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 == ®s_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, ®s_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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