Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

[Will Drewry]

On Thu, Jan 12, 2012 at 10:22 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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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