linux-kernel - [PATCH 02/17] exec: Simplify unshare

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Date:   Mon, 17 Aug 2020 17:04:10 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, criu@...nvz.org,
        bpf@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        Daniel P. Berrangé <berrange@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Matthew Wilcox <willy@...ian.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Matthew Wilcox <matthew@....cx>,
        Trond Myklebust <trond.myklebust@....uio.no>,
        Chris Wright <chrisw@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 02/17] exec: Simplify unshare_files

Now that exec no longer needs to return the unshared files to their
previous value there is no reason to return displaced.

Instead when unshare_fd creates a copy of the file table, call
put_files_struct before returning from unshare_files.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/coredump.c           |  5 +----
 fs/exec.c               |  5 +----
 include/linux/fdtable.h |  2 +-
 kernel/fork.c           | 12 ++++++------
 4 files changed, 9 insertions(+), 15 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 76e7c10edfc0..568d6e391082 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -585,7 +585,6 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 	int ispipe;
 	size_t *argv = NULL;
 	int argc = 0;
-	struct files_struct *displaced;
 	/* require nonrelative corefile path and be extra careful */
 	bool need_suid_safe = false;
 	bool core_dumped = false;
@@ -791,11 +790,9 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 	}
 
 	/* get us an unshared descriptor table; almost always a no-op */
-	retval = unshare_files(&displaced);
+	retval = unshare_files();
 	if (retval)
 		goto close_fail;
-	if (displaced)
-		put_files_struct(displaced);
 	if (!dump_interrupted()) {
 		/*
 		 * umh disabled with CONFIG_STATIC_USERMODEHELPER_PATH="" would
diff --git a/fs/exec.c b/fs/exec.c
index 17c007bba712..9b723d2560d1 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1354,7 +1354,6 @@ void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
 int begin_new_exec(struct linux_binprm * bprm)
 {
 	struct task_struct *me = current;
-	struct files_struct *displaced;
 	int retval;
 
 	/* Once we are committed compute the creds */
@@ -1375,11 +1374,9 @@ int begin_new_exec(struct linux_binprm * bprm)
 		goto out;
 
 	/* Ensure the files table is not shared. */
-	retval = unshare_files(&displaced);
+	retval = unshare_files();
 	if (retval)
 		goto out;
-	if (displaced)
-		put_files_struct(displaced);
 
 	/*
 	 * Must be called _before_ exec_mmap() as bprm->mm is
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index a32bf47c593e..f46a084b60b2 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -109,7 +109,7 @@ struct task_struct;
 struct files_struct *get_files_struct(struct task_struct *);
 void put_files_struct(struct files_struct *fs);
 void reset_files_struct(struct files_struct *);
-int unshare_files(struct files_struct **);
+int unshare_files(void);
 struct files_struct *dup_fd(struct files_struct *, unsigned, int *) __latent_entropy;
 void do_close_on_exec(struct files_struct *);
 int iterate_fd(struct files_struct *, unsigned,
diff --git a/kernel/fork.c b/kernel/fork.c
index 4d32190861bd..3049a41076f3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2995,21 +2995,21 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
  *	the exec layer of the kernel.
  */
 
-int unshare_files(struct files_struct **displaced)
+int unshare_files(void)
 {
 	struct task_struct *task = current;
-	struct files_struct *copy = NULL;
+	struct files_struct *old, *copy = NULL;
 	int error;
 
 	error = unshare_fd(CLONE_FILES, NR_OPEN_MAX, &copy);
-	if (error || !copy) {
-		*displaced = NULL;
+	if (error || !copy)
 		return error;
-	}
-	*displaced = task->files;
+
+	old = task->files;
 	task_lock(task);
 	task->files = copy;
 	task_unlock(task);
+	put_files_struct(old);
 	return 0;
 }
 
-- 
2.25.0