netdev - Re: [PATCH] net/ipv4: fix type mismatch in inet_ehash_locks

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Message-ID: <20250608201227.3970666-1-kuni1840@gmail.com>
Date: Sun,  8 Jun 2025 13:11:51 -0700
From: Kuniyuki Iwashima <kuni1840@...il.com>
To: farbere@...zon.com
Cc: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	kuniyu@...zon.com,
	kuznet@....inr.ac.ru,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	sashal@...nel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH] net/ipv4: fix type mismatch in inet_ehash_locks_alloc() causing build failure

From: Eliav Farber <farbere@...zon.com>
Date: Sun, 8 Jun 2025 06:07:26 +0000
> 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 b53d6e9525af ("tcp: bring back NUMA
> dispersion in inet_ehash_locks_alloc()") during the update from kernel
> v5.10.237 to v5.10.238.

Please use the upstream SHA1, f8ece40786c9.

> 
> It does not exist in newer kernel branches (e.g., v5.15.185 and all 6.x
> branches), because they include commit d53b5d862acd ("minmax: allow

Same here, d03eba99f5bf.

But why not backport it to stable instead ?

In the first place, f8ece40786c9 does not have Fixes: and seems
to be cherry-picked accidentally by AUTOSEL.


> 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.

The cover letter of d03eba99f5bf says:
https://lore.kernel.org/all/b97faef60ad24922b530241c5d7c933c@AcuMS.aculab.com/

---8<---
The min() (etc) functions in minmax.h require that the arguments have
exactly the same types.

However when the type check fails, rather than look at the types and
fix the type of a variable/constant, everyone seems to jump on min_t().
In reality min_t() ought to be rare - when something unusual is being
done, not normality.
---8<---

So, the typecheck variant should be rare, and it was merged 2 years ago,
so there should be more places depending on the commit.

Once someone backported such a change to stable trees again and required
this type of "fix", it would revert the less-typecheck effor gradually,
which should be avoided.


> 
> Fixes: b53d6e9525af ("tcp: bring back NUMA dispersion in inet_ehash_locks_alloc()")
> Signed-off-by: Eliav Farber <farbere@...zon.com>
> ---
>  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