linux-kernel - Re: [PATCH 02/10] udf: fix udf_build

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Message-ID: <20080131195730.GB6222@joi>
Date:	Thu, 31 Jan 2008 20:57:47 +0100
From:	Marcin Slusarz <marcin.slusarz@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] udf: fix udf_build_ustr

On Thu, Jan 31, 2008 at 11:45:32AM +0100, Jan Kara wrote:
> On Wed 30-01-08 22:03:52, marcin.slusarz@...il.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@...il.com>
> > Cc: Jan Kara <jack@...e.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)

> > -	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
- 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...
- 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

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@...il.com>
Cc: Jan Kara <jack@...e.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;
 
-	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);
 	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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