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Re: [libvirt] [PATCH] qemu: fix an off-by-one error in qemuDomainGetPercpuStats

On 02/20/2013 11:25 PM, Eric Blake wrote:
On 02/20/2013 04:51 AM, Guannan Ren wrote:
The max value of number of cpus to compute(id) should not
be equal or greater than max cpu number.
The bug ocurrs when id value is equal to max cpu number which

leads to the off-by-one error in the following for loop.

  # virsh  cpu-stats guest --start 1
  error: Failed to virDomainGetCPUStats()

  error: internal error cpuacct parse error
  src/qemu/qemu_driver.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

      /* number of cpus to compute */
-    id = max_id;
-    if (max_id - start_cpu > ncpus - 1)
The old code was trying to avoid the possibility of integer overflow, by
only using subtraction (src/libvirt.c already ensured we have
non-negative values, but not necessarily that the sum of two positive
values won't wrap around to negative).

     Get it now,  I was curious about the subtraction before.

+    if ((start_cpu + ncpus) >= max_id)
The new code is not as careful, and has redundant ().

ACK if you change this to the equivalent:

if (start_cpu >= max_id - ncpus)

    Thanks. I pushed with your changes.


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