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. and the preload process leaked two FDs, taking over
FD 0 and 1. This  which later caused issues when kernel trys to setup
stdin/stdout/stderr for init process, assuming fd 0,1,2 is 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_closenez 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: kernel-patches/bpf@cb80ddc

Signed-off-by: Yucong Sun <[email protected]>

Author: Yucong Sun

kernel/bpf/preload/bpf_preload_kern.c

@@ -54,6 +54,7 @@ static int load_skel(void)
 		err = PTR_ERR(progs_link);
 		goto out;
 	}
+	iterators_bpf__detach(skel);
 	return 0;
 out:
 	free_links_and_skel();

kernel/bpf/preload/iterators/iterators.lskel.h

@@ -28,7 +28,7 @@ iterators_bpf__dump_bpf_map__attach(struct iterators_bpf *skel)
 	int prog_fd = skel->progs.dump_bpf_map.prog_fd;
 	int fd = skel_link_create(prog_fd, 0, BPF_TRACE_ITER);
 
-	if (fd > 0)
+	if (fd >= 0)
 		skel->links.dump_bpf_map_fd = fd;
 	return fd;
 }
@@ -39,7 +39,7 @@ iterators_bpf__dump_bpf_prog__attach(struct iterators_bpf *skel)
 	int prog_fd = skel->progs.dump_bpf_prog.prog_fd;
 	int fd = skel_link_create(prog_fd, 0, BPF_TRACE_ITER);
 
-	if (fd > 0)
+	if (fd >= 0)
 		skel->links.dump_bpf_prog_fd = fd;
 	return fd;
 }
@@ -57,19 +57,21 @@ iterators_bpf__attach(struct iterators_bpf *skel)
 static inline void
 iterators_bpf__detach(struct iterators_bpf *skel)
 {
-	skel_closenz(skel->links.dump_bpf_map_fd);
-	skel_closenz(skel->links.dump_bpf_prog_fd);
+	skel_closenez(skel->links.dump_bpf_map_fd);
+	skel->links.dump_bpf_map_fd = -1;
+	skel_closenez(skel->links.dump_bpf_prog_fd);
+	skel->links.dump_bpf_prog_fd = -1;
 }
 static void
 iterators_bpf__destroy(struct iterators_bpf *skel)
 {
 	if (!skel)
 		return;
 	iterators_bpf__detach(skel);
-	skel_closenz(skel->progs.dump_bpf_map.prog_fd);
-	skel_closenz(skel->progs.dump_bpf_prog.prog_fd);
+	skel_closenez(skel->progs.dump_bpf_map.prog_fd);
+	skel_closenez(skel->progs.dump_bpf_prog.prog_fd);
 	skel_free_map_data(skel->rodata, skel->maps.rodata.initial_value, 4096);
-	skel_closenz(skel->maps.rodata.map_fd);
+	skel_closenez(skel->maps.rodata.map_fd);
 	skel_free(skel);
 }
 static inline struct iterators_bpf *

tools/bpf/bpftool/gen.c

@@ -469,7 +469,7 @@ static void codegen_attach_detach(struct bpf_object *obj, const char *obj_name)
 		codegen("\
 			\n\
 										    \n\
-				if (fd > 0)					    \n\
+				if (fd >= 0)					    \n\
 					skel->links.%1$s_fd = fd;		    \n\
 				return fd;					    \n\
 			}							    \n\
@@ -506,7 +506,8 @@ static void codegen_attach_detach(struct bpf_object *obj, const char *obj_name)
 	bpf_object__for_each_program(prog, obj) {
 		codegen("\
 			\n\
-				skel_closenz(skel->links.%1$s_fd);	    \n\
+				skel_closenez(skel->links.%1$s_fd);	    \n\
+				skel->links.%1$s_fd = -1;	    \n\
 			", bpf_program__name(prog));
 	}
 
@@ -536,7 +537,7 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 	bpf_object__for_each_program(prog, obj) {
 		codegen("\
 			\n\
-				skel_closenz(skel->progs.%1$s.prog_fd);	    \n\
+				skel_closenez(skel->progs.%1$s.prog_fd);	    \n\
 			", bpf_program__name(prog));
 	}
 
@@ -549,7 +550,7 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 			       ident, bpf_map_mmap_sz(map));
 		codegen("\
 			\n\
-				skel_closenz(skel->maps.%1$s.map_fd);	    \n\
+				skel_closenez(skel->maps.%1$s.map_fd);	    \n\
 			", ident);
 	}
 	codegen("\

tools/lib/bpf/skel_internal.h

@@ -204,11 +204,11 @@ static inline void *skel_finalize_map_data(__u64 *init_val, size_t mmap_sz, int
 }
 #endif
 
-static inline int skel_closenz(int fd)
+static inline int skel_closenez(int fd)
 {
-	if (fd > 0)
-		return close(fd);
-	return -EINVAL;
+	if (fd < 0)
+		return -EINVAL;
+	return close(fd);
 }
 
 #ifndef offsetofend