Re: [RFC][PATCH][bugfix] more checks for negative f_pos handling v4

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Sep 17, 2009 at 02:51:00PM +0800, KAMEZAWA Hiroyuki wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Now, rw_verify_area() checsk f_pos is negative or not. And if
> negative, returns -EINVAL.
> 
> But, some special files as /dev/(k)mem and /proc/<pid>/mem etc..
> has negative offsets. And we can't do any access via read/write
> to the file(device).
> 
> This patch introduce a flag S_VERYBIG and allow negative file
> offsets for big files. (usual files don't allow it.)
> 
> Changelog: v3->v4
>  - make changes in mem.c aligned.
>  - change __negative_fpos_check() to return int. 
>  - fixed bug in "pos" check.
>  - added comments.
> 
> Changelog: v2->v3
>  - fixed bug in rw_verify_area (it cannot be compiled)
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  drivers/char/mem.c |   23 +++++++++++++----------
>  fs/proc/base.c     |    2 ++
>  fs/read_write.c    |   22 ++++++++++++++++++++--
>  include/linux/fs.h |    2 ++
>  4 files changed, 37 insertions(+), 12 deletions(-)
> 
> Index: mmotm-2.6.31-Sep14/fs/read_write.c
> ===================================================================
> --- mmotm-2.6.31-Sep14.orig/fs/read_write.c
> +++ mmotm-2.6.31-Sep14/fs/read_write.c
> @@ -205,6 +205,21 @@ bad:
>  }
>  #endif
>  
> +static int
> +__negative_fpos_check(struct inode *inode, loff_t pos, size_t count)
> +{
> +	/*
> +	 * pos or pos+count is negative here, check overflow.
> +	 * too big "count" will be caught in rw_verify_area().
> +	 */
> +	if ((pos < 0) && (pos + count < pos))
> +		return -EOVERFLOW;

This returns -EOVERFLOW when pos=-10 and count=1. What's the intention?
Just to return a different error code other than -EINVAL?

Also it seems you did two behavior changes at the same time: the above
-EOVERFLOW and the below IS_VERYBIG(). Are they tightly coupled?

> +	/* If !VERYBIG inode, negative pos(pos+count) is not allowed */
> +	if (!IS_VERYBIG(inode))
> +		return -EINVAL;
> +	return 0;
> +}
> +
>  /*
>   * rw_verify_area doesn't like huge counts. We limit
>   * them to something that fits in "int" so that others
> @@ -222,8 +237,11 @@ int rw_verify_area(int read_write, struc
>  	if (unlikely((ssize_t) count < 0))
>  		return retval;
>  	pos = *ppos;
> -	if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
> -		return retval;
> +	if (unlikely((pos < 0) || (loff_t) (pos + count) < 0)) {
> +		retval = __negative_fpos_check(inode, pos, count);
> +		if (retval)
> +			return retval;
> +	}

This could look more nicer:

        retval = __negative_fpos_check(inode, pos, count);
        if (retval)
                return retval;

But they are all minor issues :)

Thanks,
Fengguang

>  
>  	if (unlikely(inode->i_flock && mandatory_lock(inode))) {
>  		retval = locks_mandatory_area(
> Index: mmotm-2.6.31-Sep14/include/linux/fs.h
> ===================================================================
> --- mmotm-2.6.31-Sep14.orig/include/linux/fs.h
> +++ mmotm-2.6.31-Sep14/include/linux/fs.h
> @@ -231,6 +231,7 @@ struct inodes_stat_t {
>  #define S_NOCMTIME	128	/* Do not update file c/mtime */
>  #define S_SWAPFILE	256	/* Do not truncate: swapon got its bmaps */
>  #define S_PRIVATE	512	/* Inode is fs-internal */
> +#define S_VERYBIG	1024	/* Allow file's loff_t can be negative */
>  
>  /*
>   * Note that nosuid etc flags are inode-specific: setting some file-system
> @@ -265,6 +266,7 @@ struct inodes_stat_t {
>  #define IS_NOCMTIME(inode)	((inode)->i_flags & S_NOCMTIME)
>  #define IS_SWAPFILE(inode)	((inode)->i_flags & S_SWAPFILE)
>  #define IS_PRIVATE(inode)	((inode)->i_flags & S_PRIVATE)
> +#define IS_VERYBIG(inode)	((inode)->i_flags & S_VERYBIG)
>  
>  /* the read-only stuff doesn't really belong here, but any other place is
>     probably as bad and I don't want to create yet another include file. */
> Index: mmotm-2.6.31-Sep14/drivers/char/mem.c
> ===================================================================
> --- mmotm-2.6.31-Sep14.orig/drivers/char/mem.c
> +++ mmotm-2.6.31-Sep14/drivers/char/mem.c
> @@ -825,22 +825,23 @@ static const struct memdev {
>  	const char *name;
>  	const struct file_operations *fops;
>  	struct backing_dev_info *dev_info;
> +	bool	verybig;
>  } devlist[] = {
> -	[ 1] = { "mem", &mem_fops, &directly_mappable_cdev_bdi },
> +	[1]  = { "mem", &mem_fops, &directly_mappable_cdev_bdi, true },
>  #ifdef CONFIG_DEVKMEM
> -	[ 2] = { "kmem", &kmem_fops, &directly_mappable_cdev_bdi },
> +	[2]  = { "kmem", &kmem_fops, &directly_mappable_cdev_bdi, true },
>  #endif
> -	[ 3] = {"null", &null_fops, NULL },
> +	[3]  = {"null", &null_fops, NULL, false },
>  #ifdef CONFIG_DEVPORT
> -	[ 4] = { "port", &port_fops, NULL },
> +	[4]  = { "port", &port_fops, NULL, false },
>  #endif
> -	[ 5] = { "zero", &zero_fops, &zero_bdi },
> -	[ 6] = { "full", &full_fops, NULL },
> -	[ 7] = { "random", &random_fops, NULL },
> -	[ 9] = { "urandom", &urandom_fops, NULL },
> -	[11] = { "kmsg", &kmsg_fops, NULL },
> +	[5]  = { "zero", &zero_fops, &zero_bdi, false },
> +	[6]  = { "full", &full_fops, NULL, false },
> +	[7]  = { "random", &random_fops, NULL, false },
> +	[9]  = { "urandom", &urandom_fops, NULL, false },
> +	[11] = { "kmsg", &kmsg_fops, NULL, false },
>  #ifdef CONFIG_CRASH_DUMP
> -	[12] = { "oldmem", &oldmem_fops, NULL },
> +	[12] = { "oldmem", &oldmem_fops, NULL, false },
>  #endif
>  };
>  
> @@ -868,6 +869,8 @@ static int memory_open(struct inode *ino
>  		ret = dev->fops->open(inode, filp);
>  	else
>  		ret = 0;
> +	if (dev->verybig)
> +		inode->i_flags |= S_VERYBIG;
>  out:
>  	unlock_kernel();
>  	return ret;
> Index: mmotm-2.6.31-Sep14/fs/proc/base.c
> ===================================================================
> --- mmotm-2.6.31-Sep14.orig/fs/proc/base.c
> +++ mmotm-2.6.31-Sep14/fs/proc/base.c
> @@ -779,6 +779,8 @@ static const struct file_operations proc
>  static int mem_open(struct inode* inode, struct file* file)
>  {
>  	file->private_data = (void*)((long)current->self_exec_id);
> +	/* this file is read only and we can catch out-pf-range */
> +	inode->i_flags |= S_VERYBIG;
>  	return 0;
>  }
>