arch/arm64/kvm/mmu.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)
Hi folks, Yet another bug I found while testing Sashiko locally with fixes to review-prompts. share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c maintain a host-side RB-tree mirroring the set of pages shared with EL2. Both invoke a hypercall that can fail (page-state mismatch, EL2 refcount still held), but neither cleans up on failure: - share_pfn_hyp() inserts the tracking node before the hypercall and leaves it in the tree on failure, leaking the allocation and presenting a phantom share to a later unshare. - unshare_pfn_hyp() erases the tracking node before the hypercall; on failure the host loses its record while EL2 still owns the share, breaking later operations on the same pfn. Severity is low (no isolation impact) and the failure paths are rare in practice, but the desync is real. Both patches are independent and apply cleanly to current mainline. In other words, this can wait for 7.2. Cheers, /fuad Fuad Tabba (2): KVM: arm64: Free hyp-share tracking node when share hypercall fails KVM: arm64: Avoid host/hyp share desync on unshare hypercall failure arch/arm64/kvm/mmu.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) -- 2.54.0.929.g9b7fa37559-goog
On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > Hi folks, > > Yet another bug I found while testing Sashiko locally with fixes to > review-prompts. > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > maintain a host-side RB-tree mirroring the set of pages shared with > EL2. Both invoke a hypercall that can fail (page-state mismatch, > EL2 refcount still held), but neither cleans up on failure: > > - share_pfn_hyp() inserts the tracking node before the hypercall > and leaves it in the tree on failure, leaking the allocation and > presenting a phantom share to a later unshare. > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > on failure the host loses its record while EL2 still owns the > share, breaking later operations on the same pfn. > > Severity is low (no isolation impact) and the failure paths are rare > in practice, but the desync is real. Both patches are independent and > apply cleanly to current mainline. In other words, this can wait for > 7.2. I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. So I haven't sent a v2. > > Cheers, > /fuad > > Fuad Tabba (2): > KVM: arm64: Free hyp-share tracking node when share hypercall fails > KVM: arm64: Avoid host/hyp share desync on unshare hypercall failure > > arch/arm64/kvm/mmu.c | 14 +++++++++++--- > 1 file changed, 11 insertions(+), 3 deletions(-) > > -- > 2.54.0.929.g9b7fa37559-goog >
On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > Hi folks, > > > > Yet another bug I found while testing Sashiko locally with fixes to > > review-prompts. > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > maintain a host-side RB-tree mirroring the set of pages shared with > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > EL2 refcount still held), but neither cleans up on failure: > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > and leaves it in the tree on failure, leaking the allocation and > > presenting a phantom share to a later unshare. > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > on failure the host loses its record while EL2 still owns the > > share, breaking later operations on the same pfn. > > > > Severity is low (no isolation impact) and the failure paths are rare > > in practice, but the desync is real. Both patches are independent and > > apply cleanly to current mainline. In other words, this can wait for > > 7.2. > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > So I haven't sent a v2. At the very least we need to add a comment, otherwise, people like me and LLMs like Sashiko would stumble upon it. That said, this fix adds no real overhead, makes the code clearer, and guards us against a future where that call might fail. Self-documenting in essense. WDYT? /fuad > > > > > Cheers, > > /fuad > > > > Fuad Tabba (2): > > KVM: arm64: Free hyp-share tracking node when share hypercall fails > > KVM: arm64: Avoid host/hyp share desync on unshare hypercall failure > > > > arch/arm64/kvm/mmu.c | 14 +++++++++++--- > > 1 file changed, 11 insertions(+), 3 deletions(-) > > > > -- > > 2.54.0.929.g9b7fa37559-goog > >
On Fri, 29 May 2026 09:05:35 +0100, Fuad Tabba <tabba@google.com> wrote: > > On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > > Hi folks, > > > > > > Yet another bug I found while testing Sashiko locally with fixes to > > > review-prompts. > > > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > > maintain a host-side RB-tree mirroring the set of pages shared with > > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > > EL2 refcount still held), but neither cleans up on failure: > > > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > > and leaves it in the tree on failure, leaking the allocation and > > > presenting a phantom share to a later unshare. > > > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > > on failure the host loses its record while EL2 still owns the > > > share, breaking later operations on the same pfn. > > > > > > Severity is low (no isolation impact) and the failure paths are rare > > > in practice, but the desync is real. Both patches are independent and > > > apply cleanly to current mainline. In other words, this can wait for > > > 7.2. > > > > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > > So I haven't sent a v2. > > At the very least we need to add a comment, otherwise, people like me > and LLMs like Sashiko would stumble upon it. > > That said, this fix adds no real overhead, makes the code clearer, and > guards us against a future where that call might fail. > Self-documenting in essense. > > WDYT? If a hypercall really cannot fail, why does it have a return value? M. -- Without deviation from the norm, progress is not possible.
