RDMA/hns: dead empty check on head->root in setup_root_hem?

[Maoyi Xie]

Hi all,

While auditing list_first_entry callsites, I noticed a place in
drivers/infiniband/hw/hns/hns_roce_hem.c where the developer
wrote a NULL check for an empty list but used the unsafe API.
The check is dead code. I would appreciate it if you could take
a look and let me know whether this is worth fixing.

The site is setup_root_hem() (linux-7.1-rc1, around line 1270):

    root_hem = list_first_entry(&head->root,
                                struct hns_roce_hem_item, list);
    if (!root_hem)
            return -ENOMEM;

    total = 0;
    for (i = 0; i < region_cnt && total <= max_ba_num; i++) {
            ...
            cpu_base = root_hem->addr + total * BA_BYTE_LEN;
            ...

list_first_entry() returns container_of(&head->root, struct
hns_roce_hem_item, list) when head->root is empty, never NULL.
The -ENOMEM early return is dead code.

With an empty head->root, the fall through pointer aliases
&head->root inside struct hns_roce_hem_head. root_hem->addr is
then read from memory at a fixed offset inside that struct,
producing a garbage DMA base address that is then handed to
the hardware.

head->root is empty if the caller fails to allocate and add the
root item before calling setup_root_hem. A future caller could
also miss that precondition.

A candidate fix is a one liner. Switch to list_first_entry_or_null
so the -ENOMEM early return runs.

Similar dead empty checks after list_first_entry have been
cleaned up in the same shape, for example commit fbb8bc408027
(net: qed: Remove redundant NULL checks after list_first_entry),
commit c708d3fad421 (crypto: atmel: use list_first_entry_or_null
to simplify find_dev) and commit 10379171f346 (ksmbd: use
list_first_entry_or_null for opinfo_get_list). The qed commit
message describes the exact shape we observe here. This hns
site appears to be missed by those cleanups.

If this is intentional or already known, please disregard.
Otherwise I am happy to send a [PATCH] or to leave the fix to
you.

Thanks,
Maoyi Xie
https://maoyixie.com/