problem is observed w/ gcc?
>> Ezra
>>
>>     
>
>
>
>