[PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

[PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

[Arnd Bergmann]

clang points out that running a 64-bit guest on a 32-bit host
would lead to uninitialized variables:

arch/x86/kvm/hyperv.c:1610:6: error: variable 'ingpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
        if (!longmode) {
            ^~~~~~~~~
arch/x86/kvm/hyperv.c:1632:55: note: uninitialized use occurs here
        trace_kvm_hv_hypercall(code, fast, rep_cnt, rep_idx, ingpa, outgpa);
                                                             ^~~~~
arch/x86/kvm/hyperv.c:1610:2: note: remove the 'if' if its condition is always true
        if (!longmode) {
        ^~~~~~~~~~~~~~~
arch/x86/kvm/hyperv.c:1595:18: note: initialize the variable 'ingpa' to silence this warning
        u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
                        ^
                         = 0
arch/x86/kvm/hyperv.c:1610:6: error: variable 'outgpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
arch/x86/kvm/hyperv.c:1610:6: error: variable 'param' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]

Since that combination is not supported anyway, change the condition
to tell the compiler how the code is actually executed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
v2: make the change inside of is_64_bit_mode().
---
 arch/x86/kvm/hyperv.c | 6 ++----
 arch/x86/kvm/x86.h    | 4 ++++
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index a39e38f13029..4c1c423a3d08 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1594,7 +1594,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 {
 	u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
 	uint16_t code, rep_idx, rep_cnt;
-	bool fast, longmode, rep;
+	bool fast, rep;
 
 	/*
 	 * hypercall generates UD from non zero cpl and real mode
@@ -1605,9 +1605,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 		return 1;
 	}
 
-	longmode = is_64_bit_mode(vcpu);
-
-	if (!longmode) {
+	if (!is_64_bit_mode(vcpu)) {
 		param = ((u64)kvm_rdx_read(vcpu) << 32) |
 			(kvm_rax_read(vcpu) & 0xffffffff);
 		ingpa = ((u64)kvm_rbx_read(vcpu) << 32) |
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index e08a12892e8b..b457b3267296 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -90,6 +90,7 @@ static inline int is_long_mode(struct kvm_vcpu *vcpu)
 #endif
 }
 
+#ifdef CONFIG_X86_64
 static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
 {
 	int cs_db, cs_l;
@@ -99,6 +100,9 @@ static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
 	kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
 	return cs_l;
 }
+#else
+#define is_64_bit_mode(vcpu) false
+#endif
 
 static inline bool is_la57_mode(struct kvm_vcpu *vcpu)
 {
-- 
2.20.0

Re: [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

[Roman Kagan]

On Fri, Jul 12, 2019 at 03:32:43PM +0200, Arnd Bergmann wrote:
> clang points out that running a 64-bit guest on a 32-bit host
> would lead to uninitialized variables:
> 
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'ingpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>         if (!longmode) {
>             ^~~~~~~~~
> arch/x86/kvm/hyperv.c:1632:55: note: uninitialized use occurs here
>         trace_kvm_hv_hypercall(code, fast, rep_cnt, rep_idx, ingpa, outgpa);
>                                                              ^~~~~
> arch/x86/kvm/hyperv.c:1610:2: note: remove the 'if' if its condition is always true
>         if (!longmode) {
>         ^~~~~~~~~~~~~~~
> arch/x86/kvm/hyperv.c:1595:18: note: initialize the variable 'ingpa' to silence this warning
>         u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
>                         ^
>                          = 0
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'outgpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> arch/x86/kvm/hyperv.c:1610:6: error: variable 'param' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> 
> Since that combination is not supported anyway, change the condition
> to tell the compiler how the code is actually executed.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> v2: make the change inside of is_64_bit_mode().
> ---
>  arch/x86/kvm/hyperv.c | 6 ++----
>  arch/x86/kvm/x86.h    | 4 ++++
>  2 files changed, 6 insertions(+), 4 deletions(-)

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>

However I still think the log message could state it more explicitly
that it was the compiler's fault, and the patch is a workaround for it.

Otherwise later on someone may decide to restore the similarity of
is_64_bit_mode() to other inlines in this file, and will be extremely
unlikely to test clang 32-bit build...

Roman.

Re: [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

[Arnd Bergmann]

On Fri, Jul 12, 2019 at 3:54 PM Roman Kagan <rkagan@virtuozzo.com> wrote:
>
> On Fri, Jul 12, 2019 at 03:32:43PM +0200, Arnd Bergmann wrote:
> > clang points out that running a 64-bit guest on a 32-bit host
> > would lead to uninitialized variables:
> >
> > arch/x86/kvm/hyperv.c:1610:6: error: variable 'ingpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> >         if (!longmode) {
> >             ^~~~~~~~~
> > arch/x86/kvm/hyperv.c:1632:55: note: uninitialized use occurs here
> >         trace_kvm_hv_hypercall(code, fast, rep_cnt, rep_idx, ingpa, outgpa);
> >                                                              ^~~~~
> > arch/x86/kvm/hyperv.c:1610:2: note: remove the 'if' if its condition is always true
> >         if (!longmode) {
> >         ^~~~~~~~~~~~~~~
> > arch/x86/kvm/hyperv.c:1595:18: note: initialize the variable 'ingpa' to silence this warning
> >         u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
> >                         ^
> >                          = 0
> > arch/x86/kvm/hyperv.c:1610:6: error: variable 'outgpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> > arch/x86/kvm/hyperv.c:1610:6: error: variable 'param' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
> >
> > Since that combination is not supported anyway, change the condition
> > to tell the compiler how the code is actually executed.
> >
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> > v2: make the change inside of is_64_bit_mode().
> > ---
> >  arch/x86/kvm/hyperv.c | 6 ++----
> >  arch/x86/kvm/x86.h    | 4 ++++
> >  2 files changed, 6 insertions(+), 4 deletions(-)
>
> Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
>
> However I still think the log message could state it more explicitly
> that it was the compiler's fault, and the patch is a workaround for it.
>
> Otherwise later on someone may decide to restore the similarity of
> is_64_bit_mode() to other inlines in this file, and will be extremely
> unlikely to test clang 32-bit build...

Fair enough. I've reworded the changelog, as well as the patch to
document this now, in a way that should make it harder to introduce
the warning again by accident. Unfortunately, that #ifdef check
cannot be turned into an if(IS_ENABLED()) because kvm_r8_read()
is not defined on i386.

Note that the 0-day bot now tests with clang as well, so you would
definitely hear about a warning appearing.

v3 coming.

       Arnd