Re: [PATCH bpf v2 2/2] selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL

["Daniel Xu" <dxu@dxuuu.xyz>]

On Thu Nov 5, 2020 at 10:30 AM PST, Song Liu wrote:
>
>
> > On Nov 4, 2020, at 6:25 PM, Daniel Xu <dxu@dxuuu.xyz> wrote:
> > 
> > Previously, bpf_probe_read_user_str() could potentially overcopy the
> > trailing bytes after the NUL due to how do_strncpy_from_user() does the
> > copy in long-sized strides. The issue has been fixed in the previous
> > commit.
> > 
> > This commit adds a selftest that ensures we don't regress
> > bpf_probe_read_user_str() again.
> > 
> > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> > ---
> > .../bpf/prog_tests/probe_read_user_str.c      | 60 +++++++++++++++++++
> > .../bpf/progs/test_probe_read_user_str.c      | 34 +++++++++++
> > 2 files changed, 94 insertions(+)
> > create mode 100644 tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> > create mode 100644 tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> > 
> > diff --git a/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> > new file mode 100644
> > index 000000000000..597a166e6c8d
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> > @@ -0,0 +1,60 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include <test_progs.h>
> > +#include "test_probe_read_user_str.skel.h"
> > +
> > +static const char str[] = "mestring";
> > +
> > +void test_probe_read_user_str(void)
> > +{
> > +	struct test_probe_read_user_str *skel;
> > +	int fd, err, duration = 0;
> > +	char buf[256];
> > +	ssize_t n;
> > +
> > +	skel = test_probe_read_user_str__open_and_load();
> > +	if (CHECK(!skel, "test_probe_read_user_str__open_and_load",
> > +		  "skeleton open and load failed\n"))
> > +		goto out;
>
> nit: we can just return here.
>
> > +
> > +	err = test_probe_read_user_str__attach(skel);
> > +	if (CHECK(err, "test_probe_read_user_str__attach",
> > +		  "skeleton attach failed: %d\n", err))
> > +		goto out;
> > +
> > +	fd = open("/dev/null", O_WRONLY);
> > +	if (CHECK(fd < 0, "open", "open /dev/null failed: %d\n", fd))
> > +		goto out;
> > +
> > +	/* Give pid to bpf prog so it doesn't read from anyone else */
> > +	skel->bss->pid = getpid();
>
> It is better to set pid before attaching skel.
>
> > +
> > +	/* Ensure bytes after string are ones */
> > +	memset(buf, 1, sizeof(buf));
> > +	memcpy(buf, str, sizeof(str));
> > +
> > +	/* Trigger tracepoint */
> > +	n = write(fd, buf, sizeof(buf));
> > +	if (CHECK(n != sizeof(buf), "write", "write failed: %ld\n", n))
> > +		goto fd_out;
> > +
> > +	/* Did helper fail? */
> > +	if (CHECK(skel->bss->ret < 0, "prog ret", "prog returned: %d\n",
>
> In most cases, we use underscore instead of spaces in the second
> argument
> of CHECK(). IOW, please use "prog_ret" instead of "prog ret".
>
> > +		  skel->bss->ret))
> > +		goto fd_out;
> > +
> > +	/* Check that string was copied correctly */
> > +	err = memcmp(skel->bss->buf, str, sizeof(str));
> > +	if (CHECK(err, "memcmp", "prog copied wrong string"))
> > +		goto fd_out;
> > +
> > +	/* Now check that no extra trailing bytes were copied */
> > +	memset(buf, 0, sizeof(buf));
> > +	err = memcmp(skel->bss->buf + sizeof(str), buf, sizeof(buf) - sizeof(str));
> > +	if (CHECK(err, "memcmp", "trailing bytes were not stripped"))
> > +		goto fd_out;
> > +
> > +fd_out:
> > +	close(fd);
> > +out:
> > +	test_probe_read_user_str__destroy(skel);
> > +}
> > diff --git a/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> > new file mode 100644
> > index 000000000000..41c3e296566e
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> > @@ -0,0 +1,34 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/bpf.h>
> > +#include <bpf/bpf_helpers.h>
> > +#include <bpf/bpf_tracing.h>
> > +
> > +#include <sys/types.h>
> > +
> > +struct sys_enter_write_args {
> > +	unsigned long long pad;
> > +	int syscall_nr;
> > +	int pad1; /* 4 byte hole */
> > +	unsigned int fd;
> > +	int pad2; /* 4 byte hole */
> > +	const char *buf;
> > +	size_t count;
> > +};
> > +
> > +pid_t pid = 0;
> > +int ret = 0;
> > +char buf[256] = {};
> > +
> > +SEC("tracepoint/syscalls/sys_enter_write")
> > +int on_write(struct sys_enter_write_args *ctx)
> > +{
> > +	if (pid != (bpf_get_current_pid_tgid() >> 32))
> > +		return 0;
> > +
> > +	ret = bpf_probe_read_user_str(buf, sizeof(buf), ctx->buf);
>
> bpf_probe_read_user_str() returns "long". Let's use "long ret;"

Thanks for review, will send v3 with these changes.

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