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

[Peter Zijlstra <peterz@infradead.org>]

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);
> +}