Kernel+LibC+Tests: Implement pwritev(2)

While this isn't really POSIX, it's needed by the Zig port and was
simple enough to implement.

Kernel/API/Syscall.h

@@ -195,7 +195,7 @@ enum class NeedsBigProcessLock {
     S(utimensat, NeedsBigProcessLock::No)                   \
     S(waitid, NeedsBigProcessLock::Yes)                     \
     S(write, NeedsBigProcessLock::Yes)                      \
-    S(writev, NeedsBigProcessLock::Yes)                     \
+    S(pwritev, NeedsBigProcessLock::Yes)                    \
     S(yield, NeedsBigProcessLock::No)
 
 namespace Syscall {

Kernel/Process.h

@@ -317,7 +317,7 @@ public:
     ErrorOr<FlatPtr> sys$pread(int fd, Userspace<u8*>, size_t, Userspace<off_t const*>);
     ErrorOr<FlatPtr> sys$readv(int fd, Userspace<const struct iovec*> iov, int iov_count);
     ErrorOr<FlatPtr> sys$write(int fd, Userspace<u8 const*>, size_t);
-    ErrorOr<FlatPtr> sys$writev(int fd, Userspace<const struct iovec*> iov, int iov_count);
+    ErrorOr<FlatPtr> sys$pwritev(int fd, Userspace<const struct iovec*> iov, int iov_count, Userspace<off_t const*>);
     ErrorOr<FlatPtr> sys$fstat(int fd, Userspace<stat*>);
     ErrorOr<FlatPtr> sys$stat(Userspace<Syscall::SC_stat_params const*>);
     ErrorOr<FlatPtr> sys$lseek(int fd, Userspace<off_t*>, int whence);
@@ -602,7 +602,7 @@ private:
     void delete_perf_events_buffer();
 
     ErrorOr<void> do_exec(NonnullLockRefPtr<OpenFileDescription> main_program_description, NonnullOwnPtrVector<KString> arguments, NonnullOwnPtrVector<KString> environment, LockRefPtr<OpenFileDescription> interpreter_description, Thread*& new_main_thread, u32& prev_flags, const ElfW(Ehdr) & main_program_header);
-    ErrorOr<FlatPtr> do_write(OpenFileDescription&, UserOrKernelBuffer const&, size_t);
+    ErrorOr<FlatPtr> do_write(OpenFileDescription&, UserOrKernelBuffer const&, size_t, Optional<off_t> = {});
 
     ErrorOr<FlatPtr> do_statvfs(FileSystem const& path, Custody const*, statvfs* buf);
 

Kernel/Syscalls/write.cpp

@@ -11,7 +11,9 @@
 
 namespace Kernel {
 
-ErrorOr<FlatPtr> Process::sys$writev(int fd, Userspace<const struct iovec*> iov, int iov_count)
+// NOTE: The offset is passed by pointer because off_t is 64bit,
+// hence it can't be passed by register on 32bit platforms.
+ErrorOr<FlatPtr> Process::sys$pwritev(int fd, Userspace<const struct iovec*> iov, int iov_count, Userspace<off_t const*> userspace_offset)
 {
     VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this);
     TRY(require_promise(Pledge::stdio));
@@ -31,26 +33,32 @@ ErrorOr<FlatPtr> Process::sys$writev(int fd, Userspace<const struct iovec*> iov,
             return EINVAL;
     }
 
+    // NOTE: Negative offset means "operate like writev" which seeks the file.
+    auto base_offset = TRY(copy_typed_from_user(userspace_offset));
     auto description = TRY(open_file_description(fd));
     if (!description->is_writable())
         return EBADF;
+    if (base_offset >= 0 && !description->file().is_seekable())
+        return EINVAL;
 
     int nwritten = 0;
+    off_t current_offset = base_offset;
     for (auto& vec : vecs) {
         auto buffer = TRY(UserOrKernelBuffer::for_user_buffer((u8*)vec.iov_base, vec.iov_len));
-        auto result = do_write(*description, buffer, vec.iov_len);
+        auto result = do_write(*description, buffer, vec.iov_len, base_offset >= 0 ? current_offset : Optional<off_t> {});
         if (result.is_error()) {
             if (nwritten == 0)
                 return result.release_error();
             return nwritten;
         }
         nwritten += result.value();
+        current_offset += result.value();
     }
 
     return nwritten;
 }
 
-ErrorOr<FlatPtr> Process::do_write(OpenFileDescription& description, UserOrKernelBuffer const& data, size_t data_size)
+ErrorOr<FlatPtr> Process::do_write(OpenFileDescription& description, UserOrKernelBuffer const& data, size_t data_size, Optional<off_t> offset)
 {
     size_t total_nwritten = 0;
 
@@ -72,7 +80,9 @@ ErrorOr<FlatPtr> Process::do_write(OpenFileDescription& description, UserOrKerne
             }
             // TODO: handle exceptions in unblock_flags
         }
-        auto nwritten_or_error = description.write(data.offset(total_nwritten), data_size - total_nwritten);
+        auto nwritten_or_error = offset.has_value()
+            ? description.write(offset.value() + total_nwritten, data.offset(total_nwritten), data_size - total_nwritten)
+            : description.write(data.offset(total_nwritten), data_size - total_nwritten);
         if (nwritten_or_error.is_error()) {
             if (total_nwritten > 0)
                 return total_nwritten;

Tests/Kernel/fuzz-syscalls.cpp

@@ -44,7 +44,7 @@ static bool is_bad_idea(int fn, size_t const* direct_sc_args, size_t const* fake
         // FIXME: Known bug: https://github.com/SerenityOS/serenity/issues/5328
         return direct_sc_args[0] == 1;
     case SC_write:
-    case SC_writev:
+    case SC_pwritev:
         // FIXME: Known bug: https://github.com/SerenityOS/serenity/issues/5328
         return direct_sc_args[0] == 0;
     case SC_pledge:

Userland/Libraries/LibC/sys/uio.cpp

@@ -13,10 +13,7 @@ extern "C" {
 
 ssize_t writev(int fd, const struct iovec* iov, int iov_count)
 {
-    __pthread_maybe_cancel();
-
-    int rc = syscall(SC_writev, fd, iov, iov_count);
-    __RETURN_WITH_ERRNO(rc, rc, -1);
+    return pwritev(fd, iov, iov_count, -1);
 }
 
 ssize_t readv(int fd, const struct iovec* iov, int iov_count)
@@ -26,4 +23,12 @@ ssize_t readv(int fd, const struct iovec* iov, int iov_count)
     int rc = syscall(SC_readv, fd, iov, iov_count);
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
+
+ssize_t pwritev(int fd, struct iovec const* iov, int iov_count, off_t offset)
+{
+    __pthread_maybe_cancel();
+
+    int rc = syscall(SC_pwritev, fd, iov, iov_count, &offset);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
 }

Userland/Libraries/LibC/sys/uio.h

@@ -13,5 +13,6 @@ __BEGIN_DECLS
 
 ssize_t writev(int fd, const struct iovec*, int iov_count);
 ssize_t readv(int fd, const struct iovec*, int iov_count);
+ssize_t pwritev(int fd, const struct iovec*, int iov_count, off_t);
 
 __END_DECLS