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    Subject[tip:sched/urgent] sched: sched_getaffinity(): Allow less than NR_CPUS length
    Commit-ID:  cd3d8031eb4311e516329aee03c79a08333141f1
    Author: KOSAKI Motohiro <>
    AuthorDate: Fri, 12 Mar 2010 16:15:36 +0900
    Committer: Ingo Molnar <>
    CommitDate: Mon, 15 Mar 2010 08:28:44 +0100

    sched: sched_getaffinity(): Allow less than NR_CPUS length

    [ Note, this commit changes the syscall ABI for > 1024 CPUs systems. ]

    Recently, some distro decided to use NR_CPUS=4096 for mysterious reasons.
    Unfortunately, glibc sched interface has the following definition:

    # define __CPU_SETSIZE 1024
    # define __NCPUBITS (8 * sizeof (__cpu_mask))
    typedef unsigned long int __cpu_mask;
    typedef struct
    __cpu_mask __bits[__CPU_SETSIZE / __NCPUBITS];
    } cpu_set_t;

    It mean, if NR_CPUS is bigger than 1024, cpu_set_t makes an
    ABI issue ...

    More recently, Sharyathi Nagesh reported following test program makes
    misterious syscall failure:

    #define _GNU_SOURCE

    int main()
    cpu_set_t set;
    if (sched_getaffinity(0, sizeof(cpu_set_t), &set) < 0)
    printf("\n Call is failing with:%d", errno);

    Because the kernel assumes len argument of sched_getaffinity() is bigger
    than NR_CPUS. But now it is not correct.

    Now we are faced with the following annoying dilemma, due to
    the limitations of the glibc interface built in years ago:

    (1) if we change glibc's __CPU_SETSIZE definition, we lost
    binary compatibility of _all_ application.

    (2) if we don't change it, we also lost binary compatibility of
    Sharyathi's use case.

    Then, I would propse to change the rule of the len argument of

    len should be bigger than NR_CPUS
    len should be bigger than maximum possible cpu id

    This creates the following behavior:

    (A) In the real 4096 cpus machine, the above test program still
    return -EINVAL.

    (B) NR_CPUS=4096 but the machine have less than 1024 cpus (almost
    all machines in the world), the above can run successfully.

    Fortunatelly, BIG SGI machine is mainly used for HPC use case. It means
    they can rebuild their programs.

    IOW we hope they are not annoyed by this issue ...

    Reported-by: Sharyathi Nagesh <>
    Signed-off-by: KOSAKI Motohiro <>
    Acked-by: Ulrich Drepper <>
    Acked-by: Peter Zijlstra <>
    Cc: Linus Torvalds <>
    Cc: Andrew Morton <>
    Cc: Jack Steiner <>
    Cc: Russ Anderson <>
    Cc: Mike Travis <>
    LKML-Reference: <>
    Signed-off-by: Ingo Molnar <>
    kernel/sched.c | 10 +++++++---
    1 files changed, 7 insertions(+), 3 deletions(-)

    diff --git a/kernel/sched.c b/kernel/sched.c
    index 9ab3cd7..6eaef3d 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -4902,7 +4902,9 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
    int ret;
    cpumask_var_t mask;

    - if (len < cpumask_size())
    + if (len < nr_cpu_ids)
    + return -EINVAL;
    + if (len & (sizeof(unsigned long)-1))
    return -EINVAL;

    if (!alloc_cpumask_var(&mask, GFP_KERNEL))
    @@ -4910,10 +4912,12 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,

    ret = sched_getaffinity(pid, mask);
    if (ret == 0) {
    - if (copy_to_user(user_mask_ptr, mask, cpumask_size()))
    + int retlen = min(len, cpumask_size());
    + if (copy_to_user(user_mask_ptr, mask, retlen))
    ret = -EFAULT;
    - ret = cpumask_size();
    + ret = retlen;

     \ /
      Last update: 2010-03-15 08:47    [W:0.025 / U:4.836 seconds]
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