Re: [PATCH] jfs: Use unsigned variable for length calculations

Kees Cook
2023-02-06T18:35:27Z

On Sat, Feb 04, 2023 at 08:25:45PM +0000, Dr. David Alan Gilbert wrote:
> * Kees Cook (keescook@chromium.org) wrote:
> > To avoid confusing the compiler about possible negative sizes, switch
> > "ssize" which can never be negative from int to u32. Seen with GCC 13:
> >
> > ../fs/jfs/namei.c: In function 'jfs_symlink': ../include/linux/fortify-string.h:57:33: warning: '__builtin_memcpy' pointer overflow between offset 0 and size [-2147483648, -1]
> > [-Warray-bounds=]
> > 57 | #define __underlying_memcpy __builtin_memcpy
> > | ^
> > ...
> > ../fs/jfs/namei.c:950:17: note: in expansion of macro 'memcpy'
> > 950 | memcpy(ip->i_link, name, ssize);
> > | ^~~~~~
> >
> > Cc: Dave Kleikamp <shaggy@kernel.org>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Dave Chinner <dchinner@redhat.com>
> > Cc: jfs-discussion@lists.sourceforge.net
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > fs/jfs/namei.c | 6 +++---
> > 1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/jfs/namei.c b/fs/jfs/namei.c
> > index b29d68b5eec5..494b9f4043cf 100644
> > --- a/fs/jfs/namei.c
> > +++ b/fs/jfs/namei.c
> > @@ -876,7 +876,7 @@ static int jfs_symlink(struct mnt_idmap *idmap, struct inode *dip,
> > tid_t tid;
> > ino_t ino = 0;
> > struct component_name dname;
> > - int ssize; /* source pathname size */
> > + u32 ssize; /* source pathname size */
>
> Had you considered using size_t - this is set from a strlen and used by a memcpy
> that both talk size_t.

I considered that, but I've had other maintainers upset about doubling
the variable size. I opted to keep the variable 32-bit here, so the
machine code would only change to lose signed-ness.

-Kees

--
Kees Cook