Re: [RFC bpf-next 1/2] bpf: Fail uprobe multi link with negative offset

[Alan Maguire <alan.maguire@oracle.com>]

On 13/12/2023 14:12, Jiri Olsa wrote:
> Currently the __uprobe_register will return 0 (success) when called with
> negative offset. The reason is that the call to register_for_each_vma and
> then build_map_info won't return error for negative offset. They just won't
> do anything - no matching vma is found so there's no registered breakpoint
> for the uprobe.
> 
> I don't think we can change the behaviour of __uprobe_register and fail
> for negative uprobe offset, because apps might depend on that already.
> 

just my view, but since passing negative offsets never made sense, I
wouldn't be as worried about breaking existing consumers. Regardless of
what a user thought would happen passing a negative value, nothing did,
so that can't have been their intent.

> But I think we can still make the change and check for it on bpf multi
> link syscall level.
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  kernel/trace/bpf_trace.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 774cf476a892..0dbf8d9b3ace 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -3397,6 +3397,11 @@ int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
>  			goto error_free;
>  		}
>  
> +		if (uprobes[i].offset < 0) {
> +			err = -EINVAL;
> +			goto error_free;
> +		}
> +
>  		uprobes[i].link = link;
>  
>  		if (flags & BPF_F_UPROBE_MULTI_RETURN)