* [linux-next:master 7731/9053] arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len' is used uninitialized whenever 'if' condition is false
@ 2025-07-12 19:29 kernel test robot
2025-07-14 12:37 ` Sascha Bischoff
0 siblings, 1 reply; 3+ messages in thread
From: kernel test robot @ 2025-07-12 19:29 UTC (permalink / raw)
To: Sascha Bischoff; +Cc: llvm, oe-kbuild-all, Oliver Upton, Timothy Hayes
tree: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
head: a62b7a37e6fcf4a675b1548e7c168b96ec836442
commit: c017e49ed1381001ba7a6521daae8f968b11cf09 [7731/9053] KVM: arm64: gic-v5: Support GICv3 compat
config: arm64-randconfig-004-20250713 (https://download.01.org/0day-ci/archive/20250713/202507130334.ddFcXn4N-lkp@intel.com/config)
compiler: clang version 21.0.0git (https://github.com/llvm/llvm-project 01c97b4953e87ae455bd4c41e3de3f0f0f29c61c)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250713/202507130334.ddFcXn4N-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202507130334.ddFcXn4N-lkp@intel.com/
All warnings (new ones prefixed by >>):
>> arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
1094 | BUG_ON(1);
| ^~~~~~~~~
include/asm-generic/bug.h:71:36: note: expanded from macro 'BUG_ON'
71 | #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
| ^~~~~~~~~~~~~~~~~~~
include/linux/compiler.h:47:23: note: expanded from macro 'unlikely'
47 | # define unlikely(x) (__branch_check__(x, 0, __builtin_constant_p(x)))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/arm64/kvm/vgic/vgic-mmio.c:1102:12: note: uninitialized use occurs here
1102 | len, &io_device->dev);
| ^~~
arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: note: remove the 'if' if its condition is always true
1094 | BUG_ON(1);
| ^
include/asm-generic/bug.h:71:32: note: expanded from macro 'BUG_ON'
71 | #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
| ^
arch/arm64/kvm/vgic/vgic-mmio.c:1084:18: note: initialize the variable 'len' to silence this warning
1084 | unsigned int len;
| ^
| = 0
1 warning generated.
vim +1094 arch/arm64/kvm/vgic/vgic-mmio.c
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1079
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1080 int vgic_register_dist_iodev(struct kvm *kvm, gpa_t dist_base_address,
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1081 enum vgic_type type)
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1082 {
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1083 struct vgic_io_device *io_device = &kvm->arch.vgic.dist_iodev;
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1084 unsigned int len;
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1085
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1086 switch (type) {
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1087 case VGIC_V2:
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1088 len = vgic_v2_init_dist_iodev(io_device);
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1089 break;
ed9b8cefa91695 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2015-12-01 1090 case VGIC_V3:
ed9b8cefa91695 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2015-12-01 1091 len = vgic_v3_init_dist_iodev(io_device);
ed9b8cefa91695 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2015-12-01 1092 break;
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 1093 default:
fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-26 @1094 BUG_ON(1);
:::::: The code at line 1094 was first introduced by commit
:::::: fb848db39661a1243f6ae939ef7e9251a765b972 KVM: arm/arm64: vgic-new: Add GICv2 MMIO handling framework
:::::: TO: Andre Przywara <andre.przywara@arm.com>
:::::: CC: Christoffer Dall <christoffer.dall@linaro.org>
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [linux-next:master 7731/9053] arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len' is used uninitialized whenever 'if' condition is false
2025-07-12 19:29 [linux-next:master 7731/9053] arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len' is used uninitialized whenever 'if' condition is false kernel test robot
@ 2025-07-14 12:37 ` Sascha Bischoff
2025-07-14 17:15 ` Nathan Chancellor
0 siblings, 1 reply; 3+ messages in thread
From: Sascha Bischoff @ 2025-07-14 12:37 UTC (permalink / raw)
To: lkp@intel.com
Cc: Timothy Hayes, llvm@lists.linux.dev,
oe-kbuild-all@lists.linux.dev, oliver.upton@linux.dev, nd
On Sun, 2025-07-13 at 03:29 +0800, kernel test robot wrote:
> tree:
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git m
> aster
