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SubjectRe: [PATCH v2] KVM: x86: avoid large stack allocations in em_fxrstor
On Tue, May 30, 2017 at 04:14:50AM +0800, kbuild test robot wrote:
> arch/x86/kvm/emulate.c: In function 'em_fxrstor':
> >> arch/x86/kvm/emulate.c:4015:5: warning: 'size' may be used uninitialized in this function [-Wmaybe-uninitialized]
> rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);
> ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I definitely thought about this before submitting. size is initialized
in the case ctxt->mode < X86EMUL_MODE_PROT64, and
ctxt->mode == X86EMUL_MODE_PROT64, but uninitialized in the case
ctxt->mode > X86EMUL_MODE_PROT64.

Paulo mentions this is impossible in:

https://lkml.org/lkml/2017/5/26/62

>>> ctxt->mode is never > X86EMUL_MODE_PROT64
>>> (see the definition of enum x86emul_mode in
>>> arch/x86/include/asm/kvm_emulate.h.)

From arch/x86/include/asm/kvm_emulate.h:

272 enum x86emul_mode {
273 X86EMUL_MODE_REAL, /* Real mode. */
274 X86EMUL_MODE_VM86, /* Virtual 8086 mode. */
275 X86EMUL_MODE_PROT16, /* 16-bit protected mode. */
276 X86EMUL_MODE_PROT32, /* 32-bit protected mode. */
277 X86EMUL_MODE_PROT64, /* 64-bit (long) mode. */
278 };

I would still rather err on the side of safety, and initialize the
variable. So I will submit a v3 initializing size to 0, and check that
size was set to a reasonable value at some point before invoke
segmented_read_std.

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