netdev - [PATCH net-next 13/15] ice: Replace one-element array with flexible-array member

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Date:   Thu, 28 Jan 2021 16:43:30 -0800
From:   Tony Nguyen <anthony.l.nguyen@...el.com>
To:     davem@...emloft.net, kuba@...nel.org
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        netdev@...r.kernel.org, sassmann@...hat.com,
        anthony.l.nguyen@...el.com, kernel test robot <lkp@...el.com>,
        Tony Brelinski <tonyx.brelinski@...el.com>
Subject: [PATCH net-next 13/15] ice: Replace one-element array with flexible-array member

From: "Gustavo A. R. Silva" <gustavoars@...nel.org>

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct ice_res_tracker, instead of a one-element array and use the
struct_size() helper to calculate the size for the allocations.

Also, notice that the code below suggests that, currently, two too many
bytes are being allocated with devm_kzalloc(), as the total number of
entries (pf->irq_tracker->num_entries) for pf->irq_tracker->list[] is
_vectors_ and sizeof(*pf->irq_tracker) also includes the size of the
one-element array _list_ in struct ice_res_tracker.

drivers/net/ethernet/intel/ice/ice_main.c:3511:
3511         /* populate SW interrupts pool with number of OS granted IRQs. */
3512         pf->num_avail_sw_msix = (u16)vectors;
3513         pf->irq_tracker->num_entries = (u16)vectors;
3514         pf->irq_tracker->end = pf->irq_tracker->num_entries;

With this change, the right amount of dynamic memory is now allocated
because, contrary to one-element arrays which occupy at least as much
space as a single object of the type, flexible-array members don't
occupy such space in the containing structure.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays

Built-tested-by: kernel test robot <lkp@...el.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
Tested-by: Tony Brelinski <tonyx.brelinski@...el.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@...el.com>
---
 drivers/net/ethernet/intel/ice/ice.h      | 2 +-
 drivers/net/ethernet/intel/ice/ice_main.c | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/intel/ice/ice.h b/drivers/net/ethernet/intel/ice/ice.h
index 56725356a17b..45bbae68b014 100644
--- a/drivers/net/ethernet/intel/ice/ice.h
+++ b/drivers/net/ethernet/intel/ice/ice.h
@@ -164,7 +164,7 @@ struct ice_tc_cfg {
 struct ice_res_tracker {
 	u16 num_entries;
 	u16 end;
-	u16 list[1];
+	u16 list[];
 };

 struct ice_qs_cfg {
diff --git a/drivers/net/ethernet/intel/ice/ice_main.c b/drivers/net/ethernet/intel/ice/ice_main.c
index 5219aa70b530..ff6190bffff6 100644
--- a/drivers/net/ethernet/intel/ice/ice_main.c
+++ b/drivers/net/ethernet/intel/ice/ice_main.c
@@ -3499,9 +3499,9 @@ static int ice_init_interrupt_scheme(struct ice_pf *pf)
 		return vectors;

 	/* set up vector assignment tracking */
-	pf->irq_tracker =
-		devm_kzalloc(ice_pf_to_dev(pf), sizeof(*pf->irq_tracker) +
-			     (sizeof(u16) * vectors), GFP_KERNEL);
+	pf->irq_tracker = devm_kzalloc(ice_pf_to_dev(pf),
+				       struct_size(pf->irq_tracker, list, vectors),
+				       GFP_KERNEL);
 	if (!pf->irq_tracker) {
 		ice_dis_msix(pf);
 		return -ENOMEM;
-- 
2.26.2