bpf: Fix issue with bpf preload module taking over stdout/stdin of kernel.

In a previous commit (1), BPF preload process was switched from user mode
process to use in-kernel light skeleton instead. However, in the kernel context
the available FD starts from 0, instead of normally 3 for user mode process.
The preload process also left two FDs open, taking over FD 0 and 1. This later
caused issues when kernel trys to setup stdin/stdout/stderr for init process,
assuming FD 0,1,2 are available.

As seen here:

Before fix:
ls -lah /proc/1/fd/*

lrwx------1 root root 64 Feb 23 17:20 /proc/1/fd/0 -> /dev/null
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/1 -> /dev/null
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/2 -> /dev/console
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/6 -> /dev/console
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/7 -> /dev/console

After Fix / Normal:

ls -lah /proc/1/fd/*

lrwx------ 1 root root 64 Feb 24 21:23 /proc/1/fd/0 -> /dev/console
lrwx------ 1 root root 64 Feb 24 21:23 /proc/1/fd/1 -> /dev/console
lrwx------ 1 root root 64 Feb 24 21:23 /proc/1/fd/2 -> /dev/console

In this patch:
  - skel_closenz was changed to skel_closegez to correctly handle
    FD=0 case.
  - various places detecting FD > 0 was changed to FD >= 0.
  - Call iterators_skel__detach() funciton to release FDs after links
  are obtained.

1: commit cb80ddc ("bpf: Convert bpf_preload.ko to use light skeleton.")

Fixes: commit cb80ddc ("bpf: Convert bpf_preload.ko to use light skeleton.")
Signed-off-by: Yucong Sun <[email protected]>

V3 -> V1: removed all changes related to handle fd=0.
V2 -> V1: rename skel_closenez to skel_closegez, added comment as
requested.

Author: Yucong Sun

kernel/bpf/preload/bpf_preload_kern.c

@@ -54,6 +54,16 @@ static int load_skel(void)
 		err = PTR_ERR(progs_link);
 		goto out;
 	}
+	/* Avoid taking over stdin/stdout/stderr of init process. This also
+	   makes skel_closenz() no-op later in free_links_and_skel(). */
+	if (skel->links.dump_bpf_map_fd < 3) {
+		close_fd(skel->links.dump_bpf_map_fd);
+		skel->links.dump_bpf_map_fd = 0;
+	}
+	if (skel->links.dump_bpf_prog_fd < 3) {
+		close_fd(skel->links.dump_bpf_prog_fd);
+		skel->links.dump_bpf_prog_fd = 0;
+	}
 	return 0;
 out:
 	free_links_and_skel();