grp/grp_merge.c alignment fix

  Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=21654

I have a test for this in my nss_tests patch (posted separately),
which is a very large patch set (the individual test requires the new
framework) and thus I chose to keep it as a separate patch in case
this patch was accepted during the freeze and the testuite change was
deferred.

Tested on x86_64, where it fails the test I wrote before, and not
after, patching.

Note: I did not try to rewrite grp_merge.c using Florian's new
alloc_buffer code, as I felt that was inappropriate for this stage of
development (post-freeze).

	[BZ #21654]
	* grp/grp_merge.c (__copy_grp): Make sure pointers-to-not-char
	are properly aligned.
	(__merge_grp): Likewise.
  

On 07/13/2017 08:36 PM, DJ Delorie wrote:
> Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=21654
> 
> I have a test for this in my nss_tests patch (posted separately),
> which is a very large patch set (the individual test requires the new
> framework) and thus I chose to keep it as a separate patch in case
> this patch was accepted during the freeze and the testuite change was
> deferred.
> 
> Tested on x86_64, where it fails the test I wrote before, and not
> after, patching.
> 
> Note: I did not try to rewrite grp_merge.c using Florian's new
> alloc_buffer code, as I felt that was inappropriate for this stage of
> development (post-freeze).
> 
> 	[BZ #21654]
> 	* grp/grp_merge.c (__copy_grp): Make sure pointers-to-not-char
> 	are properly aligned.
> 	(__merge_grp): Likewise.

LGTM.

I expect the test cases I just reviewed form the basis of the regression
test for this fix, so this has to go in first otherwise s390x will show
a test regression.
 
> diff --git a/grp/grp-merge.c b/grp/grp-merge.c
> index 77c494d..d6a53cd 100644
> --- a/grp/grp-merge.c
> +++ b/grp/grp-merge.c
> @@ -85,6 +85,14 @@ __copy_grp (const struct group srcgrp, const size_t buflen,
>      }
>    members[i] = NULL;
>  
> +  /* Align for pointers.  We can't simply align C because we need to
> +     align destbuf[c].  */
> +  if (((uintptr_t)destbuf + c) & (__alignof__(char **) - 1))

Avoid boolean coercion. Compare to zero.

> +    {
> +      uintptr_t mis_align = ((uintptr_t)destbuf + c) & (__alignof__(char **) - 1);
> +      c += __alignof__(char **) - mis_align;

OK, align the value of c up such that it is properly aligned.

> +    }
> +
>    /* Copy the pointers from the members array into the buffer and assign them
>       to the gr_mem member of destgrp.  */
>    destgrp->gr_mem = (char **) &destbuf[c];
> @@ -168,6 +176,14 @@ __merge_grp (struct group *savedgrp, char *savedbuf, char *savedend,
>    /* Add the NULL-terminator.  */
>    members[savedmemcount + memcount] = NULL;
>  
> +  /* Align for pointers.  We can't simply align C because we need to
> +     align savedbuf[c].  */
> +  if (((uintptr_t)savedbuf + c) & (__alignof__(char **) - 1))
> +    {
> +      uintptr_t mis_align = ((uintptr_t)savedbuf + c) & (__alignof__(char **) - 1);
> +      c += __alignof__(char **) - mis_align;

OK.

> +    }
> +
>    /* Copy the member array back into the buffer after the member list and free
>       the member array.  */
>    savedgrp->gr_mem = (char **) &savedbuf[c];
>