> head: a62b7a37e6fcf4a675b1548e7c168b96ec836442
> commit: c017e49ed1381001ba7a6521daae8f968b11cf09 [7731/9053] KVM:
> arm64: gic-v5: Support GICv3 compat
> config: arm64-randconfig-004-20250713
> (https://download.01.org/0day-ci/archive/20250713/202507130334.ddFcXn
> 4N-lkp@intel.com/config)
> compiler: clang version 21.0.0git
> (https://github.com/llvm/llvm-project 01c97b4953e87ae455bd4c41e3de3f0
> f0f29c61c)
> reproduce (this is a W=1 build):
> (https://download.01.org/0day-ci/archive/20250713/202507130334.ddFcXn
> 4N-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new
> version of
> the same patch/commit), kindly add following tags
> > Reported-by: kernel test robot <lkp@intel.com>
> > Closes:
> > https://lore.kernel.org/oe-kbuild-all/202507130334.ddFcXn4N-lkp@intel.com/
>
> All warnings (new ones prefixed by >>):
>
> > > arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len'
> > > is used uninitialized whenever 'if' condition is false [-
> > > Wsometimes-uninitialized]
> 1094 | BUG_ON(1);
> | ^~~~~~~~~
> include/asm-generic/bug.h:71:36: note: expanded from macro
> 'BUG_ON'
> 71 | #define BUG_ON(condition) do { if (unlikely(condition))
> BUG(); } while (0)
> | ^~~~~~~~~~~~~~~~~~~
> include/linux/compiler.h:47:23: note: expanded from macro
> 'unlikely'
> 47 | # define unlikely(x) (__branch_check__(x, 0,
> __builtin_constant_p(x)))
> |
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/arm64/kvm/vgic/vgic-mmio.c:1102:12: note: uninitialized use
> occurs here
> 1102 | len, &io_device-
> >dev);
> | ^~~
> arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: note: remove the 'if' if
> its condition is always true
> 1094 | BUG_ON(1);
> | ^
> include/asm-generic/bug.h:71:32: note: expanded from macro
> 'BUG_ON'
> 71 | #define BUG_ON(condition) do { if (unlikely(condition))
> BUG(); } while (0)
> | ^
> arch/arm64/kvm/vgic/vgic-mmio.c:1084:18: note: initialize the
> variable 'len' to silence this warning
> 1084 | unsigned int len;
> | ^
> | = 0
> 1 warning generated.
>
>
> vim +1094 arch/arm64/kvm/vgic/vgic-mmio.c
>
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1079
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1080 int vgic_register_dist_iodev(struct kvm *kvm, gpa_t
> dist_base_address,
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1081 enum vgic_type type)
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1082 {
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1083 struct vgic_io_device *io_device = &kvm-
> >arch.vgic.dist_iodev;
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1084 unsigned int len;
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1085
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1086 switch (type) {
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1087 case VGIC_V2:
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1088 len = vgic_v2_init_dist_iodev(io_device);
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1089 break;
> ed9b8cefa91695 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2015-12-
> 01 1090 case VGIC_V3:
> ed9b8cefa91695 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2015-12-
> 01 1091 len = vgic_v3_init_dist_iodev(io_device);
> ed9b8cefa91695 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2015-12-
> 01 1092 break;
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 1093 default:
> fb848db39661a1 virt/kvm/arm/vgic/vgic-mmio.c Andre Przywara 2016-04-
> 26 @1094 BUG_ON(1);
>
> :::::: The code at line 1094 was first introduced by commit
> :::::: fb848db39661a1243f6ae939ef7e9251a765b972 KVM: arm/arm64: vgic-
> new: Add GICv2 MMIO handling framework
>
> :::::: TO: Andre Przywara <andre.przywara@arm.com>
> :::::: CC: Christoffer Dall <christoffer.dall@linaro.org>
>
Hi,
There is no actual uninitialized use of len occurring here. It is
explicitly initialized in both the VGIC_V2 and VGIC_V3 cases. The only
other path is the default case, which triggers BUG_ON(1).
For arm64, BUG() itself is defined as:
#define BUG() do { \
__BUG_FLAGS(0); \
unreachable(); \
} while (0)
Since __BUG_FLAGS(0) is followed by unreachable(), the compiler should
ideally recognize that code paths beyond this point are dead.
The fact that this warning is still being emitted suggests the compiler
isn’t correctly propagating the unreachable() information in this
context.
Given that, this seems like an issue on the compiler side, or possibly
a case where BUG_ON() could be restructured to more explicitly convey
control-flow termination to the compiler.
