linux-ext4 - Re: [PATCH] ext4: fix BUG_ON() when a directory entry has an invalid rec

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Date:   Tue, 11 Oct 2022 20:57:07 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Luís Henriques <lhenriques@...e.de>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] ext4: fix BUG_ON() when a directory entry has an invalid
 rec_len

On Tue, Oct 11, 2022 at 04:57:45PM +0100, Luís Henriques wrote:
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index 3a31b662f661..06803292e394 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -2254,8 +2254,18 @@ static int make_indexed_dir(handle_t *handle, struct ext4_filename *fname,
>  	memset(de, 0, len); /* wipe old data */
>  	de = (struct ext4_dir_entry_2 *) data2;
>  	top = data2 + len;
> -	while ((char *)(de2 = ext4_next_entry(de, blocksize)) < top)
> +	while ((char *)(de2 = ext4_next_entry(de, blocksize)) < top) {
> +		if (de->rec_len & 3) {

As the kernel test bot as flaged, de->rec_len needs to be byte swapped
on big endian machines.  Also, for block sizes larger than 64k the low
2 bits are used to encode rec_len sizes 256k-4.  All of this is
encoded in ext4_rec_len_from_disk().

However, I think a better thing to do is instead of doing this one
check on rec len, that instead we call ext4_check_dir_entry(), which
will do this check, and many more besides.  It will also avoid some
code duplication, since it will take care of calling EXT4_ERROR_INODE
with the appropriate explanatory message.

					- Ted