linux-kernel - Re: [PATCH] lib: test_meminit: fix -Wmaybe-uninitialized false positive

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Date:   Mon, 17 Jun 2019 16:22:19 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Christoph Lameter <cl@...ux.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Sandeep Patil <sspatil@...roid.com>,
        Laura Abbott <labbott@...hat.com>,
        Jann Horn <jannh@...gle.com>, Marco Elver <elver@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib: test_meminit: fix -Wmaybe-uninitialized false positive

On Mon, Jun 17, 2019 at 3:12 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> The conditional logic is too complicated for the compiler to
> fully comprehend:
>
> lib/test_meminit.c: In function 'test_meminit_init':
> lib/test_meminit.c:236:5: error: 'buf_copy' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      kfree(buf_copy);
>      ^~~~~~~~~~~~~~~
> lib/test_meminit.c:201:14: note: 'buf_copy' was declared here
>
> Simplify it by splitting out the non-rcu section.
>
> Fixes: af734ee6ec85 ("lib: introduce test_meminit module")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
Acked-by: Alexander Potapenko <glider@...gle.com>
> ---
>  lib/test_meminit.c | 50 ++++++++++++++++++++++++----------------------
>  1 file changed, 26 insertions(+), 24 deletions(-)
>
> diff --git a/lib/test_meminit.c b/lib/test_meminit.c
> index ed7efec1387b..7ae2183ff1f4 100644
> --- a/lib/test_meminit.c
> +++ b/lib/test_meminit.c
> @@ -208,35 +208,37 @@ static int __init do_kmem_cache_size(size_t size, bool want_ctor,
>                 /* Check that buf is zeroed, if it must be. */
>                 fail = check_buf(buf, size, want_ctor, want_rcu, want_zero);
>                 fill_with_garbage_skip(buf, size, want_ctor ? CTOR_BYTES : 0);
> +
> +               if (!want_rcu) {
> +                       kmem_cache_free(c, buf);
> +                       continue;
> +               }
> +
>                 /*
>                  * If this is an RCU cache, use a critical section to ensure we
>                  * can touch objects after they're freed.
>                  */
> -               if (want_rcu) {
> -                       rcu_read_lock();
> -                       /*
> -                        * Copy the buffer to check that it's not wiped on
> -                        * free().
> -                        */
> -                       buf_copy = kmalloc(size, GFP_KERNEL);
> -                       if (buf_copy)
> -                               memcpy(buf_copy, buf, size);
> -               }
> -               kmem_cache_free(c, buf);
> -               if (want_rcu) {
> -                       /*
> -                        * Check that |buf| is intact after kmem_cache_free().
> -                        * |want_zero| is false, because we wrote garbage to
> -                        * the buffer already.
> -                        */
> -                       fail |= check_buf(buf, size, want_ctor, want_rcu,
> -                                         false);
> -                       if (buf_copy) {
> -                               fail |= (bool)memcmp(buf, buf_copy, size);
> -                               kfree(buf_copy);
> -                       }
> -                       rcu_read_unlock();
> +               rcu_read_lock();
> +               /*
> +                * Copy the buffer to check that it's not wiped on
> +                * free().
> +                */
> +               buf_copy = kmalloc(size, GFP_KERNEL);
> +               if (buf_copy)
> +                       memcpy(buf_copy, buf, size);
> +
> +               /*
> +                * Check that |buf| is intact after kmem_cache_free().
> +                * |want_zero| is false, because we wrote garbage to
> +                * the buffer already.
> +                */
> +               fail |= check_buf(buf, size, want_ctor, want_rcu,
> +                                 false);
> +               if (buf_copy) {
> +                       fail |= (bool)memcmp(buf, buf_copy, size);
> +                       kfree(buf_copy);
>                 }
> +               rcu_read_unlock();
>         }
>         kmem_cache_destroy(c);
>
> --
> 2.20.0
>


-- 
Alexander Potapenko
Software Engineer

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