Message-Id: <b4bfacfa-b6a0-4c6b-8ada-29763cf64ad5@app.fastmail.com>
Date: Fri, 19 Dec 2025 21:26:10 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Nathan Chancellor" <nathan@...nel.org>,
 "David Laight" <david.laight.linux@...il.com>
Cc: "Nicolas Schier" <nsc@...nel.org>, linux-kbuild@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] kbuild: Only enable
 -Wtautological-constant-out-of-range-compare for W=2

On Fri, Dec 19, 2025, at 21:12, Nathan Chancellor wrote:
> On Sun, Dec 14, 2025 at 01:15:28PM +0000, david.laight.linux@...il.com wrote:
>> From: David Laight <david.laight.linux@...il.com>
>> 
>> The kernel code style is to use !(expr) rather that (expr) == 0.
>> But clang complains that converting some constant expressions
>> (eg (0xffffu << 16)) to a boolean always evalutes to true.
>> This happens often in the validity checks in #defines.
>> Move tautological-constant-out-of-range-compare to W=2 (along with the
>> similar type-limits).
>> 
>> Signed-off-by: David Laight <david.laight.linux@...il.com>
>
> I would like Arnd to comment on this before applying because the
> reasoning of this change does not feel good enough to disable this
> warning. It is not like '== 0' is inherently uncommon in the kernel or
> hard to write to avoid the implicit conversion warning. To be honest, I
> am a bit surprised -Wtautological-constant-out-of-range-compare fires
> for that instead of some sort of -Wconversion warning...

I actually have the warning enabled by default in my randconfig
tree. I have a couple of fixup patches for the warnings we get
in the tree, but this should totally be fixable upstream as well.

See below for my patches. Are there any ones I missed?

    Arnd


>From cb753c6f52ac5b8514c95be6263b2942ea71efd8 Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@...db.de>
Date: Thu, 26 Jun 2025 19:59:06 +0200
Subject: [PATCH] riscv: limit sifive errate to CONFIG_64BIT

There are two errata for this vendor, and they both individually depend on
CONFIG_64BIT already. However, an 32-bit allmodconfig produces this warning
from clang for a condition that can never be true.

arch/riscv/errata/sifive/errata.c:29:14: error: result of comparison of constant 9223372036854775815 with expression of type 'unsigned long' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
   29 |         if (arch_id != 0x8000000000000007 ||
      |             ~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~
arch/riscv/errata/sifive/errata.c:42:14: error: result of comparison of constant 9223372036854775815 with expression of type 'unsigned long' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
   42 |         if (arch_id != 0x8000000000000007 && arch_id != 0x1)
      |             ~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~

Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
index aca9b0cfcfec..1480abe54288 100644
--- a/arch/riscv/Kconfig.errata
+++ b/arch/riscv/Kconfig.errata
@@ -46,7 +46,7 @@ config ERRATA_MIPS_P8700_PAUSE_OPCODE
 
 config ERRATA_SIFIVE
 	bool "SiFive errata"
-	depends on RISCV_ALTERNATIVE
+	depends on RISCV_ALTERNATIVE && 64BIT
 	help
 	  All SiFive errata Kconfig depend on this Kconfig. Disabling
 	  this Kconfig will disable all SiFive errata. Please say "Y"
diff --git a/arch/riscv/Kconfig.socs b/arch/riscv/Kconfig.socs
index d621b85dd63b..06ec2caaf73b 100644
--- a/arch/riscv/Kconfig.socs
+++ b/arch/riscv/Kconfig.socs
@@ -34,6 +34,7 @@ config ARCH_RENESAS
 config ARCH_SIFIVE
 	bool "SiFive SoCs"
 	select ERRATA_SIFIVE if !XIP_KERNEL
+	depends on 64BIT
 	help
 	  This enables support for SiFive SoC platform hardware.
 

>From bcd1c82d604af688b6dc3b2a67454a47150b4d6d Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@...db.de>
Date: Fri, 20 Jun 2025 23:31:30 +0200
Subject: [PATCH] scsi: ibmvscsi_tgt: fix clang build warning

This is one of a handful of drivers that trigger the
-Wtautological-constant-out-of-range-compare warning flag:

drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c:1630:32: error: result of comparison of constant 65536 with expression of type 'u16' (aka 'unsigned short') is always false [-Werror,-Wtautological-constant-out-of-range-compare]

