linux-kernel - [PATCH v3 12/12] selftests/nolibc: test

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Date:   Sat,  3 Jun 2023 16:17:25 +0800
From:   Zhangjin Wu <falcon@...ylab.org>
To:     w@....eu
Cc:     falcon@...ylab.org, arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-riscv@...ts.infradead.org,
        thomas@...ch.de,
        Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH v3 12/12] selftests/nolibc: test_fork: fix up duplicated print

running nolibc-test with glibc on x86_64 got such print issue:

    29 execve_root = -1 EACCES                                       [OK]
    30 fork30 fork = 0                                                      [OK]
    31 getdents64_root = 712                                         [OK]

The fork test case has three printf calls:

    (1) llen += printf("%d %s", test, #name);
    (2) llen += printf(" = %d %s ", expr, errorname(errno));
    (3) llen += pad_spc(llen, 64, "[FAIL]\n"); --> vfprintf()

In the following scene, the above issue happens:

    (a) The parent calls (1)
    (b) The parent calls fork()
    (c) The child runs and shares the print buffer of (1)
    (d) The child exits, flushs the print buffer and closes its own stdout/stderr
        * "30 fork" is printed at the first time.
    (e) The parent calls (2) and (3), with "\n" in (3), it flushs the whole buffer
        * "30 fork = 0 ..." is printed

Therefore, there are two "30 fork" in the stdout.

Between (a) and (b), if flush the stdout (and the sterr), the child in
stage (c) will not be able to 'see' the print buffer.

Reviewed-by: Thomas Weißschuh <linux@...ssschuh.net>
Signed-off-by: Zhangjin Wu <falcon@...ylab.org>
---
 tools/testing/selftests/nolibc/nolibc-test.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index e68c5692ec54..7dd950879161 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -490,7 +490,13 @@ static int test_getpagesize(void)
 static int test_fork(void)
 {
 	int status;
-	pid_t pid = fork();
+	pid_t pid;
+
+	/* flush the printf buffer to avoid child flush it */
+	fflush(stdout);
+	fflush(stderr);
+
+	pid = fork();
 
 	switch (pid) {
 	case -1:
-- 
2.25.1