[PATCH 1/1] proc: test lseek on /proc/net/dev

Alexey Dobriyan
2025-12-26T19:55:22Z

---
tools/testing/selftests/proc/.gitignore | 1 +
tools/testing/selftests/proc/Makefile | 1 +
.../selftests/proc/proc-net-dev-lseek.c | 68 +++++++++++++++++++
3 files changed, 70 insertions(+)
create mode 100644 tools/testing/selftests/proc/proc-net-dev-lseek.c

diff --git a/tools/testing/selftests/proc/.gitignore b/tools/testing/selftests/proc/.gitignore
index 19bb333e2485..243f4537a670 100644
--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -7,6 +7,7 @@
/proc-loadavg-001
/proc-maps-race
/proc-multiple-procfs
+/proc-net-dev-lseek
/proc-empty-vm
/proc-pid-vm
/proc-self-map-files-001
diff --git a/tools/testing/selftests/proc/Makefile b/tools/testing/selftests/proc/Makefile
index 50aba102201a..2a9547630115 100644
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -10,6 +10,7 @@ TEST_GEN_PROGS += fd-003-kthread
TEST_GEN_PROGS += proc-2-is-kthread
TEST_GEN_PROGS += proc-loadavg-001
TEST_GEN_PROGS += proc-maps-race
+TEST_GEN_PROGS += proc-net-dev-lseek
TEST_GEN_PROGS += proc-empty-vm
TEST_GEN_PROGS += proc-pid-vm
TEST_GEN_PROGS += proc-self-map-files-001
diff --git a/tools/testing/selftests/proc/proc-net-dev-lseek.c b/tools/testing/selftests/proc/proc-net-dev-lseek.c
new file mode 100644
index 000000000000..742a3e804451
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-net-dev-lseek.c
@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2025 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+#undef _GNU_SOURCE
+#define _GNU_SOURCE
+#undef NDEBUG
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <unistd.h>
+#include <sched.h>
+/*
+ * Test that lseek("/proc/net/dev/", 0, SEEK_SET)
+ * a) works,
+ * b) does what you think it does.
+ */
+int main(void)
+{
+ /* /proc/net/dev output is deterministic in fresh netns only. */
+ if (unshare(CLONE_NEWNET) == -1) {
+ if (errno == ENOSYS || errno == EPERM) {
+ return 4;
+ }
+ return 1;
+ }
+
+ const int fd = open("/proc/net/dev", O_RDONLY);
+ assert(fd >= 0);
+
+ char buf1[4096];
+ const ssize_t rv1 = read(fd, buf1, sizeof(buf1));
+ /*
+ * Not "<=", this file can't be empty:
+ * there is header, "lo" interface with some zeroes.
+ */
+ assert(0 < rv1);
+ assert(rv1 <= sizeof(buf1));
+
+ /* Believe it or not, this line broke one day. */
+ assert(lseek(fd, 0, SEEK_SET) == 0);
+
+ char buf2[4096];
+ const ssize_t rv2 = read(fd, buf2, sizeof(buf2));
+ /* Not "<=", see above. */
+ assert(0 < rv2);
+ assert(rv2 <= sizeof(buf2));
+
+ /* Test that lseek rewinds to the beginning of the file. */
+ assert(rv1 == rv2);
+ assert(memcmp(buf1, buf2, rv1) == 0);
+
+ /* Contents of the file is not validated: this test is about lseek(). */
+
+ return 0;
+}
--
2.49.1