Re: [PATCH 02/10] udf: fix udf_build_ustr

Jan Kara
2008-02-04T19:31:21Z

On Thu 31-01-08 20:57:47, Marcin Slusarz wrote:
> On Thu, Jan 31, 2008 at 11:45:32AM +0100, Jan Kara wrote:
> > On Wed 30-01-08 22:03:52, marcin.slusarz@gmail.com wrote:
> > > udf_build_ustr was completely broken when
> > > size >= UDF_NAME_LEN - 1 or size < 2
> > >
> > > nobody noticed because all callers set size
> > > to acceptable values (constants)
> > >
> > > Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
> > > Cc: Jan Kara <jack@suse.cz>
> > > ---
> > > fs/udf/unicode.c | 12 ++++++------
> > > 1 files changed, 6 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/fs/udf/unicode.c b/fs/udf/unicode.c
> > > index f969617..f4e54e5 100644
> > > --- a/fs/udf/unicode.c
> > > +++ b/fs/udf/unicode.c
> > > @@ -47,16 +47,16 @@ static int udf_char_to_ustr(struct ustr *dest, const uint8_t *src, int strlen)
> > > */
> > > int udf_build_ustr(struct ustr *dest, dstring *ptr, int size)
> > > {
> > > - int usesize;
> > > + u8 usesize;
> > What is the purpose for this? Why not just leave int there? Arithmetics
> > is usually best done in ints if that's possible...
> I made it to stress that length of string fits in one byte.
> (struct ustr->u_len is uint8_t)
I see. I don't think this is really worthwhile, please keep int there.

> > > - if ((!dest) || (!ptr) || (!size))
> > > + if (!dest || !ptr || size < 2)
> > > return -1;
> > >
> > > - memset(dest, 0, sizeof(struct ustr));
> > > - usesize = (size > UDF_NAME_LEN) ? UDF_NAME_LEN : size;
> > > + usesize = min_t(size_t, size - 2, sizeof(dest->u_name));
> > > dest->u_cmpID = ptr[0];
> > > - dest->u_len = ptr[size - 1];
> > > - memcpy(dest->u_name, ptr + 1, usesize - 1);
> > > + dest->u_len = usesize;
> > > + memcpy(dest->u_name, ptr + 1, usesize);
> > > + memset(dest->u_name + usesize, 0, sizeof(dest->u_name) - usesize);
> > Hmm, after parsing what the standard says (ugh), I don't think the code is
> > wrong (at least I think you made it incorrect). The caller of
> > udf_char_to_ustr() specifies length of the field (not length of the
> > string). Now, in the last character of the field is stored the number of
> > characters in the string and in the first character of the field is stored
> > encoding of the string. So the original code seems correct.
> You are right. I broke length calculation.
>
> But observe that:
> - when size == 1:
> dest->u_len = ptr[1 - 1], but at ptr[0] there's cmpID,
> so we create string with wrong length
Yes, but that never happens. This function should always be used for
fixed-length strings whose maximum length is defined in the standard so if
someone calls it with size == 1, it is a bug. So you can just do
BUG_ON(size < 2).

> - when size > 1 and size < UDF_NAME_LEN:
> we set u_len correctly, but memcpy copies one needless byte
> - when size == UDF_NAME_LEN - 1:
> memcpy overwrites u_len - with correct value, but...
Yes, you're right.

> - when size >= UDF_NAME_LEN:
> we copy UDF_NAME_LEN - 1 bytes, but dest->u_name is array
> of UDF_NAME_LEN - 2 bytes, so we are overwriting u_len with
> character from input string
Again, this should not happen because UDF_NAME_LEN is large enough but
you are right it's better to clean this.

> So if I didn't mess up someting, correct change would look like this:
> ---
> udf: fix udf_build_ustr
>
> udf_build_ustr was broken when
> size >= UDF_NAME_LEN or size < 2
>
> nobody noticed because all callers set size
> to acceptable values (constants whitin range)
>
> Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
> Cc: Jan Kara <jack@suse.cz>
> ---
> fs/udf/unicode.c | 13 +++++++------
> 1 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/fs/udf/unicode.c b/fs/udf/unicode.c
> index 335fc56..83ae9fc 100644
> --- a/fs/udf/unicode.c
> +++ b/fs/udf/unicode.c
> @@ -47,16 +47,17 @@ static int udf_char_to_ustr(struct ustr *dest, const uint8_t *src, int strlen)
> */
> int udf_build_ustr(struct ustr *dest, dstring *ptr, int size)
> {
> - int usesize;
> + u8 usesize;
Just use int here..

>
> - if ((!dest) || (!ptr) || (!size))
> + if (!dest || !ptr || size < 2)
> return -1;
>
> - memset(dest, 0, sizeof(struct ustr));
> - usesize = (size > UDF_NAME_LEN) ? UDF_NAME_LEN : size;
> + usesize = min_t(size_t, ptr[size - 1], sizeof(dest->u_name));
> + usesize = min_t(size_t, usesize, size - 2);
And here use just min() in both cases so that it's easier to read.

> dest->u_cmpID = ptr[0];
> - dest->u_len = ptr[size - 1];
> - memcpy(dest->u_name, ptr + 1, usesize - 1);
> + dest->u_len = usesize;
> + memcpy(dest->u_name, ptr + 1, usesize);
> + memset(dest->u_name + usesize, 0, sizeof(dest->u_name) - usesize);
>
> return 0;
> }
Otherwise it looks fine. Thanks for the cleanups.

Honza

--
Jan Kara <jack@suse.cz>
SUSE Labs, CR