Re: [PATCH bpf v3 1/3] bpf: update local storage test to check handling of null ptrs

[KP Singh <kpsingh@kernel.org>]

On Thu, Jan 28, 2021 at 2:46 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Mon, Jan 11, 2021 at 11:55 PM KP Singh <kpsingh@kernel.org> wrote:
> >
> > It was found in [1] that bpf_inode_storage_get helper did not check
> > the nullness of the passed owner ptr which caused an oops when
> > dereferenced. This change incorporates the example suggested in [1] into
> > the local storage selftest.
> >
> > The test is updated to create a temporary directory instead of just
> > using a tempfile. In order to replicate the issue this copied rm binary
> > is renamed tiggering the inode_rename with a null pointer for the
> > new_inode. The logic to verify the setting and deletion of the inode
> > local storage of the old inode is also moved to this LSM hook.
> >
> > The change also removes the copy_rm function and simply shells out
> > to copy files and recursively delete directories and consolidates the
> > logic of setting the initial inode storage to the bprm_committed_creds
> > hook and removes the file_open hook.
> >
> > [1]: https://lore.kernel.org/bpf/CANaYP3HWkH91SN=wTNO9FL_2ztHfqcXKX38SSE-JJ2voh+vssw@mail.gmail.com
> >
> > Suggested-by: Gilad Reti <gilad.reti@gmail.com>
> > Acked-by: Yonghong Song <yhs@fb.com>
> > Signed-off-by: KP Singh <kpsingh@kernel.org>
> > ---
>
> Hi KP,
>
> I'm getting a compilation warning when building selftests. Can you
> please take a look and send a fix? Thanks!
>
> /data/users/andriin/linux/tools/testing/selftests/bpf/prog_tests/test_local_storage.c:
> In function ‘test_test_local_storage’:
> /data/users/andriin/linux/tools/testing/selftests/bpf/prog_tests/test_local_storage.c:143:52:
> warning: ‘/copy_of_rm’ directive output may be truncated writing 11
> bytes into a region of size between 1 and 64 [-Wformat-truncation=]
>   143 |  snprintf(tmp_exec_path, sizeof(tmp_exec_path), "%s/copy_of_rm",
>       |                                                    ^~~~~~~~~~~
> /data/users/andriin/linux/tools/testing/selftests/bpf/prog_tests/test_local_storage.c:143:2:
> note: ‘snprintf’ output between 12 and 75 bytes into a destination of
> size 64
>   143 |  snprintf(tmp_exec_path, sizeof(tmp_exec_path), "%s/copy_of_rm",
>       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   144 |    tmp_dir_path);
>       |    ~~~~~~~~~~~~~
>

I don't seem to get this warning, so maybe we are using different compilers.

Mine is gcc 10.2.1 20201224 (from debian)

That said, I understand why it's complaining, it's for something that
cannot really happen:

tmp_dir_path cannot be 64 because we actually know its length so the
tmp_exec_path cannot really overflow 64 bytes.

Can you check if the following patch makes it go away?

diff --git a/tools/testing/selftests/bpf/prog_tests/test_local_storage.c
b/tools/testing/selftests/bpf/prog_tests/test_local_storage.c
index 3bfcf00c0a67..d2c16eaae367 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_local_storage.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_local_storage.c
@@ -113,7 +113,7 @@ static bool check_syscall_operations(int map_fd, int obj_fd)

 void test_test_local_storage(void)
 {
-       char tmp_dir_path[64] = "/tmp/local_storageXXXXXX";
+       char tmp_dir_path[] = "/tmp/local_storageXXXXXX";
        int err, serv_sk = -1, task_fd = -1, rm_fd = -1;
        struct local_storage *skel = NULL;
        char tmp_exec_path[64];

If so, I can send you a fix.

- KP

>
> >  .../bpf/prog_tests/test_local_storage.c       | 96 +++++--------------
> >  .../selftests/bpf/progs/local_storage.c       | 62 ++++++------
> >  2 files changed, 61 insertions(+), 97 deletions(-)
> >
>
> [...]