From e1093c3403dd86475dbe73c1f1ecd028de563898 Mon Sep 17 00:00:00 2001
From: Rodrigo Tobar <rtobarc@gmail.com>
Date: Tue, 12 Oct 2021 21:13:28 +0800
Subject: [PATCH] Kernel: Implement pread syscall

The OpenFileDescription class already offers the necessary functionlity,
so implementing this was only a matter of following the structure for
`read` while handling the additional `offset` argument.
---
 Kernel/API/Syscall.h     |  1 +
 Kernel/Process.h         |  1 +
 Kernel/Syscalls/read.cpp | 21 +++++++++++++++++++++
 3 files changed, 23 insertions(+)

diff --git a/Kernel/API/Syscall.h b/Kernel/API/Syscall.h
index b2d463def2..c321b4b7f9 100644
--- a/Kernel/API/Syscall.h
+++ b/Kernel/API/Syscall.h
@@ -140,6 +140,7 @@ enum class NeedsBigProcessLock {
     S(ptsname, NeedsBigProcessLock::Yes)                    \
     S(purge, NeedsBigProcessLock::Yes)                      \
     S(read, NeedsBigProcessLock::Yes)                       \
+    S(pread, NeedsBigProcessLock::Yes)                      \
     S(readlink, NeedsBigProcessLock::Yes)                   \
     S(readv, NeedsBigProcessLock::Yes)                      \
     S(realpath, NeedsBigProcessLock::Yes)                   \
diff --git a/Kernel/Process.h b/Kernel/Process.h
index 95376f3b12..2f8cceb1fa 100644
--- a/Kernel/Process.h
+++ b/Kernel/Process.h
@@ -294,6 +294,7 @@ public:
     KResultOr<FlatPtr> sys$open(Userspace<const Syscall::SC_open_params*>);
     KResultOr<FlatPtr> sys$close(int fd);
     KResultOr<FlatPtr> sys$read(int fd, Userspace<u8*>, size_t);
+    KResultOr<FlatPtr> sys$pread(int fd, Userspace<u8*>, size_t, off_t);
     KResultOr<FlatPtr> sys$readv(int fd, Userspace<const struct iovec*> iov, int iov_count);
     KResultOr<FlatPtr> sys$write(int fd, Userspace<const u8*>, size_t);
     KResultOr<FlatPtr> sys$writev(int fd, Userspace<const struct iovec*> iov, int iov_count);
diff --git a/Kernel/Syscalls/read.cpp b/Kernel/Syscalls/read.cpp
index 80dbb114f3..444c3daa02 100644
--- a/Kernel/Syscalls/read.cpp
+++ b/Kernel/Syscalls/read.cpp
@@ -91,4 +91,25 @@ KResultOr<FlatPtr> Process::sys$read(int fd, Userspace<u8*> buffer, size_t size)
     return TRY(description->read(user_buffer.value(), size));
 }
 
+KResultOr<FlatPtr> Process::sys$pread(int fd, Userspace<u8*> buffer, size_t size, off_t offset)
+{
+    VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this)
+    REQUIRE_PROMISE(stdio);
+    if (size == 0)
+        return 0;
+    if (size > NumericLimits<ssize_t>::max())
+        return EINVAL;
+    if (offset < 0)
+        return EINVAL;
+    dbgln_if(IO_DEBUG, "sys$pread({}, {}, {}, {})", fd, buffer.ptr(), size, offset);
+    auto description = TRY(open_readable_file_description(fds(), fd));
+    if (!description->file().is_seekable())
+        return EINVAL;
+    TRY(check_blocked_read(description));
+    auto user_buffer = UserOrKernelBuffer::for_user_buffer(buffer, size);
+    if (!user_buffer.has_value())
+        return EFAULT;
+    return TRY(description->read(user_buffer.value(), offset, size));
+}
+
 }