New instances of -Wenum-enum-conversion and -Wenum-compare-conditional after LLVM commit 8c2ae42b3e1c

[nathanchance]

After llvm/llvm-project@8c2ae42, I see many many instances of -Wenum-enum-conversion (although that is due to a few unique instances from headers showing up all over the build):

$ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 allmodconfig all
...
In file included from arch/x86/kernel/asm-offsets.c:14:
In file included from include/linux/suspend.h:5:
In file included from include/linux/swap.h:9:
In file included from include/linux/memcontrol.h:20:
In file included from include/linux/mm.h:2188:
include/linux/vmstat.h:508:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
  508 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
      |                            ~~~~~~~~~~~~~~~~~~~~~ ^
  509 |                            item];
      |                            ~~~~
include/linux/vmstat.h:515:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
  515 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
      |                            ~~~~~~~~~~~~~~~~~~~~~ ^
  516 |                            NR_VM_NUMA_EVENT_ITEMS +
      |                            ~~~~~~~~~~~~~~~~~~~~~~
include/linux/vmstat.h:522:36: warning: arithmetic between different enumeration types ('enum node_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
  522 |         return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_"
      |                               ~~~~~~~~~~~ ^ ~~~
include/linux/vmstat.h:527:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
  527 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
      |                            ~~~~~~~~~~~~~~~~~~~~~ ^
  528 |                            NR_VM_NUMA_EVENT_ITEMS +
      |                            ~~~~~~~~~~~~~~~~~~~~~~
include/linux/vmstat.h:536:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
  536 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
      |                            ~~~~~~~~~~~~~~~~~~~~~ ^
  537 |                            NR_VM_NUMA_EVENT_ITEMS +
      |                            ~~~~~~~~~~~~~~~~~~~~~~
5 warnings generated.
...
In file included from init/main.c:21:
In file included from include/linux/syscalls.h:93:
In file included from include/trace/syscall.h:7:
In file included from include/linux/trace_events.h:10:
In file included from include/linux/perf_event.h:25:
In file included from arch/x86/include/asm/perf_event.h:535:
arch/x86/include/asm/stacktrace.h:23:51: warning: arithmetic between different enumeration types ('enum stack_type' and 'enum exception_stack_ordering') [-Wenum-enum-conversion]
   23 |         STACK_TYPE_EXCEPTION_LAST = STACK_TYPE_EXCEPTION + N_EXCEPTION_STACKS-1,
      |                                     ~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~
...
In file included from security/security.c:14:
include/linux/bpf.h:738:48: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  738 |         ARG_PTR_TO_MAP_VALUE_OR_NULL    = PTR_MAYBE_NULL | ARG_PTR_TO_MAP_VALUE,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~
include/linux/bpf.h:739:43: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  739 |         ARG_PTR_TO_MEM_OR_NULL          = PTR_MAYBE_NULL | ARG_PTR_TO_MEM,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
include/linux/bpf.h:740:43: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  740 |         ARG_PTR_TO_CTX_OR_NULL          = PTR_MAYBE_NULL | ARG_PTR_TO_CTX,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
include/linux/bpf.h:741:45: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  741 |         ARG_PTR_TO_SOCKET_OR_NULL       = PTR_MAYBE_NULL | ARG_PTR_TO_SOCKET,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~
include/linux/bpf.h:742:44: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  742 |         ARG_PTR_TO_STACK_OR_NULL        = PTR_MAYBE_NULL | ARG_PTR_TO_STACK,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~
include/linux/bpf.h:743:45: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  743 |         ARG_PTR_TO_BTF_ID_OR_NULL       = PTR_MAYBE_NULL | ARG_PTR_TO_BTF_ID,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~
include/linux/bpf.h:747:38: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  747 |         ARG_PTR_TO_UNINIT_MEM           = MEM_UNINIT | ARG_PTR_TO_MEM,
      |                                           ~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
include/linux/bpf.h:749:45: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_arg_type') [-Wenum-enum-conversion]
  749 |         ARG_PTR_TO_FIXED_SIZE_MEM       = MEM_FIXED_SIZE | ARG_PTR_TO_MEM,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
include/linux/bpf.h:772:48: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_return_type') [-Wenum-enum-conversion]
  772 |         RET_PTR_TO_MAP_VALUE_OR_NULL    = PTR_MAYBE_NULL | RET_PTR_TO_MAP_VALUE,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~
include/linux/bpf.h:773:45: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_return_type') [-Wenum-enum-conversion]
  773 |         RET_PTR_TO_SOCKET_OR_NULL       = PTR_MAYBE_NULL | RET_PTR_TO_SOCKET,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~
include/linux/bpf.h:774:47: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_return_type') [-Wenum-enum-conversion]
  774 |         RET_PTR_TO_TCP_SOCK_OR_NULL     = PTR_MAYBE_NULL | RET_PTR_TO_TCP_SOCK,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~
include/linux/bpf.h:775:50: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_return_type') [-Wenum-enum-conversion]
  775 |         RET_PTR_TO_SOCK_COMMON_OR_NULL  = PTR_MAYBE_NULL | RET_PTR_TO_SOCK_COMMON,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~
include/linux/bpf.h:777:49: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_return_type') [-Wenum-enum-conversion]
  777 |         RET_PTR_TO_DYNPTR_MEM_OR_NULL   = PTR_MAYBE_NULL | RET_PTR_TO_MEM,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
include/linux/bpf.h:778:45: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_return_type') [-Wenum-enum-conversion]
  778 |         RET_PTR_TO_BTF_ID_OR_NULL       = PTR_MAYBE_NULL | RET_PTR_TO_BTF_ID,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~
include/linux/bpf.h:779:43: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_return_type') [-Wenum-enum-conversion]
  779 |         RET_PTR_TO_BTF_ID_TRUSTED       = PTR_TRUSTED    | RET_PTR_TO_BTF_ID,
      |                                           ~~~~~~~~~~~    ^ ~~~~~~~~~~~~~~~~~
include/linux/bpf.h:890:44: warning: bitwise operation between different enumeration types ('enum bpf_type_flag' and 'enum bpf_reg_type') [-Wenum-enum-conversion]
  890 |         PTR_TO_MAP_VALUE_OR_NULL        = PTR_MAYBE_NULL | PTR_TO_MAP_VALUE,
      |                                           ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~
...

Most of these seem intentional (one instance even notes arithmetic between types). We have worked around this warning in the past (https://git.kernel.org/linus/d7643fe6fb76, https://git.kernel.org/linus/985b1f596f9e) with integral casts but I think it may be worth just disabling this warning outright now?

cc @arndb, as you had some input on the last time this came up in #1976.

[arndb]

I have found actual bugs with this in the past, so my feeling overall is that there is enough value in this warning option to try to leave it enabled, but I have to see the entire output for some randconfig builds first. I can try that out when the patch has made it to the apt.llvm.org builds (currently on ++20240218105224+dc94eb57e39a-1exp120240218225341.536).

[nathanchance]

The full output from ARCH=x86_64 allmodconfig is 59MB, so I hid a few of the big header offenders and did another run, that shrank the output considerable:

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e30100597d0a..d98474644db8 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -735,18 +735,18 @@ enum bpf_arg_type {
 	__BPF_ARG_TYPE_MAX,
 
 	/* Extended arg_types. */
-	ARG_PTR_TO_MAP_VALUE_OR_NULL	= PTR_MAYBE_NULL | ARG_PTR_TO_MAP_VALUE,
-	ARG_PTR_TO_MEM_OR_NULL		= PTR_MAYBE_NULL | ARG_PTR_TO_MEM,
-	ARG_PTR_TO_CTX_OR_NULL		= PTR_MAYBE_NULL | ARG_PTR_TO_CTX,
-	ARG_PTR_TO_SOCKET_OR_NULL	= PTR_MAYBE_NULL | ARG_PTR_TO_SOCKET,
-	ARG_PTR_TO_STACK_OR_NULL	= PTR_MAYBE_NULL | ARG_PTR_TO_STACK,
-	ARG_PTR_TO_BTF_ID_OR_NULL	= PTR_MAYBE_NULL | ARG_PTR_TO_BTF_ID,
+	ARG_PTR_TO_MAP_VALUE_OR_NULL	= (int)PTR_MAYBE_NULL | ARG_PTR_TO_MAP_VALUE,
+	ARG_PTR_TO_MEM_OR_NULL		= (int)PTR_MAYBE_NULL | ARG_PTR_TO_MEM,
+	ARG_PTR_TO_CTX_OR_NULL		= (int)PTR_MAYBE_NULL | ARG_PTR_TO_CTX,
+	ARG_PTR_TO_SOCKET_OR_NULL	= (int)PTR_MAYBE_NULL | ARG_PTR_TO_SOCKET,
+	ARG_PTR_TO_STACK_OR_NULL	= (int)PTR_MAYBE_NULL | ARG_PTR_TO_STACK,
+	ARG_PTR_TO_BTF_ID_OR_NULL	= (int)PTR_MAYBE_NULL | ARG_PTR_TO_BTF_ID,
 	/* pointer to memory does not need to be initialized, helper function must fill
 	 * all bytes or clear them in error case.
 	 */
-	ARG_PTR_TO_UNINIT_MEM		= MEM_UNINIT | ARG_PTR_TO_MEM,
+	ARG_PTR_TO_UNINIT_MEM		= (int)MEM_UNINIT | ARG_PTR_TO_MEM,
 	/* Pointer to valid memory of size known at compile time. */
-	ARG_PTR_TO_FIXED_SIZE_MEM	= MEM_FIXED_SIZE | ARG_PTR_TO_MEM,
+	ARG_PTR_TO_FIXED_SIZE_MEM	= (int)MEM_FIXED_SIZE | ARG_PTR_TO_MEM,
 
