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Subject: Re: [PATCH bpf-next] selftests/bpf: fix prog_tests/uprobe_autoattach
 compilation error
To:     Artem Savkov <asavkov@redhat.com>,
        Alan Maguire <alan.maguire@oracle.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>, bpf@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org
References: <20220421132317.1583867-1-asavkov@redhat.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <e5919342-0697-65f0-063f-4941e74fe1ca@iogearbox.net>
Date:   Thu, 21 Apr 2022 18:53:22 +0200
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On 4/21/22 3:23 PM, Artem Savkov wrote:
> I am getting the following compilation error for prog_tests/uprobe_autoattach.c
> 
> tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c: In function ‘test_uprobe_autoattach’:
> ./test_progs.h:209:26: error: pointer ‘mem’ may be used after ‘free’ [-Werror=use-after-free]
> 
> mem variable is now used in one of the asserts so it shouldn't be freed right
> away. Move free(mem) after the assert block.

Looks good, but I rephrased this a bit to avoid confusion. It's false positive given we
only compare the addresses but don't deref mem, which the compiler might not be able to
follow in this case.

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=6a12b8e20d7e72386594a9dbe7bf2d7fae3b3aa6

Thanks,
Daniel

> Fixes: 1717e248014c ("selftests/bpf: Uprobe tests should verify param/return values")
> Signed-off-by: Artem Savkov <asavkov@redhat.com>
> ---
>   tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c b/tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c
> index d6003dc8cc99..35b87c7ba5be 100644
> --- a/tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c
> +++ b/tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c
> @@ -34,7 +34,6 @@ void test_uprobe_autoattach(void)
>   
>   	/* trigger & validate shared library u[ret]probes attached by name */
>   	mem = malloc(malloc_sz);
> -	free(mem);
>   
>   	ASSERT_EQ(skel->bss->uprobe_byname_parm1, trigger_val, "check_uprobe_byname_parm1");
>   	ASSERT_EQ(skel->bss->uprobe_byname_ran, 1, "check_uprobe_byname_ran");
> @@ -44,6 +43,8 @@ void test_uprobe_autoattach(void)
>   	ASSERT_EQ(skel->bss->uprobe_byname2_ran, 3, "check_uprobe_byname2_ran");
>   	ASSERT_EQ(skel->bss->uretprobe_byname2_rc, mem, "check_uretprobe_byname2_rc");
>   	ASSERT_EQ(skel->bss->uretprobe_byname2_ran, 4, "check_uretprobe_byname2_ran");
> +
> +	free(mem);
>   cleanup:
>   	test_uprobe_autoattach__destroy(skel);
>   }
>