On Fri, 29 May 2026 at 09:15, Marc Zyngier <maz@kernel.org> wrote: > > On Fri, 29 May 2026 09:05:35 +0100, > Fuad Tabba <tabba@google.com> wrote: > > > > On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > > > Hi folks, > > > > > > > > Yet another bug I found while testing Sashiko locally with fixes to > > > > review-prompts. > > > > > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > > > maintain a host-side RB-tree mirroring the set of pages shared with > > > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > > > EL2 refcount still held), but neither cleans up on failure: > > > > > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > > > and leaves it in the tree on failure, leaking the allocation and > > > > presenting a phantom share to a later unshare. > > > > > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > > > on failure the host loses its record while EL2 still owns the > > > > share, breaking later operations on the same pfn. > > > > > > > > Severity is low (no isolation impact) and the failure paths are rare > > > > in practice, but the desync is real. Both patches are independent and > > > > apply cleanly to current mainline. In other words, this can wait for > > > > 7.2. > > > > > > > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > > > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > > > So I haven't sent a v2. > > > > At the very least we need to add a comment, otherwise, people like me > > and LLMs like Sashiko would stumble upon it. > > > > That said, this fix adds no real overhead, makes the code clearer, and > > guards us against a future where that call might fail. > > Self-documenting in essense. > > > > WDYT? > > If a hypercall really cannot fail, why does it have a return value? Good point. If we know it cannot fail, how about just `void`? That said, Vincen't exact words are: `very much unlikely`, not the same as cannot fail :) https://lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com/ /fuad > > M. > > -- > Without deviation from the norm, progress is not possible.
On Fri, 29 May 2026 09:20:50 +0100, Fuad Tabba <tabba@google.com> wrote: > > On Fri, 29 May 2026 at 09:15, Marc Zyngier <maz@kernel.org> wrote: > > > > On Fri, 29 May 2026 09:05:35 +0100, > > Fuad Tabba <tabba@google.com> wrote: > > > > > > On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > > > > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > > > > Hi folks, > > > > > > > > > > Yet another bug I found while testing Sashiko locally with fixes to > > > > > review-prompts. > > > > > > > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > > > > maintain a host-side RB-tree mirroring the set of pages shared with > > > > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > > > > EL2 refcount still held), but neither cleans up on failure: > > > > > > > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > > > > and leaves it in the tree on failure, leaking the allocation and > > > > > presenting a phantom share to a later unshare. > > > > > > > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > > > > on failure the host loses its record while EL2 still owns the > > > > > share, breaking later operations on the same pfn. > > > > > > > > > > Severity is low (no isolation impact) and the failure paths are rare > > > > > in practice, but the desync is real. Both patches are independent and > > > > > apply cleanly to current mainline. In other words, this can wait for > > > > > 7.2. > > > > > > > > > > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > > > > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > > > > So I haven't sent a v2. > > > > > > At the very least we need to add a comment, otherwise, people like me > > > and LLMs like Sashiko would stumble upon it. > > > > > > That said, this fix adds no real overhead, makes the code clearer, and > > > guards us against a future where that call might fail. > > > Self-documenting in essense. > > > > > > WDYT? > > > > If a hypercall really cannot fail, why does it have a return value? > > Good point. If we know it cannot fail, how about just `void`? > > That said, Vincen't exact words are: `very much unlikely`, not the > same as cannot fail :) > > https://lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com/ I think the rules are simple: - if something can fail, we need to handle the failure - if something should not fail and has the potential of compromising the system, we should panic - if something absolutely cannot fail, then there is nothing to handle Thanks, M. -- Without deviation from the norm, progress is not possible.
