filp->f_pos not correctly updated in proc_task_readdir (ee6f779b) · Commits · 戴 / test

filp->f_pos only get updated at the end of the function. Thus d_off of those dirents who are in the middle will be 0, and this will cause a problem in glibc's readdir implementation, specifically endless loop. Because when overflow occurs, f_pos will be set to next dirent to read, however it will be 0, unless the next one is the last one. So it will start over again and again. There is a sample program in man 2 gendents. This is the output of the program running on a multithread program's task dir before this patch is applied: $ ./a.out /proc/3807/task --------------- nread=128 --------------- i-node# file type d_reclen d_off d_name 506442 directory 16 1 . 506441 directory 16 0 .. 506443 directory 16 0 3807 506444 directory 16 0 3809 506445 directory 16 0 3812 506446 directory 16 0 3861 506447 directory 16 0 3862 506448 directory 16 8 3863 This is the output after this patch is applied $ ./a.out /proc/3807/task --------------- nread=128 --------------- i-node# file type d_reclen d_off d_name 506442 directory 16 1 . 506441 directory 16 2 .. 506443 directory 16 3 3807 506444 directory 16 4 3809 506445 directory 16 5 3812 506446 directory 16 6 3861 506447 directory 16 7 3862 506448 directory 16 8 3863 Signed-off-by:

Zhang Le <r0bertz@gentoo.org> Acked-by:

Al Viro <viro@ZenIV.linux.org.uk> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

+7 −9

Original line number	Diff line number	Diff line
		@@ -3066,7 +3066,6 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
		int retval = -ENOENT;
		ino_t ino;
		int tid;
		unsigned long pos = filp->f_pos; /* avoiding "long long" filp->f_pos */
		struct pid_namespace *ns;

		task = get_proc_task(inode);
		@@ -3083,18 +3082,18 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
		goto out_no_task;
		retval = 0;

		switch (pos) {
		switch (filp->f_pos) {
		case 0:
		ino = inode->i_ino;
		if (filldir(dirent, ".", 1, pos, ino, DT_DIR) < 0)
		if (filldir(dirent, ".", 1, filp->f_pos, ino, DT_DIR) < 0)
		goto out;
		pos++;
		filp->f_pos++;
		/* fall through */
		case 1:
		ino = parent_ino(dentry);
		if (filldir(dirent, "..", 2, pos, ino, DT_DIR) < 0)
		if (filldir(dirent, "..", 2, filp->f_pos, ino, DT_DIR) < 0)
		goto out;
		pos++;
		filp->f_pos++;
		/* fall through */
		}

		@@ -3104,9 +3103,9 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
		ns = filp->f_dentry->d_sb->s_fs_info;
		tid = (int)filp->f_version;
		filp->f_version = 0;
		for (task = first_tid(leader, tid, pos - 2, ns);
		for (task = first_tid(leader, tid, filp->f_pos - 2, ns);
		task;
		task = next_tid(task), pos++) {
		task = next_tid(task), filp->f_pos++) {
		tid = task_pid_nr_ns(task, ns);
		if (proc_task_fill_cache(filp, dirent, filldir, task, tid) < 0) {
		/* returning this tgid failed, save it as the first
		@@ -3117,7 +3116,6 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
		}
		}
		out:
		filp->f_pos = pos;
		put_task_struct(leader);
		out_no_task:
		return retval;

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