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

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 s/ocurrs/occurs/ > 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).

> + 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)