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KVM: squelch uninitialized variable warning

PATCH In review Barret Rhoden 3 participants 3 messages Jan 9, 2020 – Jan 18, 2020 KVM

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Barret Rhoden

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#0 lore raw

PATCH 1/1 KVM: squelch uninitialized variable warning

If gfn_to_hva_many() fails, __kvm_gfn_to_hva_cache_init() will return an error. Before it does, it might use nr_pages_avail, which my compiler complained about:

	virt/kvm/kvm_main.c:2193:13: warning: 'nr_pages_avail' may be
	used uninitialized in this function [-Wmaybe-uninitialized]

	   start_gfn += nr_pages_avail;
Signed-off-by Barret Rhoden <[email protected]>
---
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
virt/kvm/kvm_main.c +1 -1
index d9aced677ddd..f8249b153d33 100644
@@ -2172,7 +2172,7 @@ static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
 	gfn_t start_gfn = gpa >> PAGE_SHIFT;
 	gfn_t end_gfn = (gpa + len - 1) >> PAGE_SHIFT;
 	gfn_t nr_pages_needed = end_gfn - start_gfn + 1;
-	gfn_t nr_pages_avail;
+	gfn_t nr_pages_avail = 0;
 	int r = start_gfn <= end_gfn ? 0 : -EINVAL;
 
 	ghc->gpa = gpa;
Signature · 1 line
2.25.0.rc1.283.g88dfdc4193-goog
Reviewed-by on v1 1/1
#1 lore raw
Quoted 8 lines · Barret Rhoden

If gfn_to_hva_many() fails, __kvm_gfn_to_hva_cache_init() will return an error. Before it does, it might use nr_pages_avail, which my compiler complained about:

	virt/kvm/kvm_main.c:2193:13: warning: 'nr_pages_avail' may be
	used uninitialized in this function [-Wmaybe-uninitialized]

	   start_gfn += nr_pages_avail;

Ugh, this whole flow is funky. The change is correct, and is certainly the minimal change if we want to backport this to stable, but IMO it's putting lipstick on a pig. I'd rather fix the underlying issues and make the code more readable in the process. I'll send a patch.

If we want to take this as a minimal fix,

Reviewed-by Sean Christopherson <[email protected]>
Quoted 21 lines
Signed-off-by Barret Rhoden <[email protected]>
---
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
virt/kvm/kvm_main.c +1 -1
index d9aced677ddd..f8249b153d33 100644
@@ -2172,7 +2172,7 @@ static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
 	gfn_t start_gfn = gpa >> PAGE_SHIFT;
 	gfn_t end_gfn = (gpa + len - 1) >> PAGE_SHIFT;
 	gfn_t nr_pages_needed = end_gfn - start_gfn + 1;
-	gfn_t nr_pages_avail;
+	gfn_t nr_pages_avail = 0;
 	int r = start_gfn <= end_gfn ? 0 : -EINVAL;
 
 	ghc->gpa = gpa;
Signature · 1 line
2.25.0.rc1.283.g88dfdc4193-goog