On 12/8/23 12:16 AM, Martin KaFai Lau wrote:
On 12/7/23 7:59 PM, Yonghong Song wrote:
I am trying to avoid making a special case for "bool has_btf_ref;"
and "bool from_map_check". It seems to a bit too much to deal with
the error path for btf_parse().
Would doing the refcount_set(&btf->refcnt, 1) earlier in btf_parse
help?
No, it does not. The core reason is what Hao is mentioned in
https://lore.kernel.org/bpf/47ee3265-23f7-2130-ff28-27bfaf3f7877@xxxxxxxxxxxxxxx/
We simply cannot take btf reference if called from btf_parse().
Let us say we move refcount_set(&btf->refcnt, 1) earlier in btf_parse()
so we take ref for btf during btf_parse_fields(), then we have
btf_put <=== expect refcount == 0 to start the destruction process
...
btf_record_free <=== in which if graph_root, a btf
reference will be hold
so btf_put will never be able to actually free btf data.
ah. There is a loop like btf->struct_meta_tab->...btf.
Yes, the kasan problem will be resolved but we leak memory.
It is also unnecessary to take a reference since the value_rec is
referring to a record in struct_meta_tab.
If we optimize for not taking a refcnt, how about not taking a
refcnt for all cases and postpone the btf_put(), instead of taking
refcnt in one case but not another. Like your fix in v1. The failed
selftest can be changed or even removed if it does not make sense
anymore.
After a couple of iterations, I think taking necessary reference
approach sounds better
and this will be consistent with how kptr is handled. For kptr,
btf_parse will ignore it.
Got it. It is why kptr.btf got away with the loop.
On the other hand, am I reading it correctly that kptr.btf only needs
to take the refcnt for btf that is btf_is_kernel()?
No. besides vmlinux and module btf, it also takes reference for prog btf, see
static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
struct btf_field_info *info)
{
...
if (id == -ENOENT) {
/* btf_parse_kptr should only be called w/ btf = program BTF */
WARN_ON_ONCE(btf_is_kernel(btf));
/* Type exists only in program BTF. Assume that it's a MEM_ALLOC
* kptr allocated via bpf_obj_new
*/
field->kptr.dtor = NULL;
id = info->kptr.type_id;
kptr_btf = (struct btf *)btf;
btf_get(kptr_btf);
goto found_dtor;
}
...
}
Unfortunately, for graph_root (list_head, rb_root), btf_parse and
map_check will both
process it and that adds a little bit complexity.
Alexei also suggested the same taking reference approach:
https://lore.kernel.org/bpf/CAADnVQL+uc6VV65_Ezgzw3WH=ME9z1Fdy8Pd6xd0oOq8rgwh7g@xxxxxxxxxxxxxx/