On Fri, May 29, 2026 at 10:29:40AM +0100, Marc Zyngier wrote: > On Fri, 29 May 2026 09:20:50 +0100, > Fuad Tabba <tabba@google.com> wrote: > > > > On Fri, 29 May 2026 at 09:15, Marc Zyngier <maz@kernel.org> wrote: > > > > > > On Fri, 29 May 2026 09:05:35 +0100, > > > Fuad Tabba <tabba@google.com> wrote: > > > > > > > > On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > > > > > > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > > > > > Hi folks, > > > > > > > > > > > > Yet another bug I found while testing Sashiko locally with fixes to > > > > > > review-prompts. > > > > > > > > > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > > > > > maintain a host-side RB-tree mirroring the set of pages shared with > > > > > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > > > > > EL2 refcount still held), but neither cleans up on failure: > > > > > > > > > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > > > > > and leaves it in the tree on failure, leaking the allocation and > > > > > > presenting a phantom share to a later unshare. > > > > > > > > > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > > > > > on failure the host loses its record while EL2 still owns the > > > > > > share, breaking later operations on the same pfn. > > > > > > > > > > > > Severity is low (no isolation impact) and the failure paths are rare > > > > > > in practice, but the desync is real. Both patches are independent and > > > > > > apply cleanly to current mainline. In other words, this can wait for > > > > > > 7.2. > > > > > > > > > > > > > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > > > > > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > > > > > So I haven't sent a v2. > > > > > > > > At the very least we need to add a comment, otherwise, people like me > > > > and LLMs like Sashiko would stumble upon it. > > > > > > > > That said, this fix adds no real overhead, makes the code clearer, and > > > > guards us against a future where that call might fail. > > > > Self-documenting in essense. > > > > > > > > WDYT? > > > > > > If a hypercall really cannot fail, why does it have a return value? > > > > Good point. If we know it cannot fail, how about just `void`? > > > > That said, Vincen't exact words are: `very much unlikely`, not the > > same as cannot fail :) > > > > https://lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com/ > > I think the rules are simple: > > - if something can fail, we need to handle the failure Looking at kvm_share_hyp() it should then rollback the shared pages. I think that is fine. > > - if something should not fail and has the potential of compromising > the system, we should panic Then kvm_unshare_hyp() being void, should BUG_ON(unshare_pfn_hyp(pfn)); > > - if something absolutely cannot fail, then there is nothing to handle > > Thanks, > > M. > > -- > Without deviation from the norm, progress is not possible.
On Fri, May 29, 2026 at 09:20:50AM +0100, Fuad Tabba wrote: > On Fri, 29 May 2026 at 09:15, Marc Zyngier <maz@kernel.org> wrote: > > > > On Fri, 29 May 2026 09:05:35 +0100, > > Fuad Tabba <tabba@google.com> wrote: > > > > > > On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > > > > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > > > > Hi folks, > > > > > > > > > > Yet another bug I found while testing Sashiko locally with fixes to > > > > > review-prompts. > > > > > > > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > > > > maintain a host-side RB-tree mirroring the set of pages shared with > > > > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > > > > EL2 refcount still held), but neither cleans up on failure: > > > > > > > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > > > > and leaves it in the tree on failure, leaking the allocation and > > > > > presenting a phantom share to a later unshare. > > > > > > > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > > > > on failure the host loses its record while EL2 still owns the > > > > > share, breaking later operations on the same pfn. > > > > > > > > > > Severity is low (no isolation impact) and the failure paths are rare > > > > > in practice, but the desync is real. Both patches are independent and > > > > > apply cleanly to current mainline. In other words, this can wait for > > > > > 7.2. > > > > > > > > > > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > > > > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > > > > So I haven't sent a v2. > > > > > > At the very least we need to add a comment, otherwise, people like me > > > and LLMs like Sashiko would stumble upon it. > > > > > > That said, this fix adds no real overhead, makes the code clearer, and > > > guards us against a future where that call might fail. > > > Self-documenting in essense. > > > > > > WDYT? > > > > If a hypercall really cannot fail, why does it have a return value? > > Good point. If we know it cannot fail, how about just `void`? > > That said, Vincen't exact words are: `very much unlikely`, not the > same as cannot fail :) > > https://lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com/ The error would happen only if the host tries to share/unshare a page with the wrong state. This would only happen in the case of a misbehaving host. And Quentin's point was that this is anyway incomplete. To handle this error properly, kvm_share_hyp/kvm_unshare_hyp would also need to rollback things... The callers of the unshare should also leak the memory which couldn't be unshared properly. This isn't the case now, (however we do WARN_ON). > > /fuad > > > > > M. > > > > -- > > Without deviation from the norm, progress is not possible.
