Re: [PATCH v2 bpf-next 10/17] libbpf: tighten BTF type ID rewriting with error checking

On Fri, Apr 23, 2021 at 9:34 AM Alexei Starovoitov
[off-list ref] wrote:

On Fri, Apr 23, 2021 at 9:31 AM Andrii Nakryiko
[off-list ref] wrote:

quoted

quoted

quoted

static int remap_type_id(__u32 *type_id, void *ctx)
{
        int *id_map = ctx;
        int new_id = id_map[*type_id];


/* Here VOID stays VOID, that's all */

        if (*type_id == 0)
                return 0;

Does it mean that id_map[0] is a garbage value?
and all other code that might be doing id_map[idx] might be reading
garbage if it doesn't have a check for idx == 0 ?

No, id_map[0] == 0 by construction (id_map is obj->btf_type_map and is
calloc()'ed) and can be used as id_map[idx].

Ok. Then why are you insisting on this micro optimization to return 0
directly?
That's the confusing part for me.

I'm not insisting:

  > but I'll rewrite it to a combined if if it makes it easier to follow

So I'm confused why you are confused.

If it was:
"if (new_id == 0 && *type_id != 0) { pr_warn"
Then it would be clear what error condition is about.
But 'return 0' messing things up in my mind,
because it's far from obvious that first check is really a combination
with the 2nd check and by itself it's a micro optimization to avoid
reading id_map[0].

I didn't try to micro optimize, that's how I naturally think about the
problem. I'll rewrite the if, don't know why we are spending emails on
this still.