lkml.org 
[lkml]   [2016]   [Jan]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] mm, gup: introduce concept of "foreign" get_user_pages()
    On 01/25, Srikar Dronamraju wrote:
    >
    > > The uprobes is_trap_at_addr() location holds mmap_sem and
    > > calls get_user_pages(current->mm) on an instruction address. This
    > > makes it a pretty unique gup caller.

    Yes, in particular is_trap_at_addr() doesn't look really nice. But we need
    to read the insn under mmap_sem to avoid the race with unregister + register
    at the same address, so that we won't send the wrong SIGTRAP in this case.

    > Changes for uprobes.c looks good to me.
    > Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

    Agreed, the changes in uprobes.c look fine.


    > > @@ -1700,7 +1700,13 @@ static int is_trap_at_addr(struct mm_str
    > > if (likely(result == 0))
    > > goto out;
    > >
    > > - result = get_user_pages(NULL, mm, vaddr, 1, 0, 1, &page, NULL);
    > > + /*
    > > + * The NULL 'tsk' here ensures that any faults that occur here
    > > + * will not be accounted to the task. 'mm' *is* current->mm,
    > > + * but we treat this as a 'foreign' access since it is
    > > + * essentially a kernel access to the memory.
    > > + */
    > > + result = get_user_pages_foreign(NULL, mm, vaddr, 1, 0, 1, &page, NULL);
    > > if (result < 0)
    > > return result;

    Yes, but perhaps we should simply remove this get_user_pages_foreign() and just
    return -EFAULT if copy_from_user_inatomic() fails. This should be very unlikely
    case, I think it would be fine to restart this insn and take another bp hit to
    fault this page in.

    Srikar what do you think? IIRC, this get_user_pages() was needed before, when
    is_trap_at_addr() had other (non-restartable) callers with mm != current->mm.

    But again, I think this patch is fine, we can do this later.

    Oleg.

    \
     
     \ /
      Last update: 2016-01-25 19:41    [W:3.583 / U:0.648 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site