[PATCH v2 5.10.y] net/ipv4: fix type mismatch in inet_ehash_locks_alloc() causing build failure

[PATCH v2 5.10.y] net/ipv4: fix type mismatch in inet_ehash_locks_alloc() causing build failure

[Eliav Farber]

Fix compilation warning:

In file included from ./include/linux/kernel.h:15,
                 from ./include/linux/list.h:9,
                 from ./include/linux/module.h:12,
                 from net/ipv4/inet_hashtables.c:12:
net/ipv4/inet_hashtables.c: In function ‘inet_ehash_locks_alloc’:
./include/linux/minmax.h:20:35: warning: comparison of distinct pointer types lacks a cast
   20 |         (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
      |                                   ^~
./include/linux/minmax.h:26:18: note: in expansion of macro ‘__typecheck’
   26 |                 (__typecheck(x, y) && __no_side_effects(x, y))
      |                  ^~~~~~~~~~~
./include/linux/minmax.h:36:31: note: in expansion of macro ‘__safe_cmp’
   36 |         __builtin_choose_expr(__safe_cmp(x, y), \
      |                               ^~~~~~~~~~
./include/linux/minmax.h:52:25: note: in expansion of macro ‘__careful_cmp’
   52 | #define max(x, y)       __careful_cmp(x, y, >)
      |                         ^~~~~~~~~~~~~
net/ipv4/inet_hashtables.c:946:19: note: in expansion of macro ‘max’
  946 |         nblocks = max(nblocks, num_online_nodes() * PAGE_SIZE / locksz);
      |                   ^~~
  CC      block/badblocks.o

When warnings are treated as errors, this causes the build to fail.

The issue is a type mismatch between the operands passed to the max()
macro. Here, nblocks is an unsigned int, while the expression
num_online_nodes() * PAGE_SIZE / locksz is promoted to unsigned long.

This happens because:
 - num_online_nodes() returns int
 - PAGE_SIZE is typically defined as an unsigned long (depending on the
   architecture)
 - locksz is unsigned int

The resulting arithmetic expression is promoted to unsigned long.

Thus, the max() macro compares values of different types: unsigned int
vs unsigned long.

This issue was introduced in commit f8ece40786c9 ("tcp: bring back NUMA
dispersion in inet_ehash_locks_alloc()") during the update from kernel
v5.10.237 to v5.10.238.

It does not exist in newer kernel branches (e.g., v5.15.185 and all 6.x
branches), because they include commit d03eba99f5bf ("minmax: allow
min()/max()/clamp() if the arguments have the same signedness.")

Fix the issue by using max_t(unsigned int, ...) to explicitly cast both
operands to the same type, avoiding the type mismatch and ensuring
correctness.

Fixes: f8ece40786c9 ("tcp: bring back NUMA dispersion in inet_ehash_locks_alloc()")
Signed-off-by: Eliav Farber <farbere@amazon.com>
---
V1 -> V2: Use upstream commit SHA1 in reference
 net/ipv4/inet_hashtables.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index fea74ab2a4be..ac2d185c04ef 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -943,7 +943,7 @@ int inet_ehash_locks_alloc(struct inet_hashinfo *hashinfo)
 	nblocks = max(2U * L1_CACHE_BYTES / locksz, 1U) * num_possible_cpus();
 
 	/* At least one page per NUMA node. */
-	nblocks = max(nblocks, num_online_nodes() * PAGE_SIZE / locksz);
+	nblocks = max_t(unsigned int, nblocks, num_online_nodes() * PAGE_SIZE / locksz);
 
 	nblocks = roundup_pow_of_two(nblocks);
 
-- 
2.47.1