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selftests/core: fix unshare_test with large fs.nr_open

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The test assumes fs.nr_open is close to the default 1048576, but some
systems set it much higher (e.g.  1073741816).  This is systemd's doing:
since systemd v240 (2018), PID 1 bumps fs.nr_open and fs.file-max to their
largest possible values on boot, as file descriptors are already accounted
for by memcg [1].

In that case, dup2() to nr_open + 64 requires the kernel to allocate a
file descriptor table with ~1 billion entries, which fails with ENOMEM.

On a kernel that already carries 04a2c4b451, dup2() no longer fails
with ENOMEM.  The allocation is now rejected up front and the caller
gets EMFILE instead, without the WARNING, but the test still fails.

Cap the nr_open value used for the test's own arithmetic to a known
reasonable base value (1048576) and restore the true original value once
the test has completed.

Link: https://lore.kernel.org/20260814165709.513263-1-khorenko@virtuozzo.com
Link: a8b627aaed [1]
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Signed-off-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: <stable@vger.kernel.org>
This commit is contained in:
Konstantin Khorenko 2026-08-14 18:57:09 +02:00 committed by Andrew Morton
parent 0a27f79503
commit df0b7e0ea5
1 changed files with 17 additions and 2 deletions
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19
tools/testing/selftests/core/unshare_test.c
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@ -40,6 +40,14 @@ TEST(unshare_EMFILE)
ASSERT_EQ(sscanf(buf, "%d", &nr_open), 1);
/*
* Cap nr_open for the duration of the test to avoid ENOMEM from a
* huge fd table allocation; buf/n keep the real original value so
* fs.nr_open can be restored to it once the test is done.
*/
if (nr_open > 1024 * 1024)
nr_open = 1024 * 1024;
ASSERT_EQ(0, getrlimit(RLIMIT_NOFILE, &rlimit));
/* bump fs.nr_open */
@ -73,10 +81,13 @@ TEST(unshare_EMFILE)
if (pid == 0) {
int err;
char buf3[32];
ssize_t n3;
/* restore fs.nr_open */
/* restore fs.nr_open to the (possibly capped) test baseline */
n3 = sprintf(buf3, "%d\n", nr_open);
lseek(fd, 0, SEEK_SET);
write(fd, buf, n);
write(fd, buf3, n3);
/* ... and now unshare(CLONE_FILES) must fail with EMFILE */
err = unshare(CLONE_FILES);
EXPECT_EQ(err, -1)
@ -89,6 +100,10 @@ TEST(unshare_EMFILE)
EXPECT_EQ(waitpid(pid, &status, 0), pid);
EXPECT_EQ(true, WIFEXITED(status));
EXPECT_EQ(0, WEXITSTATUS(status));
/* restore the real fs.nr_open value */
lseek(fd, 0, SEEK_SET);
write(fd, buf, n);
}
TEST_HARNESS_MAIN