Re: [PATCH] Fix concurrent read from /proc/videocodecs

[Jan Kara]

> Observation one: ->write_proc and ->data assignments aren't needed. Removed.
> Observation two: codecs lists are unprotected. Patch doesn't fix this.
> Observation three:
> 	/proc/videocodecs printout is done to temporary _global_ buffer which
> 	is freed in between. Consequently, two users hitting this file can
> 	screwup each other.
> 
> Steps to reproduce:
> 
> 	modprobe videocodec
> 	while true; do cat /proc/videocodecs &>/dev/null; done &
> 	while true; do cat /proc/videocodecs &>/dev/null; done &
> 
> The fix is switching to seq_files, this removes code, especially some
> line-length "logic".
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
  The patch looks fine, although seeing somebody testing it on a real
hardware would be nice :). Anyway, you can add:
  Acked-by: Jan Kara <jack@xxxxxxx>

								Honza
> ---
> 
>  Tested on empty file, I don't have hardware.
> 
>  drivers/media/video/videocodec.c |  113 ++++++---------------------------------
>  1 file changed, 19 insertions(+), 94 deletions(-)
> 
> --- a/drivers/media/video/videocodec.c
> +++ b/drivers/media/video/videocodec.c
> @@ -39,6 +39,7 @@
>  
>  #ifdef CONFIG_PROC_FS
>  #include <linux/proc_fs.h>
> +#include <linux/seq_file.h>
>  #include <asm/uaccess.h>
>  #endif
>  
> @@ -320,56 +321,22 @@ videocodec_unregister (const struct videocodec *codec)
>  }
>  
>  #ifdef CONFIG_PROC_FS
> -/* ============ */
> -/* procfs stuff */
> -/* ============ */
> -
> -static char *videocodec_buf = NULL;
> -static int videocodec_bufsize;
> -
> -static int
> -videocodec_build_table (void)
> +static int proc_videocodecs_show(struct seq_file *m, void *v)
>  {
>  	struct codec_list *h = codeclist_top;
>  	struct attached_list *a;
> -	int i = 0, size;
> -
> -	// sum up amount of slaves plus their attached masters
> -	while (h) {
> -		i += h->attached + 1;
> -		h = h->next;
> -	}
> -#define LINESIZE 100
> -	size = LINESIZE * (i + 1);
>  
> -	dprintk(3, "videocodec_build table: %d entries, %d bytes\n", i,
> -		size);
> -
> -	kfree(videocodec_buf);
> -	videocodec_buf = kmalloc(size, GFP_KERNEL);
> -
> -	if (!videocodec_buf)
> -		return 0;
> -
> -	i = 0;
> -	i += scnprintf(videocodec_buf + i, size - 1,
> -		      "<S>lave or attached <M>aster name  type flags    magic    ");
> -	i += scnprintf(videocodec_buf + i, size -i - 1, "(connected as)\n");
> +	seq_printf(m, "<S>lave or attached <M>aster name  type flags    magic    ");
> +	seq_printf(m, "(connected as)\n");
>  
>  	h = codeclist_top;
>  	while (h) {
> -		if (i > (size - LINESIZE))
> -			break;	// security check
> -		i += scnprintf(videocodec_buf + i, size -i -1,
> -			      "S %32s %04x %08lx %08lx (TEMPLATE)\n",
> +		seq_printf(m, "S %32s %04x %08lx %08lx (TEMPLATE)\n",
>  			      h->codec->name, h->codec->type,
>  			      h->codec->flags, h->codec->magic);
>  		a = h->list;
>  		while (a) {
> -			if (i > (size - LINESIZE))
> -				break;	// security check
> -			i += scnprintf(videocodec_buf + i, size -i -1,
> -				      "M %32s %04x %08lx %08lx (%s)\n",
> +			seq_printf(m, "M %32s %04x %08lx %08lx (%s)\n",
>  				      a->codec->master_data->name,
>  				      a->codec->master_data->type,
>  				      a->codec->master_data->flags,
> @@ -380,54 +347,21 @@ videocodec_build_table (void)
>  		h = h->next;
>  	}
>  
> -	return i;
> +	return 0;
>  }
>  
> -//The definition:
> -//typedef int (read_proc_t)(char *page, char **start, off_t off,
> -//                          int count, int *eof, void *data);
> -
> -static int
> -videocodec_info (char  *buffer,
> -		 char **buffer_location,
> -		 off_t  offset,
> -		 int    buffer_length,
> -		 int   *eof,
> -		 void  *data)
> +static int proc_videocodecs_open(struct inode *inode, struct file *file)
>  {
> -	int size;
> -
> -	dprintk(3, "videocodec_info: offset: %ld, len %d / size %d\n",
> -		offset, buffer_length, videocodec_bufsize);
> -
> -	if (offset == 0) {
> -		videocodec_bufsize = videocodec_build_table();
> -	}
> -	if ((offset < 0) || (offset >= videocodec_bufsize)) {
> -		dprintk(4,
> -			"videocodec_info: call delivers no result, return 0\n");
> -		*eof = 1;
> -		return 0;
> -	}
> -
> -	if (buffer_length < (videocodec_bufsize - offset)) {
> -		dprintk(4, "videocodec_info: %ld needed, %d got\n",
> -			videocodec_bufsize - offset, buffer_length);
> -		size = buffer_length;
> -	} else {
> -		dprintk(4, "videocodec_info: last reading of %ld bytes\n",
> -			videocodec_bufsize - offset);
> -		size = videocodec_bufsize - offset;
> -		*eof = 1;
> -	}
> -
> -	memcpy(buffer, videocodec_buf + offset, size);
> -	/* doesn't work...                           */
> -	/* copy_to_user(buffer, videocodec_buf+offset, size); */
> -	/* *buffer_location = videocodec_buf+offset; */
> -
> -	return size;
> +	return single_open(file, proc_videocodecs_show, NULL);
>  }
> +
> +static const struct file_operations videocodecs_proc_fops = {
> +	.owner		= THIS_MODULE,
> +	.open		= proc_videocodecs_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= single_release,
> +};
>  #endif
>  
>  /* ===================== */
> @@ -444,16 +378,8 @@ videocodec_init (void)
>  	       VIDEOCODEC_VERSION);
>  
>  #ifdef CONFIG_PROC_FS
> -	videocodec_buf = NULL;
> -	videocodec_bufsize = 0;
> -
> -	videocodec_proc_entry = create_proc_entry("videocodecs", 0, NULL);
> -	if (videocodec_proc_entry) {
> -		videocodec_proc_entry->read_proc = videocodec_info;
> -		videocodec_proc_entry->write_proc = NULL;
> -		videocodec_proc_entry->data = NULL;
> -		videocodec_proc_entry->owner = THIS_MODULE;
> -	} else {
> +	videocodec_proc_entry = proc_create("videocodecs", 0, NULL, &videocodecs_proc_fops);
> +	if (!videocodec_proc_entry) {
>  		dprintk(1, KERN_ERR "videocodec: can't init procfs.\n");
>  	}
>  #endif
> @@ -465,7 +391,6 @@ videocodec_exit (void)
>  {
>  #ifdef CONFIG_PROC_FS
>  	remove_proc_entry("videocodecs", NULL);
> -	kfree(videocodec_buf);
>  #endif
>  }
>  
> 
> --
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-- 
Jan Kara <jack@xxxxxxx>
SuSE CR Labs
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