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

[Marcin Slusarz]

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@xxxxxxxxx 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@xxxxxxxxx>
> > > > Cc: Jan Kara <jack@xxxxxxx>
> > > > ---
> > > >  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@xxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > ---
> >  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@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
---
 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

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