On Thu, Dec 17, 2020 at 8:33 PM Song Liu <songliubrav...@fb.com> wrote:
> >
> > ahh. I missed that. Makes sense.
> > vm_file needs to be accurate, but vm_area_struct should be accessed as
> > ptr_to_btf_id.
>
> Passing pointer of vm_area_struct into BPF will be tricky. For example, shall
> we
> allow the user to access vma->vm_file? IIUC, with ptr_to_btf_id the verifier
> will
> allow access of vma->vm_file as a valid pointer to struct file. However,
> since the
> vma might be freed, vma->vm_file could point to random data.
I don't think so. The proposed patch will do get_file() on it.
There is actually no need to assign it into a different variable.
Accessing it via vma->vm_file is safe and cleaner.
> >> [1] ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on
> >> mmap_lock")
> >
> > Thanks for this link. With "if (mmap_lock_is_contended())" check it should
> > work indeed.
>
> To make sure we are on the same page: I am using slightly different mechanism
> in
> task_vma_iter, which doesn't require checking mmap_lock_is_contended(). In the
> smaps_rollup case, the code only unlock mmap_sem when the lock is contended.
> In
> task_iter, we always unlock mmap_sem between two iterations. This is because
> we
> don't want to hold mmap_sem while calling the BPF program, which may sleep
> (calling
> bpf_d_path).
That part is clear. I had to look into mmap_read_lock_killable() implementation
to realize that it's checking for lock_is_contended after acquiring
and releasing
if there is a contention. So it's the same behavior at the end.
