Re: t1450-fsck (sometimes/often) failes on Mac OS X

From: Thomas Rast <trast@inf.ethz.ch>
Date: Jul 28, 2012, 16:00 UTC
Message-ID: <87hasrdgej.fsf@thomas.inf.ethz.ch>
In-Reply-To: <20120728154320.GA98893@book.hvoigt.net>

Heiko Voigt <hvoigt@hvoigt.net> writes:

Show 5 quoted lines

>         if (!git_index_file) {
> -               git_index_file = xmalloc(strlen(git_dir) + 7);
> +               git_index_file = xmalloc(strlen(git_dir) + 7 + 8);
>                 sprintf(git_index_file, "%s/index", git_dir);
>         }

[...]

> -       if (!memcmp(ent->base, objdir, pfxlen)) {
> +       objdirlen = strlen(objdir);
> +       if (!memcmp(ent->base, objdir, pfxlen > objdirlen ? objdirlen : pfxlen)) {

[...]

Show 5 quoted lines

> Initialized empty Git repository in /Users/hvoigt/Repository/git/t/trash directory.t1450-fsck/another/.git/
> ==42686== Invalid read of size 8
> ==42686==    at 0x100625064: bcmp (in /usr/lib/libSystem.B.dylib)
> ==42686==    by 0x100112846: link_alt_odb_entries (in /Users/hvoigt/Repository/git/t/valgrind/../../git)
> ==42686==    by 0x1001129C0: read_info_alternates (in /Users/hvoigt/Repository/git/t/valgrind/../../git)

[...]

> ==42686==  Address 0x100faca78 is 8 bytes inside a block of size 13 alloc'd
> ==42686==    at 0x10029C679: malloc (vg_replace_malloc.c:266)
> ==42686==    by 0x1001349CD: xmalloc (in /Users/hvoigt/Repository/git/t/valgrind/../../git)
> ==42686==    by 0x1000C23F5: setup_git_env (in /Users/hvoigt/Repository/git/t/valgrind/../../git)

To me that looks just like a false positive. memcmp (which seems to be the same as bcmp) can load 8 bytes from an aligned address even if these are only partially within the block being compared, since an aligned load can never partially fault (it must all be within the same page). Valgrind normally substitutes its own routines for memcmp etc. to correctly handle this, but this does not seem to happen in your case for some reason.

Then again I am not entirely sure how you could verify that this theory is correct :-)

-- 
Thomas Rast
trast@{inf,student}.ethz.ch