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    SubjectRe: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF
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    On Thu, Jan 12, 2012 at 10:22 AM, Oleg Nesterov <oleg@redhat.com> wrote:
    > On 01/11, Will Drewry wrote:
    >>
    >> +__weak u8 *seccomp_get_regs(u8 *scratch, size_t *available)
    >> +{
    >> +     /* regset is usually returned based on task personality, not current
    >> +      * system call convention.  This behavior makes it unsafe to execute
    >> +      * BPF programs over regviews if is_compat_task or the personality
    >> +      * have changed since the program was installed.
    >> +      */
    >> +     const struct user_regset_view *view = task_user_regset_view(current);
    >> +     const struct user_regset *regset = &view->regsets[0];
    >> +     size_t scratch_size = *available;
    >> +     if (regset->core_note_type != NT_PRSTATUS) {
    >> +             /* The architecture should override this method for speed. */
    >> +             regset = find_prstatus(view);
    >> +             if (!regset)
    >> +                     return NULL;
    >> +     }
    >> +     *available = regset->n * regset->size;
    >> +     /* Make sure the scratch space isn't exceeded. */
    >> +     if (*available > scratch_size)
    >> +             *available = scratch_size;
    >> +     if (regset->get(current, regset, 0, *available, scratch, NULL))
    >> +             return NULL;
    >> +     return scratch;
    >> +}
    >> +
    >> +/**
    >> + * seccomp_test_filters - tests 'current' against the given syscall
    >> + * @syscall: number of the system call to test
    >> + *
    >> + * Returns 0 on ok and non-zero on error/failure.
    >> + */
    >> +int seccomp_test_filters(int syscall)
    >> +{
    >> +     struct seccomp_filter *filter;
    >> +     u8 regs_tmp[sizeof(struct user_regs_struct)], *regs;
    >> +     size_t regs_size = sizeof(struct user_regs_struct);
    >> +     int ret = -EACCES;
    >> +
    >> +     filter = current->seccomp.filter; /* uses task ref */
    >> +     if (!filter)
    >> +             goto out;
    >> +
    >> +     /* All filters in the list are required to share the same system call
    >> +      * convention so only the first filter is ever checked.
    >> +      */
    >> +     if (seccomp_check_personality(filter))
    >> +             goto out;
    >> +
    >> +     /* Grab the user_regs_struct.  Normally, regs == &regs_tmp, but
    >> +      * that is not mandatory.  E.g., it may return a point to
    >> +      * task_pt_regs(current).  NULL checking is mandatory.
    >> +      */
    >> +     regs = seccomp_get_regs(regs_tmp, &regs_size);
    >
    > Stupid question. I am sure you know what are you doing ;) and I know
    > nothing about !x86 arches.
    >
    > But could you explain why it is designed to use user_regs_struct ?
    > Why we can't simply use task_pt_regs() and avoid the (costly) regsets?

    So on x86 32, it would work since user_regs_struct == task_pt_regs
    (iirc), but on x86-64
    and others, that's not true. I don't think it's kosher to expose
    pt_regs to the userspace, but if, let's say, x86-32 overrides the weak
    linkage, then it could just return task_pt_regs and be the fastest
    path.

    If it would be appropriate to expose pt_regs to userspace, then I'd
    happily do so :)
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