linux-kernel - Re: [PATCH bpf-next v2] selftests/bpf: Fix htab_update/reenter

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Message-ID: <aRdkp7ztSM1JNZME@linux.ibm.com>
Date: Fri, 14 Nov 2025 22:49:35 +0530
From: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
To: bot+bpf-ci@...nel.org
Cc: bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, hbathini@...ux.ibm.com,
        sachinpb@...ux.ibm.com, venkat88@...ux.ibm.com, andrii@...nel.org,
        eddyz87@...il.com, ast@...nel.org, daniel@...earbox.net,
        martin.lau@...ux.dev, song@...nel.org, yonghong.song@...ux.dev,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
        haoluo@...gle.com, jolsa@...nel.org, shuah@...nel.org,
        martin.lau@...nel.org, clm@...a.com, ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v2] selftests/bpf: Fix
 htab_update/reenter_update selftest failure

On Fri, Nov 14, 2025 at 03:50:44PM +0000, bot+bpf-ci@...nel.org wrote:
> > diff --git a/tools/testing/selftests/bpf/prog_tests/htab_update.c b/tools/testing/selftests/bpf/prog_tests/htab_update.c
> > index 2bc85f481..96b65c1a3 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/htab_update.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/htab_update.c
> > @@ -15,17 +15,17 @@ struct htab_update_ctx {
> >  static void test_reenter_update(void)
> >  {
> >  	struct htab_update *skel;
> > -	unsigned int key, value;
> > +	void *value = NULL;
> > +	unsigned int key, value_size;
>                            ^^^
> 
> Is key supposed to be initialized? The old code had 'unsigned int key = 0,
> value = 0;' but the new code drops the initialization. Later uses of key
> at the bpf_map_update_elem() calls will pass uninitialized stack memory.
> 
> >  	int err;
> >
> >  	skel = htab_update__open();
> >  	if (!ASSERT_OK_PTR(skel, "htab_update__open"))
> >  		return;
> 
> [ ... ]
> 
> > @@ -33,14 +33,32 @@ static void test_reenter_update(void)
> >  	if (!ASSERT_OK(err, "htab_update__attach"))
> >  		goto out;
> >
> > -	/* Will trigger the reentrancy of bpf_map_update_elem() */
> > -	key = 0;
> > -	value = 0;
> > -	err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, 0);
> > -	if (!ASSERT_OK(err, "add element"))
> > +	value_size = bpf_map__value_size(skel->maps.htab);
> > +
> > +	value = calloc(1, value_size);
> > +	if (!ASSERT_OK_PTR(value, "calloc value"))
> > +		goto out;
> > +	/*
> > +	 * First update: plain insert. This should NOT trigger the re-entrancy
> > +	 * path, because there is no old element to free yet.
> > +	 */
> > +	err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, BPF_ANY);
> > +	if (!ASSERT_OK(err, "first update (insert)"))
> > +		goto out;
> > +
> > +	/*
> > +	 * Second update: replace existing element with same key and trigger
> > +	 * the reentrancy of bpf_map_update_elem().
> > +	 * check_and_free_fields() calls bpf_obj_free_fields() on the old
> > +	 * value, which is where fentry program runs and performs a nested
> > +	 * bpf_map_update_elem(), triggering -EDEADLK.
> > +	 */
> > +	memset(&value, 0, sizeof(value));
>                ^^^^^^^
> 
> Does this memset zero the allocated memory? It looks like this zeros the
> pointer variable itself (8 bytes) rather than the memory it points to.
> Should this be 'memset(value, 0, value_size)' instead?
> 
> > +	err = bpf_map_update_elem(bpf_map__fd(skel->maps.htab), &key, &value, BPF_ANY);
> > +	if (!ASSERT_OK(err, "second update (replace)"))
> >  		goto out;
> >
> > -	ASSERT_EQ(skel->bss->update_err, -EBUSY, "no reentrancy");
> > +	ASSERT_EQ(skel->bss->update_err, -EDEADLK, "no reentrancy");
> >  out:
> >  	htab_update__destroy(skel);
> >  }
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/19369517166
Will fix these.