[PATCH] Implement leftpad syscall

[Richard Weinberger]

From: David Gstir <david@xxxxxxxxxxxxx>

Implement the leftpad() system call such that userspace,
especially node.js applications, can in the near future directly
use it and no longer depend on fragile npm packages.

Signed-off-by: David Gstir <david@xxxxxxxxxxxxx>
Signed-off-by: Richard Weinberger <richard@xxxxxx>
---
 arch/x86/entry/syscalls/syscall_64.tbl |  1 +
 include/linux/syscalls.h               |  1 +
 kernel/sys.c                           | 35 ++++++++++++++++++++++++++++++++++
 kernel/sys_ni.c                        |  1 +
 4 files changed, 38 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index cac6d17..f287712 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -335,6 +335,7 @@
 326	common	copy_file_range		sys_copy_file_range
 327	64	preadv2			sys_preadv2
 328	64	pwritev2		sys_pwritev2
+329	common	leftpad			sys_leftpad
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index d795472..a0850bb 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -898,4 +898,5 @@ asmlinkage long sys_copy_file_range(int fd_in, loff_t __user *off_in,
 
 asmlinkage long sys_mlock2(unsigned long start, size_t len, int flags);
 
+asmlinkage long sys_leftpad(char *str, char pad, char *dst, size_t dst_len);
 #endif
diff --git a/kernel/sys.c b/kernel/sys.c
index cf8ba54..e42d972 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2432,3 +2432,38 @@ COMPAT_SYSCALL_DEFINE1(sysinfo, struct compat_sysinfo __user *, info)
 	return 0;
 }
 #endif /* CONFIG_COMPAT */
+
+
+SYSCALL_DEFINE4(leftpad, char *, src, char, pad, char *, dst, size_t, dst_len)
+{
+	char *buf;
+	long ret;
+	size_t len = strlen_user(src);
+	size_t pad_len = dst_len - len;
+
+	if (dst_len <= len || dst_len > 4096) {
+		return -EINVAL;
+	}
+
+	buf = kmalloc(dst_len, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	memset(buf, pad, pad_len);
+	ret = copy_from_user(buf + pad_len, src, len);
+	if (ret) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	ret = copy_to_user(dst, buf, dst_len);
+	if (ret) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	ret = pad_len;
+out:
+	kfree(buf);
+	return ret;
+}
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 2c5e3a8..262608d 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -175,6 +175,7 @@ cond_syscall(sys_setfsgid);
 cond_syscall(sys_capget);
 cond_syscall(sys_capset);
 cond_syscall(sys_copy_file_range);
+cond_syscall(sys_leftpad);
 
 /* arch-specific weak syscall entries */
 cond_syscall(sys_pciconfig_read);
-- 
1.8.4.5