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    SubjectRe: cpumask: fix compat getaffinity
    > On Wed, 12 May 2010 06:00:45 pm Milton Miller wrote:
    > >
    > > At least for parsing, we need to allocate and parse NR_CPUS until
    > > all places like arch/powerpc/platforms/pseries/xics.c that compare a
    > > user-supplied mask to CPUMASK_ALL are eliminated.
    > Good point. Anton will want to fix those anyway for CONFIG_CPUMASK_OFFSTACK,
    > too, but that's the reason the parsing uses nr_cpumask_bits.
    > > > Would it make sense to use my initial patch for -stable, which reverts
    > > > the ABI back to before the change that caused the problem, but apply
    > > > the correct fix (changing the ABI throughout) for future releases?
    > >
    > > This would definitly be the conservative fix.
    > Instead of changing back to NR_CPUS which will break libnuma for
    > CPUMASK_OFFSTACK, how about changing it to nr_cpumask_bits and having an
    > explicit comment above it:

    Yes and No.

    1) sched_getaffinity syscall is used from glibc and libnuma.
    2) glibc doesn't use the return value almostly. glibc emulate it as NR_CPUS=1024.
    3) Now, both sched_getaffinity() and compat_sys_sched_getaffinity() have nr_cpu_ids thing.
    4) But only compat_sys_sched_getaffinity() hit libnuma problem.

    I think It mean compat_sys_sched_getaffinity() should behave as sched_getaffinity().
    IOW, libnuma assume compat_sys_sched_getaffinity() return len args or NR_CPUS.
    then, following patch do it. I confirmed the patch works with or without CPUMASK_OFFSTACK.

    So, My proposal is
    1) merge both mine and yours to linus tree
    2) but backport only mine

    How do you think?

    From: KOSAKI Motohiro <>
    Subject: [PATCH] cpumask: fix compat getaffinity

    Commit a45185d2d "cpumask: convert kernel/compat.c" broke
    libnuma, which abuses sched_getaffinity to find out NR_CPUS
    in order to parse /sys/devices/system/node/node*/cpumap.

    On NUMA systems with less than 32 possibly CPUs, the
    current compat_sys_sched_getaffinity now returns '4'
    instead of the actual NR_CPUS/8, which makes libnuma
    bail out when parsing the cpumap.

    The libnuma call sched_getaffinity(0, bitmap, 4096)
    at first. It mean the libnuma expect the return value of
    sched_getaffinity() is either len argument or NR_CPUS.
    But it doesn't expect to return nr_cpu_ids.

    Strictly speaking, userland requirement are

    1) Glibc assume the return value mean the lengh of initialized
    of mask argument. E.g. if sched_getaffinity(1024) return 128,
    glibc make zero fill rest 896 byte.
    2) Libnuma assume the return value can be used to guess NR_CPUS
    in kernel. It assume len-arg<NR_CPUS makes -EINVAL. But
    it try len=4096 at first and 4096 is always bigger than
    NR_CPUS. Then, if we remove strange min_length normalization,
    we never hit -EINVAL case.

    sched_getaffinity() already solved this issue. This patch
    adapt compat_sys_sched_getaffinity() as it.

    Signed-off-by: KOSAKI Motohiro <>
    kernel/compat.c | 25 +++++++++++--------------
    1 files changed, 11 insertions(+), 14 deletions(-)

    diff --git a/kernel/compat.c b/kernel/compat.c
    index 7f40e92..5adab05 100644
    --- a/kernel/compat.c
    +++ b/kernel/compat.c
    @@ -495,29 +495,26 @@ asmlinkage long compat_sys_sched_getaffinity(compat_pid_t pid, unsigned int len,
    int ret;
    cpumask_var_t mask;
    - unsigned long *k;
    - unsigned int min_length = cpumask_size();
    - if (nr_cpu_ids <= BITS_PER_COMPAT_LONG)
    - min_length = sizeof(compat_ulong_t);

    - if (len < min_length)
    + if ((len * BITS_PER_BYTE) < nr_cpu_ids)
    + return -EINVAL;
    + if (len & (sizeof(compat_ulong_t)-1))
    return -EINVAL;

    if (!alloc_cpumask_var(&mask, GFP_KERNEL))
    return -ENOMEM;

    ret = sched_getaffinity(pid, mask);
    - if (ret < 0)
    - goto out;
    + if (ret == 0) {
    + size_t retlen = min_t(size_t, len, cpumask_size());

    - k = cpumask_bits(mask);
    - ret = compat_put_bitmap(user_mask_ptr, k, min_length * 8);
    - if (ret == 0)
    - ret = min_length;
    + if (compat_put_bitmap(user_mask_ptr, cpumask_bits(mask), retlen * 8))
    + ret = -EFAULT;
    + else
    + ret = retlen;
    + }
    return ret;


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