Messages in this thread | | | From | Kees Cook <> | Date | Mon, 4 Dec 2017 10:37:15 -0800 | Subject | Re: [PATCH] exec: avoid gcc-8 warning for get_task_comm |
| |
On Mon, Dec 4, 2017 at 6:49 AM, Arnd Bergmann <arnd@arndb.de> wrote: > gcc-8 warns about using strncpy() with the source size as the limit: > > fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess] > > This is indeed slightly suspicious, as it protects us from source > arguments without NUL-termination, but does not guarantee that the > destination is terminated. > > This changes it to strlcpy with a hardcoded length, to guarantee > a properly terminated string. Since we already use strlcpy() for > __set_task_comm(), the source should always be terminated properly,
It's terminated but it's not padded. This change _might_ result in leaving a string buffer uninitialized (and if that buffer is passed as a whole instead of just its strlen(), this could be a problem: proc_comm_connector() e.g. may suffer from this, I haven't checked closely).
> so this patch won't change the behavior, but make it a bit more robust.
I would rather we change get_task_comm() to include the size of buf... maybe just convert it to a macro:
-char *get_task_comm(char *buf, struct task_struct *tsk) +char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
...
#define get_task_comm(buf, tsk) __get_task_comm(buf, sizeof(buf), tsk)
?
That will only work for preallocated arrays though. Maybe a build bug when sizeof(buf) != TASK_COMM_LEN ?
I think the strncpy() should stay, but for other conversions where NULL-padding isn't an issue, strscpy() is likely preferred to strlcpy() to avoid the potential "overread" due to the strlen() call in strlcpy().
-Kees
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de> > --- > fs/exec.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/fs/exec.c b/fs/exec.c > index 6be2aa0ab26f..3e8012afe440 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1218,9 +1218,9 @@ static int de_thread(struct task_struct *tsk) > > char *get_task_comm(char *buf, struct task_struct *tsk) > { > - /* buf must be at least sizeof(tsk->comm) in size */ > + /* buf must be at least TASK_COMM_LEN in size */ > task_lock(tsk); > - strncpy(buf, tsk->comm, sizeof(tsk->comm)); > + strlcpy(buf, tsk->comm, TASK_COMM_LEN); > task_unlock(tsk); > return buf; > } > -- > 2.9.0 >
-- Kees Cook Pixel Security
| |