Re: [PATCH] Fix seq_read dead loop and trigger memory allocation failure.

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Apr 21, 2014 at 10:12:42PM +0800, Fengwei Yin wrote:
> When dump /proc/xxx/maps, if d_path return error in seq_path, the
> buffer will be exhaust and trigger dead loop in seq_read. Till
> kmalloc fails with -ENOMEM.

*WHAT* d_path error?  -ENAMETOOLONG, aka. "you've got too little space"?

> @@ -295,8 +295,16 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma, int is_pid)
>  	 * special [heap] marker for the heap:
>  	 */
>  	if (file) {
> +		size_t sz;
>  		seq_pad(m, ' ');
> -		seq_path(m, &file->f_path, "\n");
> +		/* Save current count. Once seq_path return negtive value,
> +		 * we need to restore saved count. Otherwise, seq_path will
> +		 * exhaust the buffer and make seq_read dead loop till
> +		 * m->buff allocation failure.
> +		 */
> +		sz = m->count;
> +		if (seq_path(m, &file->f_path, "\n") < 0)
> +			m->count = sz;

NAK.  No way in hell.  Any code playing with m->count that way is broken.
Post the reproducer for that infinite loop; then we'll be able to see
what triggers an impossible error from d_path().  _That_ is where the bug
is, assuming it exists at all.