Re: [viro-vfs:work.fdtable 13/13] kernel/fork.c:3242 unshare_fd() warn: passing a valid pointer to 'PTR_ERR'

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Aug 13, 2024 at 07:16:00PM +0100, Al Viro wrote:
> On Tue, Aug 13, 2024 at 11:00:04AM +0300, Dan Carpenter wrote:
> > 3f4b0acefd818e Al Viro           2024-08-06  3240  		if (IS_ERR(*new_fdp)) {
> > 3f4b0acefd818e Al Viro           2024-08-06  3241  			*new_fdp = NULL;
> > 3f4b0acefd818e Al Viro           2024-08-06 @3242  			return PTR_ERR(new_fdp);
> >                                                                                ^^^^^^^^^^^^^^^^
> > 	err = PTR_ERR(*new_fdp);
> > 	*new_fdp = NULL;
> > 	return err;
> 
> Argh...  Obvious braino, but what it shows is that failures of that
> thing are not covered by anything in e.g. LTP.  Or in-kernel
> self-tests, for that matter...

FWIW, this does exercise that codepath, but I would really like to
have kselftest folks to comment on the damn thing - I'm pretty sure
that it's _not_ a good style for those.

diff --git a/tools/testing/selftests/core/Makefile b/tools/testing/selftests/core/Makefile
index ce262d097269..8e99f87f5d7c 100644
--- a/tools/testing/selftests/core/Makefile
+++ b/tools/testing/selftests/core/Makefile
@@ -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
 
diff --git a/tools/testing/selftests/core/unshare_test.c b/tools/testing/selftests/core/unshare_test.c
new file mode 100644
index 000000000000..7fec9dfb1b0e
--- /dev/null
+++ b/tools/testing/selftests/core/unshare_test.c
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/kernel.h>
+#include <limits.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+#include <unistd.h>
+#include <sys/resource.h>
+#include <linux/close_range.h>
+
+#include "../kselftest_harness.h"
+#include "../clone3/clone3_selftests.h"
+
+TEST(unshare_EMFILE)
+{
+	pid_t pid;
+	int status;
+	struct __clone_args args = {
+		.flags = CLONE_FILES,
+		.exit_signal = SIGCHLD,
+	};
+	int fd;
+	ssize_t n, n2;
+	static char buf[512], buf2[512];
+	struct rlimit rlimit;
+	int nr_open;
+
+	fd = open("/proc/sys/fs/nr_open", O_RDWR);
+	ASSERT_GE(fd, 0);
+
+	n = read(fd, buf, sizeof(buf));
+	ASSERT_GT(n, 0);
+	ASSERT_EQ(buf[n - 1], '\n');
+
+	ASSERT_EQ(sscanf(buf, "%d", &nr_open), 1);
+
+	ASSERT_EQ(0, getrlimit(RLIMIT_NOFILE, &rlimit));
+
+	/* bump fs.nr_open */
+	n2 = sprintf(buf2, "%d\n", nr_open + 1024);
+	lseek(fd, 0, SEEK_SET);
+	write(fd, buf2, n2);
+
+	/* bump ulimit -n */
+	rlimit.rlim_cur = nr_open + 1024;
+	rlimit.rlim_max = nr_open + 1024;
+	EXPECT_EQ(0, setrlimit(RLIMIT_NOFILE, &rlimit)) {
+		lseek(fd, 0, SEEK_SET);
+		write(fd, buf, n);
+		exit(EXIT_FAILURE);
+	}
+
+	/* get a descriptor past the old fs.nr_open */
+	EXPECT_GE(dup2(2, nr_open + 64), 0) {
+		lseek(fd, 0, SEEK_SET);
+		write(fd, buf, n);
+		exit(EXIT_FAILURE);
+	}
+
+	/* get descriptor table shared */
+	pid = sys_clone3(&args, sizeof(args));
+	EXPECT_GE(pid, 0) {
+		lseek(fd, 0, SEEK_SET);
+		write(fd, buf, n);
+		exit(EXIT_FAILURE);
+	}
+
+	if (pid == 0) {
+		int err;
+
+		/* restore fs.nr_open */
+		lseek(fd, 0, SEEK_SET);
+		write(fd, buf, n);
+		/* ... and now unshare(CLONE_FILES) must fail with EMFILE */
+		err = unshare(CLONE_FILES);
+		EXPECT_EQ(err, -1)
+			exit(EXIT_FAILURE);
+		EXPECT_EQ(errno, EMFILE)
+			exit(EXIT_FAILURE);
+		exit(EXIT_SUCCESS);
+	}
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
+TEST_HARNESS_MAIN