[PATCH] binfmt_flat: Fix corruption when not offsetting data start

[Greg Ungerer]

Hi Stefan,

On 26/3/24 13:20, Stefan O'Rear wrote:
> Commit 04d82a6d0881 ("binfmt_flat: allow not offsetting data start")
> introduced a RISC-V specific variant of the FLAT format which does not
> allocate any space for the (obsolescent) array of shared library
> pointers. However, it did not disable the code which initializes the
> array, resulting in the corruption of sizeof(long) bytes before the DATA
> segment, generally the end of the TEXT segment.
> 
> Use CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET to guard initialization the
> shared library pointer region so that it will only be initialized if
> space is reserved for it.
> 
> Fixes: 04d82a6d0881 ("binfmt_flat: allow not offsetting data start")
> Signed-off-by: Stefan O'Rear <sorear at fastmail.com>
> Tested-by: Waldemar Brodkorb <wbx at openadk.org>
> ---
>   fs/binfmt_flat.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
> index c26545d71d39..70c2b68988f4 100644
> --- a/fs/binfmt_flat.c
> +++ b/fs/binfmt_flat.c
> @@ -879,6 +879,7 @@ static int load_flat_binary(struct linux_binprm *bprm)
>   	if (res < 0)
>   		return res;
>   
> +#ifndef CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET
>   	/* Update data segment pointers for all libraries */
>   	for (i = 0; i < MAX_SHARED_LIBS; i++) {
                         ^^^^^^^^^^^^^^^

It may be cleaner (ie no # conditional required) if this was changed to
DATA_START_OFFSET_WORDS. So becoming:

         for (i = 0; i < DATA_START_OFFSET_WORDS; i++) {

My only concern is does this make it less clear what the code is doing?

Regards
Greg


>   		if (!libinfo.lib_list[i].loaded)
> @@ -893,6 +894,7 @@ static int load_flat_binary(struct linux_binprm *bprm)
>   				return -EFAULT;
>   		}
>   	}
> +#endif
>   
>   	set_binfmt(&flat_format);
>