linux-kernel - [PATCH] Revert "fs: remove ksys

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Date:   Wed, 1 Jan 2020 20:05:03 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     torvalds@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
        youling257@...il.com, Arvind Sankar <nivedita@...m.mit.edu>
Subject: [PATCH] Revert "fs: remove ksys_dup()"

This reverts commit 8243186f0cc7c57cf9d6a110cd7315c44e3e0be8 and
additionally the fix in commit 2d3145f8d2809592ef803a30c8a342b5a9e2de9a.

Trying to use filp_open() and f_dupfd() instead of pseudo-syscalls
caused more trouble than what is worth it: it requires accessing
vfs internals and causes a strange issue on Androidx86 long after
boot.

Reported-by: youling 257 <youling257@...il.com>
Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>

---

Instead of that (seemingly suboptimal) approach in trying to get the
fake __user char out of init/main.c, I'll prepare some patches which
have ksys_open() [and a few other ksys_*() functions] operate on
kernelspace pointers instead. That will be more compatible with
keeping vfs internals internal, and actually move us faster in the
direction to let kernel init run with USER_DS. Moreover, that approach
doesn't seem to break youling 257's setup.

Thanks,
	Dominik


diff --git a/fs/file.c b/fs/file.c
index 2f4fcf985079..3da91a112bab 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -960,7 +960,7 @@ SYSCALL_DEFINE2(dup2, unsigned int, oldfd, unsigned int, newfd)
 	return ksys_dup3(oldfd, newfd, 0);
 }
 
-SYSCALL_DEFINE1(dup, unsigned int, fildes)
+int ksys_dup(unsigned int fildes)
 {
 	int ret = -EBADF;
 	struct file *file = fget_raw(fildes);
@@ -975,6 +975,11 @@ SYSCALL_DEFINE1(dup, unsigned int, fildes)
 	return ret;
 }
 
+SYSCALL_DEFINE1(dup, unsigned int, fildes)
+{
+	return ksys_dup(fildes);
+}
+
 int f_dupfd(unsigned int from, struct file *file, unsigned flags)
 {
 	int err;
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 2960dedcfde8..5262b7a76d39 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1232,6 +1232,7 @@ asmlinkage long sys_ni_syscall(void);
  */
 
 int ksys_umount(char __user *name, int flags);
+int ksys_dup(unsigned int fildes);
 int ksys_chroot(const char __user *filename);
 ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count);
 int ksys_chdir(const char __user *filename);
diff --git a/init/main.c b/init/main.c
index 1ecfd43ed464..2cd736059416 100644
--- a/init/main.c
+++ b/init/main.c
@@ -93,7 +93,6 @@
 #include <linux/rodata_test.h>
 #include <linux/jump_label.h>
 #include <linux/mem_encrypt.h>
-#include <linux/file.h>
 
 #include <asm/io.h>
 #include <asm/bugs.h>
@@ -1158,26 +1157,13 @@ static int __ref kernel_init(void *unused)
 
 void console_on_rootfs(void)
 {
-	struct file *file;
-	unsigned int i;
-
-	/* Open /dev/console in kernelspace, this should never fail */
-	file = filp_open("/dev/console", O_RDWR, 0);
-	if (IS_ERR(file))
-		goto err_out;
-
-	/* create stdin/stdout/stderr, this should never fail */
-	for (i = 0; i < 3; i++) {
-		if (f_dupfd(i, file, 0) != i)
-			goto err_out;
-	}
-
-	return;
+	/* Open the /dev/console as stdin, this should never fail */
+	if (ksys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
+		pr_err("Warning: unable to open an initial console.\n");
 
-err_out:
-	/* no panic -- this might not be fatal */
-	pr_err("Warning: unable to open an initial console.\n");
-	return;
+	/* create stdout/stderr */
+	(void) ksys_dup(0);
+	(void) ksys_dup(0);
 }
 
 static noinline void __init kernel_init_freeable(void)