Extend the type appropriately to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c b/drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c
index f259746bc804..9ef6fefbf0ad 100644
--- a/drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c
+++ b/drivers/scsi/ibmvscsi_tgt/ibmvscsi_tgt.c
@@ -1616,7 +1616,8 @@ static int ibmvscsis_cap_mad(struct scsi_info *vscsi, struct iu_entry *iue)
 	struct capabilities *cap;
 	struct mad_capability_common *common;
 	dma_addr_t token;
-	u16 olen, len, status, min_len, cap_len;
+	u16 len, status, min_len, cap_len;
+	u32 olen;
 	u32 flag;
 	uint flag_bits = 0;
 	long rc = 0;

>From 523a92edf0b8e32a9b1da160d3edaebeb12cfccc Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@...db.de>
Date: Tue, 30 Jul 2024 16:25:07 +0200
Subject: [PATCH] fuse: fs-verity: aoid out-of-range comparison

clang points out that comparing the 16-bit size of the digest against
SIZE_MAX is not a helpful comparison:

fs/fuse/ioctl.c:130:18: error: result of comparison of constant 18446744073709551611 with expression of type '__u16' (aka 'unsigned short') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        if (digest_size > SIZE_MAX - sizeof(struct fsverity_digest))
            ~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This either means tha tthe check can be removed entirely, or that the
intended comparison was for the 16-bit range. Assuming the latter was
intended, compare against U16_MAX instead.

Fixes: 9fe2a036a23c ("fuse: Add initial support for fs-verity")
Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/fs/fuse/ioctl.c b/fs/fuse/ioctl.c
index fdc175e93f74..c85b51ca7c86 100644
--- a/fs/fuse/ioctl.c
+++ b/fs/fuse/ioctl.c
@@ -129,7 +129,7 @@ static int fuse_setup_measure_verity(unsigned long arg, struct iovec *iov)
 	if (copy_from_user(&digest_size, &uarg->digest_size, sizeof(digest_size)))
 		return -EFAULT;
 
-	if (digest_size > SIZE_MAX - sizeof(struct fsverity_digest))
+	if (digest_size > U16_MAX - sizeof(struct fsverity_digest))
 		return -EINVAL;
 
 	iov->iov_len = sizeof(struct fsverity_digest) + digest_size;

>From bd340f4cb7fc43a8594f91de8b9305cc9f7a0f89 Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@...db.de>
Date: Thu, 11 Mar 2021 16:52:40 +0100
Subject: [PATCH] [REVISIT 20210323] ARM: delay: avoid clang
 -Wtautological-constant warning

Passing an 8-bit constant into delay() triggers a warning when building
with 'make W=1' using clang:

drivers/clk/actions/owl-pll.c:182:2: error: result of comparison of constant 2000 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        udelay(pll_hw->delay);
        ^~~~~~~~~~~~~~~~~~~~~
arch/arm/include/asm/delay.h:84:9: note: expanded from macro 'udelay'
          ((n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() :              \
           ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~
arch/arm/mach-omap2/wd_timer.c:89:3: error: result of comparison of constant 2000 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                udelay(oh->class->sysc->srst_udelay);
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Shut up the warning by adding a cast to a 64-bit number. A cast to 'int'
would usually be sufficient, but would fail to cause a link-time error
for large 64-bit constants.

Link: https://lore.kernel.org/all/20210323133005.GC1463@shell.armlinux.org.uk/
Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
index 1d069e558d8d..05a6a347d703 100644
--- a/arch/arm/include/asm/delay.h
+++ b/arch/arm/include/asm/delay.h
@@ -81,7 +81,7 @@ extern void __bad_udelay(void);
 
 #define udelay(n)							\
 	(__builtin_constant_p(n) ?					\
-	  ((n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() :		\
+	  ((u64)(n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() :		\
 			__const_udelay((n) * UDELAY_MULT)) :		\
 	  __udelay(n))
 

>From 3080c47aa770504f1f9bb5fcd9c4c60147b6f48d Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@...db.de>
Date: Wed, 22 Sep 2021 13:11:34 +0200
Subject: [PATCH] [SUBMITTED 20240328] infiniband: uverbs: avoid out-of-range
 warnings

clang warns for comparisons that are always true, which is the case
for these two page size checks on architectures with 64KB pages:

drivers/infiniband/core/uverbs_ioctl.c:90:39: error: result of comparison of constant 65536 with expression of type 'u16' (aka 'unsigned short') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        WARN_ON_ONCE(method_elm->bundle_size > PAGE_SIZE);
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
include/asm-generic/bug.h:104:25: note: expanded from macro 'WARN_ON_ONCE'
        int __ret_warn_on = !!(condition);                      \
                               ^~~~~~~~~
drivers/infiniband/core/uverbs_ioctl.c:621:17: error: result of comparison of constant 65536 with expression of type '__u16' (aka 'unsigned short') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        if (hdr.length > PAGE_SIZE ||
            ~~~~~~~~~~ ^ ~~~~~~~~~

Add a cast to u32 in both cases, so it never warns about this.

Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/drivers/infiniband/core/uverbs_ioctl.c b/drivers/infiniband/core/uverbs_ioctl.c
index f80da6a67e24..e0cc3ddae71b 100644
--- a/drivers/infiniband/core/uverbs_ioctl.c
+++ b/drivers/infiniband/core/uverbs_ioctl.c
@@ -90,7 +90,7 @@ void uapi_compute_bundle_size(struct uverbs_api_ioctl_method *method_elm,
 		ALIGN(bundle_size + 256, sizeof(*pbundle->internal_buffer));
 
 	/* Do not want order-2 allocations for this. */
-	WARN_ON_ONCE(method_elm->bundle_size > PAGE_SIZE);
+	WARN_ON_ONCE((u32)method_elm->bundle_size > PAGE_SIZE);
 }
 
 /**
@@ -636,7 +636,7 @@ long ib_uverbs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	if (err)
 		return -EFAULT;
 
-	if (hdr.length > PAGE_SIZE ||
+	if ((u32)hdr.length > PAGE_SIZE ||
 	    hdr.length != struct_size(&hdr, attrs, hdr.num_attrs))
 		return -EINVAL;
 

>From c47a0d27c29bcac2ea5ac9d8b0313258b19bcee4 Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@...db.de>
Date: Wed, 22 Sep 2021 13:41:53 +0200
Subject: [PATCH] [SUBMITTED 20240328] ipv4: tcp_output: avoid warning about
 NET_ADD_STATS

Clang warns about a range check in percpu_add_op() being impossible
to hit for an u8 variable:

net/ipv4/tcp_output.c:188:3: error: result of comparison of constant -1 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/net/ip.h:291:41: note: expanded from macro 'NET_ADD_STATS'
 #define NET_ADD_STATS(net, field, adnd) SNMP_ADD_STATS((net)->mib.net_statistics, field, adnd)
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/net/snmp.h:143:4: note: expanded from macro 'SNMP_ADD_STATS'
                        this_cpu_add(mib->mibs[field], addend)
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/percpu-defs.h:509:33: note: expanded from macro 'this_cpu_add'
 #define this_cpu_add(pcp, val)          __pcpu_size_call(this_cpu_add_, pcp, val)
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
<scratch space>:187:1: note: expanded from here
this_cpu_add_8
^
arch/x86/include/asm/percpu.h:326:35: note: expanded from macro 'this_cpu_add_8'
 #define this_cpu_add_8(pcp, val)                percpu_add_op(8, volatile, (pcp), val)
                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/include/asm/percpu.h:127:31: note: expanded from macro 'percpu_add_op'
                              ((val) == 1 || (val) == -1)) ?            \
                                             ~~~~~ ^  ~~

Avoid this warning with a cast to a signed 'int'.

Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 479afb714bdf..00bf6c1639be 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -188,7 +188,7 @@ static inline void tcp_event_ack_sent(struct sock *sk, u32 rcv_nxt)
 
 	if (unlikely(tp->compressed_ack)) {
 		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
-			      tp->compressed_ack);
+			      (int)tp->compressed_ack);
 		tp->compressed_ack = 0;
 		if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
 			__sock_put(sk);

>From 8493af46865dd6a7f77dd7d8379181ac2b07e723 Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@...db.de>
Date: Fri, 24 Sep 2021 12:50:07 +0200
Subject: [PATCH] [SUBMITTED 20240328] rbd: avoid out-of-range warning

clang-14 points out that the range check is always true on 64-bit
architectures since a u32 is not greater than the allowed size:

drivers/block/rbd.c:6079:17: error: result of comparison of constant 2305843009213693948 with expression of type 'u32' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        if (snap_count > (SIZE_MAX - sizeof (struct ceph_snap_context))
            ~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is harmless, so just change the type of the temporary to size_t
to shut up that warning.

Fixes: bb23e37acb2a ("rbd: refactor rbd_header_from_disk()")
Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index af0e21149dbc..1bb55827d18d 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -6054,7 +6054,7 @@ static int rbd_dev_v2_snap_context(struct rbd_device *rbd_dev,
 	void *p;
 	void *end;
 	u64 seq;
-	u32 snap_count;
+	size_t snap_count;
 	struct ceph_snap_context *snapc;
 	u32 i;
 

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