From 72231b405ac9f92922d933d2c73098881488fbcd Mon Sep 17 00:00:00 2001
From: Liav A <liavalb@gmail.com>
Date: Wed, 23 Aug 2023 21:10:02 +0300
Subject: [PATCH] AK+Kernel: Introduce StdLib function to copy
 FixedStringBuffer to user

This new Kernel StdLib function will be used to copy contents of a
FixedStringBuffer with a null character to a user process.

The first user of this new function is the prctl option of
PR_GET_PROCESS_NAME which would copy a process name including a null
character to a user provided buffer.
---
 AK/FixedStringBuffer.h    |  7 ++++++-
 Kernel/Library/StdLib.h   | 11 +++++++++++
 Kernel/Syscalls/prctl.cpp |  6 ++----
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/AK/FixedStringBuffer.h b/AK/FixedStringBuffer.h
index ec34f0e4df..20e03a6d31 100644
--- a/AK/FixedStringBuffer.h
+++ b/AK/FixedStringBuffer.h
@@ -95,8 +95,13 @@ public:
         return m_storage.span();
     }
     StringView representable_view() const { return StringView(m_storage.data(), m_stored_length); }
+    Span<u8 const> span_view_ensuring_ending_null_char()
+    {
+        VERIFY(m_stored_length + 1 <= Size);
+        m_storage[m_stored_length] = '\0';
+        return Span<u8 const>(m_storage.data(), m_stored_length + 1);
+    }
 
-    size_t fixed_length() const { return Size; }
     size_t stored_length() const { return m_stored_length; }
 
     FixedStringBuffer()
diff --git a/Kernel/Library/StdLib.h b/Kernel/Library/StdLib.h
index c1aa94ff9f..9887400068 100644
--- a/Kernel/Library/StdLib.h
+++ b/Kernel/Library/StdLib.h
@@ -201,3 +201,14 @@ inline ErrorOr<T> copy_typed_from_user(Userspace<T*> user_data)
     TRY(copy_from_user(&data, user_data));
     return data;
 }
+
+template<size_t Size>
+ErrorOr<void> copy_fixed_string_buffer_including_null_char_to_user(Userspace<char*> dest, size_t buffer_size, FixedStringBuffer<Size> const& buffer)
+{
+    FixedStringBuffer<Size + 1> name_with_null_char {};
+    name_with_null_char.store_characters(buffer.representable_view());
+    if (name_with_null_char.stored_length() + 1 > buffer_size)
+        return ENAMETOOLONG;
+    auto name_with_null_char_view = name_with_null_char.span_view_ensuring_ending_null_char();
+    return copy_to_user(dest, name_with_null_char_view.data(), name_with_null_char_view.size());
+}
diff --git a/Kernel/Syscalls/prctl.cpp b/Kernel/Syscalls/prctl.cpp
index 8de0d7d72c..bbcb902b1e 100644
--- a/Kernel/Syscalls/prctl.cpp
+++ b/Kernel/Syscalls/prctl.cpp
@@ -66,10 +66,8 @@ ErrorOr<FlatPtr> Process::sys$prctl(int option, FlatPtr arg1, FlatPtr arg2)
             Userspace<char*> buffer = arg1;
             size_t buffer_size = static_cast<size_t>(arg2);
             TRY(m_name.with([&buffer, buffer_size](auto& name) -> ErrorOr<void> {
-                auto view = name.representable_view();
-                if (view.length() + 1 > buffer_size)
-                    return ENAMETOOLONG;
-                return copy_to_user(buffer, view.characters_without_null_termination(), view.length() + 1);
+                VERIFY(!name.representable_view().is_null());
+                return copy_fixed_string_buffer_including_null_char_to_user(buffer, buffer_size, name);
             }));
             return 0;
         }