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

Marcin Slusarz
2008-02-04T21:28:30Z

On Mon, Feb 04, 2008 at 08:31:07PM +0100, Jan Kara wrote:
> 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..
Ok

> >
> > - 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.
I used min_t because gcc and sparse warned about different types.

> > 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.

Updated patch:
---
udf: fix udf_build_ustr

udf_build_ustr was broken:

- size == 1:
dest->u_len = ptr[1 - 1], but at ptr[0] there's cmpID,
so we created string with wrong length
it should not happen, so we BUG() it
- size > 1 and size < UDF_NAME_LEN:
we set u_len correctly, but memcpy copied one needless byte
- size == UDF_NAME_LEN - 1:
memcpy overwrited u_len - with correct value, but...
- size >= UDF_NAME_LEN:
we copied UDF_NAME_LEN - 1 bytes, but dest->u_name is array
of UDF_NAME_LEN - 2 bytes, so we were overwriting u_len with
character from input string

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 | 12 +++++++-----
1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/udf/unicode.c b/fs/udf/unicode.c
index 335fc56..442d38a 100644
--- a/fs/udf/unicode.c
+++ b/fs/udf/unicode.c
@@ -49,14 +49,16 @@ int udf_build_ustr(struct ustr *dest, dstring *ptr, int size)
{
int usesize;

- if ((!dest) || (!ptr) || (!size))
+ if (!dest || !ptr || !size)
return -1;
+ BUG_ON(size < 2);

- 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(usesize, size - 2);
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;
}
--
1.5.3.7