Thanks,
Sascha
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [linux-next:master 7731/9053] arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len' is used uninitialized whenever 'if' condition is false
2025-07-14 12:37 ` Sascha Bischoff
@ 2025-07-14 17:15 ` Nathan Chancellor
0 siblings, 0 replies; 3+ messages in thread
From: Nathan Chancellor @ 2025-07-14 17:15 UTC (permalink / raw)
To: Sascha Bischoff
Cc: lkp@intel.com, Timothy Hayes, llvm@lists.linux.dev,
oe-kbuild-all@lists.linux.dev, oliver.upton@linux.dev, nd
Hi Sascha,
On Mon, Jul 14, 2025 at 12:37:35PM +0000, Sascha Bischoff wrote:
> On Sun, 2025-07-13 at 03:29 +0800, kernel test robot wrote:
> > commit: c017e49ed1381001ba7a6521daae8f968b11cf09 [7731/9053] KVM: arm64: gic-v5: Support GICv3 compat
> > config: arm64-randconfig-004-20250713 (https://download.01.org/0day-ci/archive/20250713/202507130334.ddFcXn4N-lkp@intel.com/config)
> > compiler: clang version 21.0.0git (https://github.com/llvm/llvm-project 01c97b4953e87ae455bd4c41e3de3f0f0f29c61c)
...
> > arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> > 1094 | BUG_ON(1);
> > | ^~~~~~~~~
> > include/asm-generic/bug.h:71:36: note: expanded from macro 'BUG_ON'
> > 71 | #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
> > | ^~~~~~~~~~~~~~~~~~~
> > include/linux/compiler.h:47:23: note: expanded from macro 'unlikely'
> > 47 | # define unlikely(x) (__branch_check__(x, 0, __builtin_constant_p(x)))
> > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...
> There is no actual uninitialized use of len occurring here. It is
> explicitly initialized in both the VGIC_V2 and VGIC_V3 cases. The only
> other path is the default case, which triggers BUG_ON(1).
>
> For arm64, BUG() itself is defined as:
>
> #define BUG() do { \
> __BUG_FLAGS(0); \
> unreachable(); \
> } while (0)
>
> Since __BUG_FLAGS(0) is followed by unreachable(), the compiler should
> ideally recognize that code paths beyond this point are dead.
> The fact that this warning is still being emitted suggests the compiler
> isn’t correctly propagating the unreachable() information in this
> context.
>
> Given that, this seems like an issue on the compiler side, or possibly
> a case where BUG_ON() could be restructured to more explicitly convey
> control-flow termination to the compiler.
In a normal configuration, it is easy for the compiler to see that
BUG_ON(1) is the same thing as BUG() and take into account the
__builtin_unreachable() at the end to avoid warnings.
However, the key to what is going on here is in the second note: this
configuration has CONFIG_TRACE_BRANCH_PROFILING=y, which turns
unlikely() into
#define __branch_check__(x, expect, is_constant) ({ \
long ______r; \
static struct ftrace_likely_data \
__aligned(4) \
__section("_ftrace_annotated_branch") \
______f = { \
.data.func = __func__, \
.data.file = __FILE__, \
.data.line = __LINE__, \
}; \
______r = __builtin_expect(!!(x), expect); \
ftrace_likely_update(&______f, ______r, \
expect, is_constant); \
______r; \
})
Clang's semantic analysis phase happens early in the front end, so it is
not able to value track ______r yet, meaning that it thinks BUG() is
only conditionally called, leaving len uninitialized in the other
imaginary branch.
I see two ways to fix this. The most obvious is just turn this BUG_ON(1)
into BUG() directly to avoid having an unlikely() in the first place.
diff --git a/arch/arm64/kvm/vgic/vgic-mmio.c b/arch/arm64/kvm/vgic/vgic-mmio.c
index e416e433baff..a573b1f0c6cb 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio.c
@@ -1091,7 +1091,7 @@ int vgic_register_dist_iodev(struct kvm *kvm, gpa_t dist_base_address,
len = vgic_v3_init_dist_iodev(io_device);
break;
default:
- BUG_ON(1);
+ BUG();
}
io_device->base_addr = dist_base_address;
---
The other option would be to turn the unlikely() in the generic BUG_ON()
to unlikely_notrace(), so that the '1' condition can always be
constantly evaluated. I am not sure having branch checks on BUG_ON() is
super useful since these are conditions that are intentionally hard to
hit and you will know when they are :)
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 2d9f61346dab..35f3e914b068 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -74,7 +74,7 @@ struct bug_entry {
#endif
#ifndef HAVE_ARCH_BUG_ON
-#define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
+#define BUG_ON(condition) do { if (unlikely_notrace(condition)) BUG(); } while (0)
#endif
/*
---
Both of these resolve the warning from the original configuration for
me.
Cheers,
Nathan
^ permalink raw reply related [flat|nested] 3+ messages in thread
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2025-07-12 19:29 [linux-next:master 7731/9053] arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: warning: variable 'len' is used uninitialized whenever 'if' condition is false kernel test robot
2025-07-14 12:37 ` Sascha Bischoff
2025-07-14 17:15 ` Nathan Chancellor
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