在 2021/9/9 上午2:13, Linus Torvalds 写道:
> On Tue, Sep 7, 2021 at 7:57 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>> After looking at the kcore code, it is found that kcore does not implement
>> mmap, resulting in frequent context switching triggered by read.
>> Therefore, we want to add mmap interface to optimize performance.  Since
>> vmalloc and module areas will change with allocation and release,
>> consistency cannot be guaranteed, so mmap interface only maps KCORE_TEXT
>> and KCORE_RAM.
> Honestly, I still hate this patch.
>
> The last time people wanted to speed up /dev/kcore accesses, it was
> all for black-hat reasons and speeding up kernel attacks.
>
> And this code just makes me nervous even aside from that, because I do
> not understand what the heck it's doing.
>
>
>> +       if (kern_addr_valid(start)) {
>> +               if (m->type == KCORE_RAM)
>> +                       pfn = __pa(start) >> PAGE_SHIFT;
>> +               else if (m->type == KCORE_TEXT)
>> +                       pfn = __pa_symbol(start) >> PAGE_SHIFT;
> Why is "__pa(start)" right in one situation, and "__pa_symbol(start)"
> in another.

Hi, Linus

The use here is refer to "read_kcore" in fs/proc/kcore.c.

list_for_each_entry(m, &kclist_head, list) {
...
if (m->type == KCORE_RAM || m->type == KCORE_REMAP)
phdr->p_paddr = __pa(m->addr);
else if (m->type == KCORE_TEXT)
phdr->p_paddr = __pa_symbol(m->addr);
...
}

Ensure consistency of usage.


>
> So this just makes me go "this is all confusing, dangerous, and the
> use-case is dubious".
>
> Mapping kernel memory is dangerous. The use-cases for it are dubious.
> The patch isn't obvious.

Kcore mmap kernel memory just has read permission.

+	if (vma->vm_flags & (VM_WRITE | VM_EXEC)) {
+		ret = -EPERM;
+		goto out;
+	}
+
+	vma->vm_flags &= ~(VM_MAYWRITE | VM_MAYEXEC);
+	vma->vm_flags |= VM_MIXEDMAP;
+	vma->vm_ops = &kcore_mmap_ops;

Compared to the read interface, kcore mmap has no increased risk, just
reduce context switching.

>
> All of that screams "I'll skip this".
>
>             Linus