Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

Alexei Starovoitov
2023-12-09T04:51:59Z

On Fri, Dec 8, 2023 at 2:46 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
>
> Ok, did that. Current patches (on top of bpf-next) are here:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/cfi

Looks really great. The last patch is cleaner than I expected. Good idea.

> (really should try and write better changelogs, but it's too late)

commit logs look fine except the "pilfer" word that I had to look up
in the dictionary :)

> [ 247.721063] ? bpf_throw+0x9b/0xf0
> [ 247.721126] ? bpf_test_run+0x108/0x350
> [ 247.721191] ? bpf_prog_5555714b685bf0cf_exception_throw_always_1+0x26/0x26
> [ 247.721301] ? bpf_test_run+0x108/0x350
> [ 247.721368] bpf_test_run+0x212/0x350
> [ 247.721433] ? slab_build_skb+0x22/0x110
> [ 247.721503] bpf_prog_test_run_skb+0x347/0x4a0
>
> But I'm too tired to think staight. Is this a bpf_callback_t vs
> bpf_exception_cb difference?

Yep.
It's easy to fix:
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 0e162eae8639..e36b3f41751e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1484,7 +1484,7 @@ struct bpf_prog_aux {
int cgroup_atype; /* enum cgroup_bpf_attach_type */
struct bpf_map *cgroup_storage[MAX_BPF_CGROUP_STORAGE_TYPE];
char name[BPF_OBJ_NAME_LEN];
- unsigned int (*bpf_exception_cb)(u64 cookie, u64 sp, u64 bp);
+ u64 (*bpf_exception_cb)(u64 cookie, u64 sp, u64 bp, u64, u64);
#ifdef CONFIG_SECURITY
void *security;
#endif
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index fe229b28e4a9..650ebe8ff183 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2537,7 +2537,7 @@ __bpf_kfunc void bpf_throw(u64 cookie)
* which skips compiler generated instrumentation to do the same.
*/
kasan_unpoison_task_stack_below((void *)(long)ctx.sp);
- ctx.aux->bpf_exception_cb(cookie, ctx.sp, ctx.bp);
+ ctx.aux->bpf_exception_cb(cookie, ctx.sp, ctx.bp, 0, 0);
WARN(1, "A call to BPF exception callback should never return\n");
}

and with that all of test_progs runs successfully without CFI panics.
*happy dance*

Only test_progs -t btf/line_info fails suspiciously.
There we check that line info embedded in the prog looks sane.
New cfi preamble is probably tripping something.
It could be a test issue. I'll investigate. It's not a blocker.

Do you mind resending the whole set so that BPF CI can test it
on different archs ?