 	/* This must be the last entry. Its purpose is to ensure the enum is
 	 * wide enough to hold the higher bits reserved for bpf_type_flag.
@@ -769,14 +769,14 @@ enum bpf_return_type {
 	__BPF_RET_TYPE_MAX,
 
 	/* Extended ret_types. */
-	RET_PTR_TO_MAP_VALUE_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_MAP_VALUE,
-	RET_PTR_TO_SOCKET_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_SOCKET,
-	RET_PTR_TO_TCP_SOCK_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_TCP_SOCK,
-	RET_PTR_TO_SOCK_COMMON_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_SOCK_COMMON,
-	RET_PTR_TO_RINGBUF_MEM_OR_NULL	= PTR_MAYBE_NULL | MEM_RINGBUF | RET_PTR_TO_MEM,
-	RET_PTR_TO_DYNPTR_MEM_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_MEM,
-	RET_PTR_TO_BTF_ID_OR_NULL	= PTR_MAYBE_NULL | RET_PTR_TO_BTF_ID,
-	RET_PTR_TO_BTF_ID_TRUSTED	= PTR_TRUSTED	 | RET_PTR_TO_BTF_ID,
+	RET_PTR_TO_MAP_VALUE_OR_NULL	= (int)PTR_MAYBE_NULL | RET_PTR_TO_MAP_VALUE,
+	RET_PTR_TO_SOCKET_OR_NULL	= (int)PTR_MAYBE_NULL | RET_PTR_TO_SOCKET,
+	RET_PTR_TO_TCP_SOCK_OR_NULL	= (int)PTR_MAYBE_NULL | RET_PTR_TO_TCP_SOCK,
+	RET_PTR_TO_SOCK_COMMON_OR_NULL	= (int)PTR_MAYBE_NULL | RET_PTR_TO_SOCK_COMMON,
+	RET_PTR_TO_RINGBUF_MEM_OR_NULL	= (int)PTR_MAYBE_NULL | MEM_RINGBUF | RET_PTR_TO_MEM,
+	RET_PTR_TO_DYNPTR_MEM_OR_NULL	= (int)PTR_MAYBE_NULL | RET_PTR_TO_MEM,
+	RET_PTR_TO_BTF_ID_OR_NULL	= (int)PTR_MAYBE_NULL | RET_PTR_TO_BTF_ID,
+	RET_PTR_TO_BTF_ID_TRUSTED	= (int)PTR_TRUSTED	 | RET_PTR_TO_BTF_ID,
 
 	/* This must be the last entry. Its purpose is to ensure the enum is
 	 * wide enough to hold the higher bits reserved for bpf_type_flag.
@@ -887,11 +887,11 @@ enum bpf_reg_type {
 	__BPF_REG_TYPE_MAX,
 
 	/* Extended reg_types. */
-	PTR_TO_MAP_VALUE_OR_NULL	= PTR_MAYBE_NULL | PTR_TO_MAP_VALUE,
-	PTR_TO_SOCKET_OR_NULL		= PTR_MAYBE_NULL | PTR_TO_SOCKET,
-	PTR_TO_SOCK_COMMON_OR_NULL	= PTR_MAYBE_NULL | PTR_TO_SOCK_COMMON,
-	PTR_TO_TCP_SOCK_OR_NULL		= PTR_MAYBE_NULL | PTR_TO_TCP_SOCK,
-	PTR_TO_BTF_ID_OR_NULL		= PTR_MAYBE_NULL | PTR_TO_BTF_ID,
+	PTR_TO_MAP_VALUE_OR_NULL	= (int)PTR_MAYBE_NULL | PTR_TO_MAP_VALUE,
+	PTR_TO_SOCKET_OR_NULL		= (int)PTR_MAYBE_NULL | PTR_TO_SOCKET,
+	PTR_TO_SOCK_COMMON_OR_NULL	= (int)PTR_MAYBE_NULL | PTR_TO_SOCK_COMMON,
+	PTR_TO_TCP_SOCK_OR_NULL		= (int)PTR_MAYBE_NULL | PTR_TO_TCP_SOCK,
+	PTR_TO_BTF_ID_OR_NULL		= (int)PTR_MAYBE_NULL | PTR_TO_BTF_ID,
 
 	/* This must be the last entry. Its purpose is to ensure the enum is
 	 * wide enough to hold the higher bits reserved for bpf_type_flag.
diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
index 343906a98d6e..8e0526b4d574 100644
--- a/include/linux/vmstat.h
+++ b/include/linux/vmstat.h
@@ -132,7 +132,7 @@ static inline void vm_events_fold_cpu(int cpu)
 #endif
 
 #define __count_zid_vm_events(item, zid, delta) \
-	__count_vm_events(item##_NORMAL - ZONE_NORMAL + zid, delta)
+	__count_vm_events((int)item##_NORMAL - ZONE_NORMAL + zid, delta)
 
 /*
  * Zone and node-based page accounting with per cpu differentials.
@@ -505,38 +505,38 @@ static inline const char *zone_stat_name(enum zone_stat_item item)
 #ifdef CONFIG_NUMA
 static inline const char *numa_stat_name(enum numa_stat_item item)
 {
-	return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
+	return vmstat_text[(int)NR_VM_ZONE_STAT_ITEMS +
 			   item];
 }
 #endif /* CONFIG_NUMA */
 
 static inline const char *node_stat_name(enum node_stat_item item)
 {
-	return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
-			   NR_VM_NUMA_EVENT_ITEMS +
+	return vmstat_text[(int)NR_VM_ZONE_STAT_ITEMS +
+			   (int)NR_VM_NUMA_EVENT_ITEMS +
 			   item];
 }
 
 static inline const char *lru_list_name(enum lru_list lru)
 {
-	return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_"
+	return node_stat_name((int)NR_LRU_BASE + lru) + 3; // skip "nr_"
 }
 
 static inline const char *writeback_stat_name(enum writeback_stat_item item)
 {
-	return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
-			   NR_VM_NUMA_EVENT_ITEMS +
-			   NR_VM_NODE_STAT_ITEMS +
+	return vmstat_text[(int)NR_VM_ZONE_STAT_ITEMS +
+			   (int)NR_VM_NUMA_EVENT_ITEMS +
+			   (int)NR_VM_NODE_STAT_ITEMS +
 			   item];
 }
 
 #if defined(CONFIG_VM_EVENT_COUNTERS) || defined(CONFIG_MEMCG)
 static inline const char *vm_event_name(enum vm_event_item item)
 {
-	return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
-			   NR_VM_NUMA_EVENT_ITEMS +
-			   NR_VM_NODE_STAT_ITEMS +
-			   NR_VM_WRITEBACK_STAT_ITEMS +
+	return vmstat_text[(int)NR_VM_ZONE_STAT_ITEMS +
+			   (int)NR_VM_NUMA_EVENT_ITEMS +
+			   (int)NR_VM_NODE_STAT_ITEMS +
+			   (int)NR_VM_WRITEBACK_STAT_ITEMS +
 			   item];
 }
 #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */
diff --git a/net/devlink/devl_internal.h b/net/devlink/devl_internal.h
index c7a8e13f917c..8e7572c16838 100644
--- a/net/devlink/devl_internal.h
+++ b/net/devlink/devl_internal.h
@@ -23,7 +23,7 @@ struct devlink_rel;
 #define DEVLINK_REGISTERED XA_MARK_1
 
 #define DEVLINK_RELOAD_STATS_ARRAY_SIZE \
-	(__DEVLINK_RELOAD_LIMIT_MAX * __DEVLINK_RELOAD_ACTION_MAX)
+	((int)__DEVLINK_RELOAD_LIMIT_MAX * __DEVLINK_RELOAD_ACTION_MAX)
 
 struct devlink_dev_stats {
 	u32 reload_stats[DEVLINK_RELOAD_STATS_ARRAY_SIZE];

https://gist.github.com/nathanchance/971e5abeba504d3017cd6ed4517bbda6

I tried to hide all warnings to see what a final diff would look like but it got too tiring due to how many instances there are.

