Re: [PATCH v5 bpf-next 5/5] bpf/selftests: Add a selftest for bpf_getxattr

[Christian Brauner <brauner@kernel.org>]

On Tue, Jun 28, 2022 at 04:19:48PM +0000, KP Singh wrote:
> A simple test that adds an xattr on a copied /bin/ls and reads it back
> when the copied ls is executed.
> 
> Signed-off-by: KP Singh <kpsingh@kernel.org>
> ---
>  .../testing/selftests/bpf/prog_tests/xattr.c  | 54 +++++++++++++++++++
>  tools/testing/selftests/bpf/progs/xattr.c     | 37 +++++++++++++
>  2 files changed, 91 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/xattr.c
>  create mode 100644 tools/testing/selftests/bpf/progs/xattr.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/xattr.c b/tools/testing/selftests/bpf/prog_tests/xattr.c
> new file mode 100644
> index 000000000000..ef07fa8a1763
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/xattr.c
> @@ -0,0 +1,54 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Copyright 2022 Google LLC.
> + */
> +
> +#include <test_progs.h>
> +#include <sys/xattr.h>
> +#include "xattr.skel.h"
> +
> +#define XATTR_NAME "security.bpf"
> +#define XATTR_VALUE "test_progs"
> +
> +void test_xattr(void)
> +{
> +	struct xattr *skel = NULL;
> +	char tmp_dir_path[] = "/tmp/xattrXXXXXX";
> +	char tmp_exec_path[64];
> +	char cmd[256];
> +	int err;
> +
> +	if (CHECK_FAIL(!mkdtemp(tmp_dir_path)))
> +		goto close_prog;
> +
> +	snprintf(tmp_exec_path, sizeof(tmp_exec_path), "%s/copy_of_ls",
> +		 tmp_dir_path);
> +	snprintf(cmd, sizeof(cmd), "cp /bin/ls %s", tmp_exec_path);
> +	if (CHECK_FAIL(system(cmd)))
> +		goto close_prog_rmdir;
> +
> +	if (CHECK_FAIL(setxattr(tmp_exec_path, XATTR_NAME, XATTR_VALUE,
> +			   sizeof(XATTR_VALUE), 0)))
> +		goto close_prog_rmdir;
> +
> +	skel = xattr__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "skel_load"))
> +		goto close_prog_rmdir;
> +
> +	err = xattr__attach(skel);
> +	if (!ASSERT_OK(err, "xattr__attach failed"))
> +		goto close_prog_rmdir;
> +
> +	snprintf(cmd, sizeof(cmd), "%s -l", tmp_exec_path);
> +	if (CHECK_FAIL(system(cmd)))
> +		goto close_prog_rmdir;
> +
> +	ASSERT_EQ(skel->bss->result, 1, "xattr result");
> +
> +close_prog_rmdir:
> +	snprintf(cmd, sizeof(cmd), "rm -rf %s", tmp_dir_path);
> +	system(cmd);
> +close_prog:
> +	xattr__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/xattr.c b/tools/testing/selftests/bpf/progs/xattr.c
> new file mode 100644
> index 000000000000..ccc078fb8ebd
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/xattr.c
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Copyright 2022 Google LLC.
> + */
> +
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +#define XATTR_NAME "security.bpf"
> +#define XATTR_VALUE "test_progs"
> +
> +__u64 result = 0;
> +
> +extern ssize_t bpf_getxattr(struct dentry *dentry, struct inode *inode,
> +			    const char *name, void *value, int size) __ksym;
> +
> +SEC("lsm.s/bprm_committed_creds")
> +void BPF_PROG(bprm_cc, struct linux_binprm *bprm)
> +{
> +	struct task_struct *current = bpf_get_current_task_btf();
> +	char dir_xattr_value[64] = {0};
> +	int xattr_sz = 0;
> +
> +	xattr_sz = bpf_getxattr(bprm->file->f_path.dentry,
> +				bprm->file->f_path.dentry->d_inode, XATTR_NAME,
> +				dir_xattr_value, 64);

Yeah, this isn't right. You're not accounting for the caller's userns
nor for the idmapped mount. If this is supposed to work you will need a
variant of vfs_getxattr() that takes the mount's idmapping into account
afaict. See what needs to happen after do_getxattr().