On Fri, 29 May 2026 at 10:21, Vincent Donnefort <vdonnefort@google.com> wrote: > > On Fri, May 29, 2026 at 09:20:50AM +0100, Fuad Tabba wrote: > > On Fri, 29 May 2026 at 09:15, Marc Zyngier <maz@kernel.org> wrote: > > > > > > On Fri, 29 May 2026 09:05:35 +0100, > > > Fuad Tabba <tabba@google.com> wrote: > > > > > > > > On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > > > > > > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > > > > > Hi folks, > > > > > > > > > > > > Yet another bug I found while testing Sashiko locally with fixes to > > > > > > review-prompts. > > > > > > > > > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > > > > > maintain a host-side RB-tree mirroring the set of pages shared with > > > > > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > > > > > EL2 refcount still held), but neither cleans up on failure: > > > > > > > > > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > > > > > and leaves it in the tree on failure, leaking the allocation and > > > > > > presenting a phantom share to a later unshare. > > > > > > > > > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > > > > > on failure the host loses its record while EL2 still owns the > > > > > > share, breaking later operations on the same pfn. > > > > > > > > > > > > Severity is low (no isolation impact) and the failure paths are rare > > > > > > in practice, but the desync is real. Both patches are independent and > > > > > > apply cleanly to current mainline. In other words, this can wait for > > > > > > 7.2. > > > > > > > > > > > > > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > > > > > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > > > > > So I haven't sent a v2. > > > > > > > > At the very least we need to add a comment, otherwise, people like me > > > > and LLMs like Sashiko would stumble upon it. > > > > > > > > That said, this fix adds no real overhead, makes the code clearer, and > > > > guards us against a future where that call might fail. > > > > Self-documenting in essense. > > > > > > > > WDYT? > > > > > > If a hypercall really cannot fail, why does it have a return value? > > > > Good point. If we know it cannot fail, how about just `void`? > > > > That said, Vincen't exact words are: `very much unlikely`, not the > > same as cannot fail :) > > > > https://lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com/ > > The error would happen only if the host tries to share/unshare a page with the > wrong state. This would only happen in the case of a misbehaving host. > > And Quentin's point was that this is anyway incomplete. To handle this error > properly, kvm_share_hyp/kvm_unshare_hyp would also need to rollback things... > The callers of the unshare should also leak the memory which couldn't be > unshared properly. This isn't the case now, (however we do WARN_ON). If we WARN_ON() in hyp, then I argue we shouldn't have a return value. Or at least add a comment, BUG_ON() here. Think of the poor LLMs and the people who run them :) /fuad > > > > > /fuad > > > > > > > > M. > > > > > > -- > > > Without deviation from the norm, progress is not possible.
On Fri, May 29, 2026 at 10:23:22AM +0100, Fuad Tabba wrote: > On Fri, 29 May 2026 at 10:21, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > On Fri, May 29, 2026 at 09:20:50AM +0100, Fuad Tabba wrote: > > > On Fri, 29 May 2026 at 09:15, Marc Zyngier <maz@kernel.org> wrote: > > > > > > > > On Fri, 29 May 2026 09:05:35 +0100, > > > > Fuad Tabba <tabba@google.com> wrote: > > > > > > > > > > On Fri, 29 May 2026 at 09:02, Vincent Donnefort <vdonnefort@google.com> wrote: > > > > > > > > > > > > On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote: > > > > > > > Hi folks, > > > > > > > > > > > > > > Yet another bug I found while testing Sashiko locally with fixes to > > > > > > > review-prompts. > > > > > > > > > > > > > > share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c > > > > > > > maintain a host-side RB-tree mirroring the set of pages shared with > > > > > > > EL2. Both invoke a hypercall that can fail (page-state mismatch, > > > > > > > EL2 refcount still held), but neither cleans up on failure: > > > > > > > > > > > > > > - share_pfn_hyp() inserts the tracking node before the hypercall > > > > > > > and leaves it in the tree on failure, leaking the allocation and > > > > > > > presenting a phantom share to a later unshare. > > > > > > > > > > > > > > - unshare_pfn_hyp() erases the tracking node before the hypercall; > > > > > > > on failure the host loses its record while EL2 still owns the > > > > > > > share, breaking later operations on the same pfn. > > > > > > > > > > > > > > Severity is low (no isolation impact) and the failure paths are rare > > > > > > > in practice, but the desync is real. Both patches are independent and > > > > > > > apply cleanly to current mainline. In other words, this can wait for > > > > > > > 7.2. > > > > > > > > > > > > > > > > > > I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but > > > > > > as Quentin pointed-out, there's absolutely no reason for the hypercall to fail. > > > > > > So I haven't sent a v2. > > > > > > > > > > At the very least we need to add a comment, otherwise, people like me > > > > > and LLMs like Sashiko would stumble upon it. > > > > > > > > > > That said, this fix adds no real overhead, makes the code clearer, and > > > > > guards us against a future where that call might fail. > > > > > Self-documenting in essense. > > > > > > > > > > WDYT? > > > > > > > > If a hypercall really cannot fail, why does it have a return value? > > > > > > Good point. If we know it cannot fail, how about just `void`? > > > > > > That said, Vincen't exact words are: `very much unlikely`, not the > > > same as cannot fail :) > > > > > > https://lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com/ > > > > The error would happen only if the host tries to share/unshare a page with the > > wrong state. This would only happen in the case of a misbehaving host. > > > > And Quentin's point was that this is anyway incomplete. To handle this error > > properly, kvm_share_hyp/kvm_unshare_hyp would also need to rollback things... > > The callers of the unshare should also leak the memory which couldn't be > > unshared properly. This isn't the case now, (however we do WARN_ON). > > If we WARN_ON() in hyp, then I argue we shouldn't have a return value. I meant the WARN_ON in the host's kvm_hyp_unshare() > Or at least add a comment, BUG_ON() here. Think of the poor LLMs and > the people who run them :) > > /fuad > > > > > > > > > /fuad > > > > > > > > > > > M. > > > > > > > > -- > > > > Without deviation from the norm, progress is not possible.
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