From: Randy Dunlap
> Sent: 23 December 2020 07:19
>
> On 12/22/20 9:04 PM, Josh Poimboeuf wrote:
> > GCC 7 has a known bug where UBSAN ignores '-fwrapv' and generates false
> > signed-overflow-UB warnings. The type mismatch between 'i' and
> > 'nr_segs' in copy_compat_iovec_from_user() is causing such a warning,
> > which also happens to violate uaccess rules:
> >
> > lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to
> > __ubsan_handle_add_overflow()
> with UACCESS enabled
> >
> > Fix it by making the variable types match.
> >
> > This is similar to a previous commit:
> >
> > 29da93fea3ea ("mm/uaccess: Use 'unsigned long' to placate UBSAN warnings
> > on older GCC versions")
> >
> > Reported-by: Randy Dunlap <[email protected]>
> > Signed-off-by: Josh Poimboeuf <[email protected]>
>
> All good. Thanks.
>
> Acked-by: Randy Dunlap <[email protected]> # build-tested
FWIW I've tested the equivalent change locally.
It generates rather better code on amd64.
(You don't want to index arrays with 'int' unless you really have to.)
So probably:
Acked-by: David Laight <[email protected]>
David
>
>
> > ---
> > lib/iov_iter.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> > index 1635111c5bd2..2e6a42f5d1df 100644
> > --- a/lib/iov_iter.c
> > +++ b/lib/iov_iter.c
> > @@ -1656,7 +1656,8 @@ static int copy_compat_iovec_from_user(struct iovec
> > *iov,
> > {
> > const struct compat_iovec __user *uiov =
> > (const struct compat_iovec __user *)uvec;
> > - int ret = -EFAULT, i;
> > + int ret = -EFAULT;
> > + unsigned long i;
> >
> > if (!user_access_begin(uvec, nr_segs * sizeof(*uvec)))
> > return -EFAULT;
> >
>
>
> --
> ~Randy
-
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