Re: [PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is never 0
From: Andrii Nakryiko <hidden>
Date: 2021-10-06 04:41:43
Also in:
bpf
On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi [off-list ref] wrote:
quoted hunk ↗ jump to hunk
Since the code assumes in various places that BTF fd for modules is never 0, if we end up getting fd as 0, obtain a new fd > 0. Even though fd 0 being free for allocation is usually an application error, it is still possible that we end up getting fd 0 if the application explicitly closes its stdin. Deal with this by getting a new fd using dup and closing fd 0. Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> --- tools/lib/bpf/libbpf.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index d286dec73b5f..3e5e460fe63e 100644 --- a/tools/lib/bpf/libbpf.c +++ b/tools/lib/bpf/libbpf.c@@ -4975,6 +4975,20 @@ static int load_module_btfs(struct bpf_object *obj) pr_warn("failed to get BTF object #%d FD: %d\n", id, err); return err; } + /* Make sure module BTF fd is never 0, as kernel depends on it + * being > 0 to distinguish between vmlinux and module BTFs, + * e.g. for BPF_PSEUDO_BTF_ID ld_imm64 insns (ksyms). + */ + if (!fd) { + fd = dup(0);
This is not the only place where we make assumptions that fd > 0 but technically can get fd == 0. Instead of doing such a check in every such place, would it be possible to open (cheaply) some FD (/dev/null or whatever, don't know what's the best file to open), if we detect that FD == 0 is not allocated? Can we detect that fd 0 is not allocated? Doing something like that in bpf_object__open() or bpf_object__load() would make everything much simpler and we'll have a guarantee that fd == 0 is not going to be allocated (unless someone accidentally or not accidentally does close(0), but that's entirely different story).
+ if (fd < 0) {
+ err = -errno;
+ pr_warn("failed to dup BTF object #%d FD 0 to FD > 0: %d\n", id, err);
+ close(0);
+ return err;
+ }
+ close(0);
+ }
len = sizeof(info);
memset(&info, 0, sizeof(info));
--
2.33.0