bpf: Handle show_fdinfo for the parameterized task BPF iterators

Show information of iterators in the respective files under
/proc/<pid>/fdinfo/.

For example, for a task file iterator with 1723 as the value of tid
parameter, its fdinfo would look like the following lines.

    pos:    0
    flags:  02000000
    mnt_id: 14
    ino:    38
    link_type:      iter
    link_id:        51
    prog_tag:       a590ac96db22b825
    prog_id:        299
    target_name:    task_file
    task_type:      TID
    tid: 1723

This patch add the last three fields.  task_type is the type of the
task parameter.  TID means the iterator visit only the thread
specified by tid.  The value of tid in the above example is 1723.  For
the case of PID task_type, it means the iterator visits only threads
of a process and will show the pid value of the process instead of a
tid.
Signed-off-by: Kui-Feng Lee <kuifeng@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20220926184957.208194-4-kuifeng@fb.com

kernel/bpf/task_iter.c

@@ -10,6 +10,12 @@
 #include <linux/btf_ids.h>
 #include "mmap_unlock_work.h"
 
+static const char * const iter_task_type_names[] = {
+	"ALL",
+	"TID",
+	"PID",
+};
+
 struct bpf_iter_seq_task_common {
 	struct pid_namespace *ns;
 	enum bpf_iter_task_type	type;
@@ -687,6 +693,15 @@ static int bpf_iter_fill_link_info(const struct bpf_iter_aux_info *aux, struct b
 	return 0;
 }
 
+static void bpf_iter_task_show_fdinfo(const struct bpf_iter_aux_info *aux, struct seq_file *seq)
+{
+	seq_printf(seq, "task_type:\t%s\n", iter_task_type_names[aux->task.type]);
+	if (aux->task.type == BPF_TASK_ITER_TID)
+		seq_printf(seq, "tid:\t%u\n", aux->task.pid);
+	else if (aux->task.type == BPF_TASK_ITER_TGID)
+		seq_printf(seq, "pid:\t%u\n", aux->task.pid);
+}
+
 static struct bpf_iter_reg task_reg_info = {
 	.target			= "task",
 	.attach_target		= bpf_iter_attach_task,
@@ -698,6 +713,7 @@ static struct bpf_iter_reg task_reg_info = {
 	},
 	.seq_info		= &task_seq_info,
 	.fill_link_info		= bpf_iter_fill_link_info,
+	.show_fdinfo		= bpf_iter_task_show_fdinfo,
 };
 
 static const struct bpf_iter_seq_info task_file_seq_info = {
@@ -720,6 +736,7 @@ static struct bpf_iter_reg task_file_reg_info = {
 	},
 	.seq_info		= &task_file_seq_info,
 	.fill_link_info		= bpf_iter_fill_link_info,
+	.show_fdinfo		= bpf_iter_task_show_fdinfo,
 };
 
 static const struct bpf_iter_seq_info task_vma_seq_info = {
@@ -742,6 +759,7 @@ static struct bpf_iter_reg task_vma_reg_info = {
 	},
 	.seq_info		= &task_vma_seq_info,
 	.fill_link_info		= bpf_iter_fill_link_info,
+	.show_fdinfo		= bpf_iter_task_show_fdinfo,
 };
 
 BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,