This problem was caught when running KVM PPC selftests on big-endian.
__ffs() doc clearly says:
"Undefined if no bit exists, so code should check against 0 first"
This cause KVM to deliever an arbitary interrupt to the guest and was
causing guest to hang up while running these selftests.
On checking further - GCC was emitting 0 in this case, which caused
System Reset in guest while running kvm selftests.
Use find_first_bit() instead, which reads the word once and returns the
bitmap size when no bit is set, instead of reading *pending twice
thereby causing TOCTOU as pointed by Sashiko.
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
arch/powerpc/kvm/book3s.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
index 2efbe05caed7..5de4b6e86656 100644
--- a/arch/powerpc/kvm/book3s.c
+++ b/arch/powerpc/kvm/book3s.c
@@ -401,7 +401,7 @@ int kvmppc_core_prepare_to_enter(struct kvm_vcpu *vcpu)
if (vcpu->arch.pending_exceptions)
printk(KERN_EMERG "KVM: Check pending: %lx\n", vcpu->arch.pending_exceptions);
#endif
- priority = __ffs(*pending);
+ priority = find_first_bit(pending, BITS_PER_BYTE * sizeof(*pending));
while (priority < BOOK3S_IRQPRIO_MAX) {
if (kvmppc_book3s_irqprio_deliver(vcpu, priority) &&
clear_irqprio(vcpu, priority)) {--
2.39.5