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

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Message-ID: <20250608203054.3982608-1-kuni1840@gmail.com>
Date: Sun,  8 Jun 2025 13:30:18 -0700
From: Kuniyuki Iwashima <kuni1840@...il.com>
To: kuni1840@...il.com
Cc: davem@...emloft.net,
	edumazet@...gle.com,
	farbere@...zon.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: Kuniyuki Iwashima <kuni1840@...il.com>
Date: Sun,  8 Jun 2025 13:11:51 -0700
> 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 ?

I just checked the 5.10.238 thread.
https://lore.kernel.org/stable/2025060412-cursor-navigate-126d@gregkh/

---8<---
> > For both of these, I'll just let them be as they are ok, it's just the
> > mess of our min/max macro unwinding causes these issues.
> > 
> > Unless they really bother someone, and in that case, a patch to add the
> > correct type to the backport to make the noise go away would be greatly
> > appreciated.
> 
> Yeah that's a reasonable resolution, I will try to track down the missing
> patches for minmax.h so we are warning free for the stable kernels.

I tried in the past, it's non-trivial.  What would be easier is to just
properly cast the variables in the places where this warning is showing
up to get rid of that warning.  We've done that in some backports in the
past as well.
---8<---

So this should be fixed up in the backport, and I guess this patch
targeted the stable trees ?

If so, please clarify that by specifying the stable version in the
subject and CCing the stable mainling list:

  Subject: [PATCH 5.10.y] tcp: ...
  Cc: stable@...r.kernel.org, ...