I have an LLVM main snapshot available with this change at https://mirrors.edge.kernel.org/pub/tools/llvm/files/prerelease/ (for a separate reason but it helps out here too).

Just to make sure there is no confusion here, -Wenum-enum-conversion is a different warning from -Wenum-conversion, which is definitely useful and should stay on due to the bug finding value. However, given the warnings and the potential diff needed to resolve all of them, I really fail to see the value of keeping -Wenum-enum-conversion enabled. In my opinion, when people are performing arithmetic with enums, they have to be thinking about the underlying integral value, rather than the actual meaning of the enum, so I don't think the warning has much value. I don't think -Wenum-conversion has that same problem because it is looking for enum mismatches in assignments and function calls, where the underlying integral value is likely not thought much about. I could be missing something though.

[nathanchance]

I hacked up my toolchain to remove -Wenum-enum-conversion from -Wenum-conversion, which allowed me to find some more new warnings from that change.

In file included from drivers/media/platform/renesas/rcar-vin/rcar-v4l2.c:20:
drivers/media/platform/renesas/rcar-vin/rcar-vin.h:296:12: warning: conditional expression between different enumeration types ('enum rvin_csi_id' and 'enum rvin_isp_id') [-Wenum-compare-conditional]
  296 |         } remotes[RVIN_REMOTES_MAX];
      |                   ^~~~~~~~~~~~~~~~
drivers/media/platform/renesas/rcar-vin/rcar-vin.h:61:63: note: expanded from macro 'RVIN_REMOTES_MAX'
   61 |         (((unsigned int)RVIN_CSI_MAX) > ((unsigned int)RVIN_ISP_MAX) ? \
      |                                                                      ^
   62 |          RVIN_CSI_MAX : RVIN_ISP_MAX)
      |          ~~~~~~~~~~~~   ~~~~~~~~~~~~
1 warning generated.

drivers/media/platform/mediatek/vcodec/encoder/venc/venc_h264_if.c:596:29: warning: conditional expression between different enumeration types ('enum scp_ipi_id' and 'enum ipi_id') [-Wenum-compare-conditional]
  596 |         inst->vpu_inst.id = is_ext ? SCP_IPI_VENC_H264 : IPI_VENC_H264;
      |                                    ^ ~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~
1 warning generated.

drivers/net/wireless/realtek/rtw89/core.c:1686:14: warning: conditional expression between different enumeration types ('enum nl80211_eht_gi' and 'enum nl80211_he_gi') [-Wenum-compare-conditional]
 1686 |                 return eht ? NL80211_RATE_INFO_EHT_GI_0_8 :
      |                            ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1687 |                              NL80211_RATE_INFO_HE_GI_0_8;
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/realtek/rtw89/core.c:1690:14: warning: conditional expression between different enumeration types ('enum nl80211_eht_gi' and 'enum nl80211_he_gi') [-Wenum-compare-conditional]
 1690 |                 return eht ? NL80211_RATE_INFO_EHT_GI_1_6 :
      |                            ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1691 |                              NL80211_RATE_INFO_HE_GI_1_6;
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/realtek/rtw89/core.c:1693:14: warning: conditional expression between different enumeration types ('enum nl80211_eht_gi' and 'enum nl80211_he_gi') [-Wenum-compare-conditional]
 1693 |                 return eht ? NL80211_RATE_INFO_EHT_GI_3_2 :
      |                            ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1694 |                              NL80211_RATE_INFO_HE_GI_3_2;
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/realtek/rtw89/core.c:1698:15: warning: conditional expression between different enumeration types ('enum nl80211_eht_gi' and 'enum nl80211_he_gi') [-Wenum-compare-conditional]
 1698 |                         return eht ? NL80211_RATE_INFO_EHT_GI_3_2 :
      |                                    ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1699 |                                      NL80211_RATE_INFO_HE_GI_3_2;
      |                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 warnings generated.

