| Message ID | 20220909035758.17434-2-chenyi.qiang@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Update linux headers to v6.0-rc4 and fix the clang build error | expand |
On Fri, Sep 09 2022, Chenyi Qiang <chenyi.qiang@intel.com> wrote: > In recent linux headers update to v6.0-rc, it switched GNU Maybe "A Linux headers update to v6.0-rc switches some definitions from the GNU..." ? > 'zero-length-array' extension to the C-standard-defined flexible array > member. e.g. > > struct kvm_msrs { > __u32 nmsrs; /* number of msrs in entries */ > __u32 pad; > > - struct kvm_msr_entry entries[0]; > + struct kvm_msr_entry entries[]; > }; > > Those (unlike the GNU zero-length-array) have some extra restrictions like > 'this must be put at the end of a struct', which clang build would complain > about. e.g. the current code > > struct { > struct kvm_msrs info; > struct kvm_msr_entry entries[1]; > } msr_data = { } > > generates the warning like: > > target/i386/kvm/kvm.c:2868:25: error: field 'info' with variable sized > type 'struct kvm_msrs' not at the end of a struct or class is a GNU > extension [-Werror,-Wgnu-variable-sized-type-not-at-end] > struct kvm_msrs info; > ^ > In fact, the variable length 'entries[]' field in 'info' is zero-sized in > GNU defined semantics, which can give predictable offset for 'entries[1]' > in local msr_data. The local defined struct is just there to force a stack > allocation large enough for 1 kvm_msr_entry, a clever trick but requires to > turn off this clang warning. > > Suggested-by: Daniel P. Berrangé <berrange@redhat.com> > Reviewed-by: Richard Henderson <richard.henderson@linaro.org> > Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com> > --- > configure | 1 + > 1 file changed, 1 insertion(+) Reviewed-by: Cornelia Huck <cohuck@redhat.com>
diff --git a/configure b/configure index 575dde1c1f..7e0a1a4187 100755 --- a/configure +++ b/configure @@ -1258,6 +1258,7 @@ add_to nowarn_flags -Wno-string-plus-int add_to nowarn_flags -Wno-typedef-redefinition add_to nowarn_flags -Wno-tautological-type-limit-compare add_to nowarn_flags -Wno-psabi +add_to nowarn_flags -Wno-gnu-variable-sized-type-not-at-end gcc_flags="$warn_flags $nowarn_flags"
In recent linux headers update to v6.0-rc, it switched GNU 'zero-length-array' extension to the C-standard-defined flexible array member. e.g. struct kvm_msrs { __u32 nmsrs; /* number of msrs in entries */ __u32 pad; - struct kvm_msr_entry entries[0]; + struct kvm_msr_entry entries[]; }; Those (unlike the GNU zero-length-array) have some extra restrictions like 'this must be put at the end of a struct', which clang build would complain about. e.g. the current code struct { struct kvm_msrs info; struct kvm_msr_entry entries[1]; } msr_data = { } generates the warning like: target/i386/kvm/kvm.c:2868:25: error: field 'info' with variable sized type 'struct kvm_msrs' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end] struct kvm_msrs info; ^ In fact, the variable length 'entries[]' field in 'info' is zero-sized in GNU defined semantics, which can give predictable offset for 'entries[1]' in local msr_data. The local defined struct is just there to force a stack allocation large enough for 1 kvm_msr_entry, a clever trick but requires to turn off this clang warning. Suggested-by: Daniel P. Berrangé <berrange@redhat.com> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com> --- configure | 1 + 1 file changed, 1 insertion(+)