linux-kernel - Re: [bpf-next v3 02/12] selftests/bpf: Avoid a clobbering of errno

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Date:   Thu, 11 Jul 2019 17:59:46 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Krzesimir Nowak <krzesimir@...volk.io>
Cc:     open list <linux-kernel@...r.kernel.org>,
        Alban Crequy <alban@...volk.io>,
        Iago López Galeiras <iago@...volk.io>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        xdp-newbies@...r.kernel.org
Subject: Re: [bpf-next v3 02/12] selftests/bpf: Avoid a clobbering of errno

On Thu, Jul 11, 2019 at 5:04 AM Krzesimir Nowak <krzesimir@...volk.io> wrote:
>
> On Thu, Jul 11, 2019 at 1:52 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Mon, Jul 8, 2019 at 3:42 PM Krzesimir Nowak <krzesimir@...volk.io> wrote:
> > >
> > > Save errno right after bpf_prog_test_run returns, so we later check
> > > the error code actually set by bpf_prog_test_run, not by some libcap
> > > function.
> > >
> > > Changes since v1:
> > > - Fix the "Fixes:" tag to mention actual commit that introduced the
> > >   bug
> > >
> > > Changes since v2:
> > > - Move the declaration so it fits the reverse christmas tree style.
> > >
> > > Cc: Daniel Borkmann <daniel@...earbox.net>
> > > Fixes: 832c6f2c29ec ("bpf: test make sure to run unpriv test cases in test_verifier")
> > > Signed-off-by: Krzesimir Nowak <krzesimir@...volk.io>
> > > ---
> > >  tools/testing/selftests/bpf/test_verifier.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> > > index b8d065623ead..3fe126e0083b 100644
> > > --- a/tools/testing/selftests/bpf/test_verifier.c
> > > +++ b/tools/testing/selftests/bpf/test_verifier.c
> > > @@ -823,16 +823,18 @@ static int do_prog_test_run(int fd_prog, bool unpriv, uint32_t expected_val,
> > >         __u8 tmp[TEST_DATA_LEN << 2];
> > >         __u32 size_tmp = sizeof(tmp);
> > >         uint32_t retval;
> > > +       int saved_errno;
> > >         int err;
> > >
> > >         if (unpriv)
> > >                 set_admin(true);
> > >         err = bpf_prog_test_run(fd_prog, 1, data, size_data,
> > >                                 tmp, &size_tmp, &retval, NULL);
> >
> > Given err is either 0 or -1, how about instead making err useful right
> > here without extra variable?
> >
> > if (bpf_prog_test_run(...))
> >         err = errno;
>
> I change it later to bpf_prog_test_run_xattr, which can also return
> -EINVAL and then errno is not set. But this one probably should not be

This is wrong. bpf_prog_test_run/bpf_prog_test_run_xattr should either
always return -1 and set errno to actual error (like syscalls do), or
always use return code with proper error. Give they are pretending to
be just pure syscall, it's probably better to set errno to EINVAL and
return -1 on invalid input args?

> triggered by the test code. So not sure, probably would be better to
> keep it as is for consistency?
>
> >
> > > +       saved_errno = errno;
> > >         if (unpriv)
> > >                 set_admin(false);
> > >         if (err) {
> > > -               switch (errno) {
> > > +               switch (saved_errno) {
> > >                 case 524/*ENOTSUPP*/:
> >
> > ENOTSUPP is defined in include/linux/errno.h, is there any problem
> > with using this in selftests?
>
> I just used whatever there was earlier. Seems like <linux/errno.h> is
> not copied to tools include directory.

Ok, let's leave it as is, thanks!

>
> >
> > >                         printf("Did not run the program (not supported) ");
> > >                         return 0;
> > > --
> > > 2.20.1
> > >
>
>
>
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