[PATCH v2 1/1] ethdev: fix int overflow in descriptor count logic

[Niall Meade <niall.meade@intel.com>]

Addressed a specific overflow issue in the eth_dev_adjust_nb_desc()
function where the uint16_t variable nb_desc would overflow when its
value was greater than (2^16 - nb_align). This overflow caused nb_desc
to incorrectly wrap around between 0 and nb_align-1, leading to the
function setting nb_desc to nb_min instead of the expected nb_max.

To give an example, let nb_desc=UINT16_MAX, nb_align=32, nb_max=4096 and
nb_min=64. RTE_ALIGN_CEIL(nb_desc, nb_align) calls
RTE_ALIGN_FLOOR(nb_desc + nb_align - 1, nb_align). This results in an
overflow of nb_desc, leading to nb_desc being set to 30 and then 0 when
the macros return. As a result of this, nb_desc is then set to nb_min
later on.

The resolution involves upcasting nb_desc to a uint32_t before the
RTE_ALIGN_CEIL macro is applied. This change ensures that the subsequent
call to RTE_ALIGN_FLOOR(nb_desc + (nb_align - 1), nb_align) does not
result in an overflow, as it would when nb_desc is a uint16_t. By using
a uint32_t for these operations, the correct behavior is maintained
without the risk of overflow.

Fixes: 0f67fc3baeb9 ("ethdev: add function to adjust number of descriptors")

Signed-off-by: Niall Meade <niall.meade@intel.com>

---
v2:
* add example of issue to commit message
---
 .mailmap                |  1 +
 lib/ethdev/rte_ethdev.c | 12 +++++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/.mailmap b/.mailmap
index 4a508bafad..c1941e78bb 100644
--- a/.mailmap
+++ b/.mailmap
@@ -1053,6 +1053,7 @@ Nelson Escobar <neescoba@cisco.com>
 Nemanja Marjanovic <nemanja.marjanovic@intel.com>
 Netanel Belgazal <netanel@amazon.com>
 Netanel Gonen <netanelg@mellanox.com>
+Niall Meade <niall.meade@intel.com>
 Niall Power <niall.power@intel.com>
 Nicholas Pratte <npratte@iol.unh.edu>
 Nick Connolly <nick.connolly@arm.com> <nick.connolly@mayadata.io>
diff --git a/lib/ethdev/rte_ethdev.c b/lib/ethdev/rte_ethdev.c
index f1c658f49e..f978283edf 100644
--- a/lib/ethdev/rte_ethdev.c
+++ b/lib/ethdev/rte_ethdev.c
@@ -6577,13 +6577,19 @@ static void
 eth_dev_adjust_nb_desc(uint16_t *nb_desc,
 		const struct rte_eth_desc_lim *desc_lim)
 {
+	/* Upcast to uint32 to avoid potential overflow with RTE_ALIGN_CEIL(). */
+	uint32_t nb_desc_32 = *nb_desc;
+
 	if (desc_lim->nb_align != 0)
-		*nb_desc = RTE_ALIGN_CEIL(*nb_desc, desc_lim->nb_align);
+		nb_desc_32 = RTE_ALIGN_CEIL(nb_desc_32, desc_lim->nb_align);
 
 	if (desc_lim->nb_max != 0)
-		*nb_desc = RTE_MIN(*nb_desc, desc_lim->nb_max);
+		nb_desc_32 = RTE_MIN(nb_desc_32, desc_lim->nb_max);
+
+	nb_desc_32 = RTE_MAX(nb_desc_32, desc_lim->nb_min);
 
-	*nb_desc = RTE_MAX(*nb_desc, desc_lim->nb_min);
+	/* Assign clipped u32 back to u16. */
+	*nb_desc = nb_desc_32;
 }
 
 int
-- 
2.34.1

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