proc: test lseek on /proc/net/dev

This line in tools/testing/selftests/proc/read.c was added to catch
oopses, not to verify lseek correctness:

        (void)lseek(fd, 0, SEEK_SET);

Oh, well. Prevent more embarassement with simple test.

Link: https://lkml.kernel.org/r/aKTCfMuRXOpjBXxI@p183
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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

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

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;
+}