FAILED: patch "[PATCH] selftests/core: fix unshare_test with large fs.nr_open" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x df0b7e0ea59c43708cff8c7e34da5e2f8d430e5e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From df0b7e0ea59c43708cff8c7e34da5e2f8d430e5e Mon Sep 17 00:00:00 2001
From: Konstantin Khorenko <[email protected]>
Date: Fri, 14 Aug 2026 18:57:09 +0200
Subject: [PATCH] selftests/core: fix unshare_test with large fs.nr_open

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 04a2c4b4511d1, 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.

---
 tools/testing/selftests/core/unshare_test.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

index ffce75a6c228f..d40e963dd5205 100644
@@ -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

2.53.0

From: Al Viro <[email protected]>

[ Upstream commit 611fbeb44a777e5ab54ab3127ec85f72147911d8 ]

At some point there'd been a dumb braino during the dup_fd() calling conventions change; caught by smatch and immediately fixed. The trouble is, there had been no test coverage for the dup_fd() failure handling - neither in kselftests nor in LTP. Fortunately, it can be triggered on stock kernel - ENOMEM would require fault injection, but EMFILE can be had with sysctl alone (fs.nr_open).

Add a test for dup_fd() failure.
Fixed up commit log and short log - Shuah Khan

Stable-dep-of: df0b7e0ea59c ("selftests/core: fix unshare_test with large fs.nr_open")

---
 tools/testing/selftests/core/Makefile       |  2 +-
 tools/testing/selftests/core/unshare_test.c | 94 +++++++++++++++++++++
 2 files changed, 95 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/core/unshare_test.c

index ce262d0972699..8e99f87f5d7c2 100644
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -g $(KHDR_INCLUDES)
 
-TEST_GEN_PROGS := close_range_test
+TEST_GEN_PROGS := close_range_test unshare_test
 
 include ../lib.mk
 

2.53.0

From: Konstantin Khorenko <[email protected]>

[ Upstream commit df0b7e0ea59c43708cff8c7e34da5e2f8d430e5e ]

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 04a2c4b4511d1, 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.

---
 tools/testing/selftests/core/unshare_test.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

index 7fec9dfb1b0e3..6716e0f74d7da 100644
@@ -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

2.53.0