drivers/net/wireless/intel/iwlwifi/mvm/mac-ctxt.c:955:24: warning: conditional expression between different enumeration types ('enum iwl_mac_beacon_flags' and 'enum iwl_mac_beacon_flags_v1') [-Wenum-compare-conditional]
  955 |                 flags |= is_new_rate ? IWL_MAC_BEACON_CCK
      |                                      ^ ~~~~~~~~~~~~~~~~~~
  956 |                           : IWL_MAC_BEACON_CCK_V1;
      |                             ~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/intel/iwlwifi/mvm/mac-ctxt.c:1120:21: warning: conditional expression between different enumeration types ('enum iwl_mac_beacon_flags' and 'enum iwl_mac_beacon_flags_v1') [-Wenum-compare-conditional]
 1120 |                                                0) > 10 ?
      |                                                        ^
 1121 |                         IWL_MAC_BEACON_FILS :
      |                         ~~~~~~~~~~~~~~~~~~~
 1122 |                         IWL_MAC_BEACON_FILS_V1;
      |                         ~~~~~~~~~~~~~~~~~~~~~~
2 warnings generated.

drivers/net/wireless/intel/iwlwifi/mvm/sta.c:4310:17: warning: conditional expression between different enumeration types ('enum iwl_fw_sta_type' and 'enum iwl_sta_type') [-Wenum-compare-conditional]
 4310 |         u32 type = mld ? STATION_TYPE_PEER : IWL_STA_LINK;
      |                        ^ ~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~
1 warning generated.

Most of these seem to be aware of converting from one enumerated type to another and I can't see a fix other than casting or moving to #define which are both ugly but maybe this one is worth doing that for to keep it on? Could disable it otherwise.

[yonghong-song]

Indeed, we could disable the warning with -Wno-enum-enum-conversion. The following build command can build kernel successfully with latest llvm19:

make LLVM=1 -j KCFLAGS=-Wno-enum-enum-conversion

[yonghong-song]

I have found actual bugs with this in the past, so my feeling overall is that there is enough value in this warning option to try to leave it enabled, but I have to see the entire output for some randconfig builds first. I can try that out when the patch has made it to the apt.llvm.org builds (currently on ++20240218105224+dc94eb57e39a-1exp120240218225341.536).

@arndb Could you give some examples what actual bugs are? Are these bugs related to linux kernel? @nickdesaulniers what is your suggestion on this?

[nathanchance]

This diff should work for disabling it, I can send it once there is some consensus:

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index a9e552a1e910..6053aa22b8f5 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -81,6 +81,14 @@ KBUILD_CFLAGS += $(call cc-option,-Werror=designated-init)

 # Warn if there is an enum types mismatch
 KBUILD_CFLAGS += $(call cc-option,-Wenum-conversion)
+ifdef CONFIG_CC_IS_CLANG
+# Clang enables these extra warnings under -Wenum-conversion but the kernel
+# performs arithmetic using or has conditionals returning enums of different
+# types in several different places, which is rarely a bug in the kernel's
+# case, so disable the warnings.
+KBUILD_CFLAGS += -Wno-enum-compare-conditional
+KBUILD_CFLAGS += -Wno-enum-enum-conversion
+endif

 #
 # W=1 - warnings which may be relevant and do not occur too often

[yonghong-song]

In my kernel config, '-Wno-enum-enum-conversion' seems enough. For example, to workaround the build issue, I am doing the following

$ make LLVM=1 -j KCFLAGS=-Wno-enum-enum-conversion

and the build is successful.
But mentioned in the above, -Wno-enum-compare-conditional is also needed. My config file does not enable all possible options.

@nathanchance In my opinion, the above fix is reasonable. Please cc me in the patch ('Yonghong Song yonghong.song@linux.dev'). Thanks!

[nathanchance]

I've sent that patch to the mailing list for further comments: https://lore.kernel.org/20240305-disable-extra-clang-enum-warnings-v1-1-6a93ef3d35ff@kernel.org/

[nathanchance]

v2: https://lore.kernel.org/20240305-disable-extra-clang-enum-warnings-v2-1-ba529ec15f95@kernel.org/

[nathanchance]

Patch accepted: https://git.kernel.org/masahiroy/linux-kbuild/c/75b5ab134bb5f657ef7979a59106dce0657e8d87