On Thu, Jun 18, 2020 at 4:52 PM Andrey Ignatov <rdna@xxxxxx> wrote:
>
> Andrey Ignatov <rdna@xxxxxx> [Thu, 2020-06-18 12:42 -0700]:
> > Martin KaFai Lau <kafai@xxxxxx> [Wed, 2020-06-17 23:18 -0700]:
> > > On Wed, Jun 17, 2020 at 01:43:45PM -0700, Andrey Ignatov wrote:
> > > [ ... ]
> > > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > > index e5c5305e859c..fa21b1e766ae 100644
> > > > --- a/kernel/bpf/btf.c
> > > > +++ b/kernel/bpf/btf.c
> > > > @@ -3577,6 +3577,67 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, struct btf *btf,
> > > > return ctx_type;
> > > > }
> > > >
> > > > +#define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type)
> > > > +#define BPF_LINK_TYPE(_id, _name)
> > > > +static const struct bpf_map_ops * const btf_vmlinux_map_ops[] = {
> > > > +#define BPF_MAP_TYPE(_id, _ops) \
> > > > + [_id] = &_ops,
> > > > +#include <linux/bpf_types.h>
> > > > +#undef BPF_MAP_TYPE
> > > > +};
> > > > +static u32 btf_vmlinux_map_ids[] = {
> > > > +#define BPF_MAP_TYPE(_id, _ops) \
> > > > + [_id] = (u32)-1,
> > > > +#include <linux/bpf_types.h>
> > > > +#undef BPF_MAP_TYPE
> > > > +};
> > > > +#undef BPF_PROG_TYPE
> > > > +#undef BPF_LINK_TYPE
> > > > +
> > > > +static int btf_vmlinux_map_ids_init(const struct btf *btf,
> > > > + struct bpf_verifier_log *log)
> > > > +{
> > > > + int base_btf_id, btf_id, i;
> > > > + const char *btf_name;
> > > > +
> > > > + base_btf_id = btf_find_by_name_kind(btf, "bpf_map", BTF_KIND_STRUCT);
> > > > + if (base_btf_id < 0)
> > > > + return base_btf_id;
> > > > +
> > > > + BUILD_BUG_ON(ARRAY_SIZE(btf_vmlinux_map_ops) !=
> > > > + ARRAY_SIZE(btf_vmlinux_map_ids));
> > > > +
> > > > + for (i = 0; i < ARRAY_SIZE(btf_vmlinux_map_ops); ++i) {
> > > > + if (!btf_vmlinux_map_ops[i])
> > > > + continue;
> > > > + btf_name = btf_vmlinux_map_ops[i]->map_btf_name;
> > > > + if (!btf_name) {
> > > > + btf_vmlinux_map_ids[i] = base_btf_id;
> > > > + continue;
> > > > + }
> > > > + btf_id = btf_find_by_name_kind(btf, btf_name, BTF_KIND_STRUCT);
> > > > + if (btf_id < 0)
> > > > + return btf_id;
> > > > + btf_vmlinux_map_ids[i] = btf_id;
> > > Since map_btf_name is already in map_ops, how about storing map_btf_id in
> > > map_ops also?
> > > btf_id 0 is "void" which is as good as -1, so there is no need
> > > to modify all map_ops to init map_btf_id to -1.
> >
> > Yeah, btf_id == 0 being a valid id was the reason I used -1.
> >
> > I realized that having a map type specific struct with btf_id == 0
> > should be practically impossible, but is it guaranteed to always be
> > "void" or it just happened so and can change in the future?
> >
> > If both this and having one more field in bpf_map_ops is not a problem,
> > I'll move it to bpf_map_ops.
>
> Nope, I can't do it. All `struct bpf_map_ops` are global `const`, i.e.
> rodata and a try cast `const` away and change them causes a panic.
>
> Simple user space repro:
>
> % cat 1.c
> #include <stdio.h>
>
> struct map_ops {
> int a;
> };
>
> const struct map_ops ops = {
> .a = 1,
> };
>
> int main(void)
> {
> struct map_ops *ops_rw = (struct map_ops *)&ops;
>
> printf("before a=%d\n", ops_rw->a);
> ops_rw->a = 3;
> printf(" afrer a=%d\n", ops_rw->a);
> }
> % clang -O2 -Wall -Wextra -pedantic -pedantic-errors -g 1.c && ./a.out
> before a=1
> Segmentation fault (core dumped)
> % objdump -t a.out | grep -w ops
> 0000000000400600 g O .rodata 0000000000000004 ops
>
> --
> Andrey Ignatov
See the trick that helper prototypes do for BTF ids. Fictional example:
static int hash_map_btf_id;
const struct bpf_map_ops hash_map_opss = {
...
.btf_id = &hash_map_btf_id,
};
then *hash_map_ops.btf_id = <final_btf_id>;
