From 0695ff8282eca40ab08300c5a9be8716d453aee6 Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Fri, 10 Jan 2020 20:13:23 +0100
Subject: [PATCH] Kernel: Pass characters+length to readlink()

Note that I'm developing some helper types in the Syscall namespace as
I go here. Once I settle on some nice types, I will convert all the
other syscalls to use them as well.
---
 Kernel/Process.cpp        | 31 +++++++++++++++++++++----------
 Kernel/Process.h          |  4 +++-
 Kernel/Syscall.h          | 11 +++++++++++
 Libraries/LibC/unistd.cpp |  3 ++-
 4 files changed, 37 insertions(+), 12 deletions(-)

diff --git a/Kernel/Process.cpp b/Kernel/Process.cpp
index 5bd8de15e6..0430aba5d7 100644
--- a/Kernel/Process.cpp
+++ b/Kernel/Process.cpp
@@ -1558,17 +1558,27 @@ int Process::sys$stat(const char* user_path, size_t path_length, stat* user_stat
     return 0;
 }
 
-int Process::sys$readlink(const char* path, char* buffer, ssize_t size)
+template<typename DataType, typename SizeType>
+bool Process::validate(const Syscall::MutableBufferArgument<DataType, SizeType>& buffer)
 {
-    if (size < 0)
-        return -EINVAL;
-    SmapDisabler disabler;
-    if (!validate_read_str(path))
+    return validate_write(buffer.data, buffer.size);
+}
+
+int Process::sys$readlink(const Syscall::SC_readlink_params* user_params)
+{
+    if (!validate_read_typed(user_params))
         return -EFAULT;
-    if (!validate_write(buffer, size))
+    Syscall::SC_readlink_params params;
+    copy_from_user(&params, user_params, sizeof(params));
+
+    if (!validate(params.buffer))
         return -EFAULT;
 
-    auto result = VFS::the().open(path, O_RDONLY | O_NOFOLLOW_NOERROR, 0, current_directory());
+    auto path = get_syscall_path_argument(params.path.characters, params.path.length);
+    if (path.is_error())
+        return path.error();
+
+    auto result = VFS::the().open(path.value(), O_RDONLY | O_NOFOLLOW_NOERROR, 0, current_directory());
     if (result.is_error())
         return result.error();
     auto description = result.value();
@@ -1580,9 +1590,10 @@ int Process::sys$readlink(const char* path, char* buffer, ssize_t size)
     if (!contents)
         return -EIO; // FIXME: Get a more detailed error from VFS.
 
-    copy_to_user(buffer, contents.data(), min(size, (ssize_t)contents.size()));
-    if (contents.size() + 1 < size)
-        buffer[contents.size()] = '\0';
+    auto link_target = String::copy(contents);
+    if (link_target.length() + 1 > params.buffer.size)
+        return -ENAMETOOLONG;
+    copy_to_user(params.buffer.data, link_target.characters(), link_target.length() + 1);
     return 0;
 }
 
diff --git a/Kernel/Process.h b/Kernel/Process.h
index 5c9980b557..d3aac8d661 100644
--- a/Kernel/Process.h
+++ b/Kernel/Process.h
@@ -151,7 +151,7 @@ public:
     int sys$clock_nanosleep(const Syscall::SC_clock_nanosleep_params*);
     int sys$gethostname(char*, ssize_t);
     int sys$uname(utsname*);
-    int sys$readlink(const char*, char*, ssize_t);
+    int sys$readlink(const Syscall::SC_readlink_params*);
     int sys$ttyname_r(int fd, char*, ssize_t);
     int sys$ptsname_r(int fd, char*, ssize_t);
     pid_t sys$fork(RegisterDump&);
@@ -260,6 +260,8 @@ public:
     bool validate_read_typed(T* value, size_t count = 1) { return validate_read(value, sizeof(T) * count); }
     template<typename T>
     bool validate_write_typed(T* value, size_t count = 1) { return validate_write(value, sizeof(T) * count); }
+    template<typename DataType, typename SizeType>
+    bool validate(const Syscall::MutableBufferArgument<DataType, SizeType>&);
 
     Custody& current_directory();
     Custody* executable() { return m_executable.ptr(); }
diff --git a/Kernel/Syscall.h b/Kernel/Syscall.h
index 8c973d3660..9396a97d5e 100644
--- a/Kernel/Syscall.h
+++ b/Kernel/Syscall.h
@@ -305,6 +305,12 @@ struct SyscallString {
     size_t length { 0 };
 };
 
+template<typename DataType, typename SizeType>
+struct MutableBufferArgument {
+    DataType* data { nullptr };
+    SizeType size { 0 };
+};
+
 struct SyscallStringList {
     SyscallString* strings { nullptr };
     size_t length { 0 };
@@ -316,6 +322,11 @@ struct SC_execve_params {
     SyscallStringList environment;
 };
 
+struct SC_readlink_params {
+    SyscallString path;
+    MutableBufferArgument<char, size_t> buffer;
+};
+
 void initialize();
 int sync();
 
diff --git a/Libraries/LibC/unistd.cpp b/Libraries/LibC/unistd.cpp
index 2f4debf7b3..658291b478 100644
--- a/Libraries/LibC/unistd.cpp
+++ b/Libraries/LibC/unistd.cpp
@@ -326,7 +326,8 @@ int gethostname(char* buffer, size_t size)
 
 ssize_t readlink(const char* path, char* buffer, size_t size)
 {
-    int rc = syscall(SC_readlink, path, buffer, size);
+    Syscall::SC_readlink_params params { { path, strlen(path) }, { buffer, size } };
+    int rc = syscall(SC_readlink, &params);
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }