From d96a44a738b89a97ba6cb6b10324831f3b23c01e Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Wed, 3 Mar 2021 20:21:24 +0100
Subject: [PATCH] Kernel: Avoid transient kmalloc heap allocations in
 sys$select()

Dynamic Vector allocations in sys$select() were showing up in the
full-system profile and since there will never be more than FD_SETSIZE
file descriptors to worry about, we can confidently add enough inline
capacity to this Vector that it never has to kmalloc.

To compensate for the increased stack usage, reduce the size of the
FDInfo struct while we're here. :^)
---
 Kernel/Syscalls/select.cpp | 2 +-
 Kernel/Thread.h            | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/Kernel/Syscalls/select.cpp b/Kernel/Syscalls/select.cpp
index 0b7014eb8b..def1cc2c95 100644
--- a/Kernel/Syscalls/select.cpp
+++ b/Kernel/Syscalls/select.cpp
@@ -74,7 +74,7 @@ KResultOr<int> Process::sys$select(Userspace<const Syscall::SC_select_params*> u
         return EFAULT;
 
     Thread::SelectBlocker::FDVector fds_info;
-    Vector<int> fds;
+    Vector<int, FD_SETSIZE> fds;
     for (int fd = 0; fd < params.nfds; fd++) {
         u32 block_flags = (u32)Thread::FileBlocker::BlockFlags::None;
         if (params.readfds && FD_ISSET(fd, &fds_read))
diff --git a/Kernel/Thread.h b/Kernel/Thread.h
index a40ee39748..2fd7f2ea18 100644
--- a/Kernel/Thread.h
+++ b/Kernel/Thread.h
@@ -530,7 +530,7 @@ public:
 
     class FileBlocker : public Blocker {
     public:
-        enum class BlockFlags : u32 {
+        enum class BlockFlags : u16 {
             None = 0,
 
             Read = 1 << 0,
@@ -632,7 +632,7 @@ public:
     public:
         struct FDInfo {
             NonnullRefPtr<FileDescription> description;
-            BlockFlags block_flags;
+            BlockFlags block_flags { BlockFlags::None };
             BlockFlags unblocked_flags { BlockFlags::None };
         };