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Re: grp/grp_merge.c alignment fix
- From: Carlos O'Donell <carlos at redhat dot com>
- To: DJ Delorie <dj at redhat dot com>, libc-alpha at sourceware dot org
- Date: Fri, 14 Jul 2017 15:58:06 -0400
- Subject: Re: grp/grp_merge.c alignment fix
- Authentication-results: sourceware.org; auth=none
- References: <xn7ezbx3ao.fsf@greed.delorie.com>
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];
>
--
Cheers,
Carlos.