Re: [PATCH bpf-next v4 1/2] bpf: extend stackmap to save binary_build_id+offset instead of address

[Peter Zijlstra]

On Mon, Mar 12, 2018 at 01:39:56PM -0700, Song Liu wrote:
> +static void stack_map_get_build_id_offset(struct bpf_map *map,
> +                                       struct stack_map_bucket *bucket,
> +                                       u64 *ips, u32 trace_nr)
> +{
> +     int i;
> +     struct vm_area_struct *vma;
> +     struct bpf_stack_build_id *id_offs;
> +
> +     bucket->nr = trace_nr;
> +     id_offs = (struct bpf_stack_build_id *)bucket->data;
> +
> +     if (!current || !current->mm ||
> +         down_read_trylock(&current->mm->mmap_sem) == 0) {

You probably want an in_nmi() before the down_read_trylock(). Doing
up_read() is an absolute no-no from NMI context.

And IIUC its 'trivial' to use this stuff with hardware counters.

> +             /* cannot access current->mm, fall back to ips */
> +             for (i = 0; i < trace_nr; i++) {
> +                     id_offs[i].status = BPF_STACK_BUILD_ID_IP;
> +                     id_offs[i].ip = ips[i];
> +             }
> +             return;
> +     }
> +
> +     for (i = 0; i < trace_nr; i++) {
> +             vma = find_vma(current->mm, ips[i]);
> +             if (!vma || stack_map_get_build_id(vma, id_offs[i].build_id)) {
> +                     /* per entry fall back to ips */
> +                     id_offs[i].status = BPF_STACK_BUILD_ID_IP;
> +                     id_offs[i].ip = ips[i];
> +                     continue;
> +             }
> +             id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + ips[i]
> +                     - vma->vm_start;
> +             id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
> +     }
> +     up_read(&current->mm->mmap_sem);
> +}