[libc-commits] [libc] [libc] Add the implementation of the fdopen function (PR #94186)

[Xu Zhang via libc-commits]

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@@ -0,0 +1,94 @@
+//===-- Implementation of internal fcntl ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/OSUtil/fcntl.h"
+
+#include "hdr/fcntl_macros.h"
+#include "hdr/types/struct_f_owner_ex.h"
+#include "hdr/types/struct_flock.h"
+#include "hdr/types/struct_flock64.h"
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+#include "src/errno/libc_errno.h"
+
+#include <stdarg.h>
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace LIBC_NAMESPACE::internal {
+
+int fcntl(int fd, int cmd, void *arg) {
+  switch (cmd) {
+  case F_OFD_SETLKW: {
+    struct flock *flk = reinterpret_cast<struct flock *>(arg);
+    // convert the struct to a flock64
+    struct flock64 flk64;
+    flk64.l_type = flk->l_type;
+    flk64.l_whence = flk->l_whence;
+    flk64.l_start = flk->l_start;
+    flk64.l_len = flk->l_len;
+    flk64.l_pid = flk->l_pid;
+    // create a syscall
+    return LIBC_NAMESPACE::syscall_impl<int>(SYS_fcntl, fd, cmd, &flk64);
+  }
+  case F_OFD_GETLK:
+  case F_OFD_SETLK: {
+    struct flock *flk = reinterpret_cast<struct flock *>(arg);
+    // convert the struct to a flock64
+    struct flock64 flk64;
+    flk64.l_type = flk->l_type;
+    flk64.l_whence = flk->l_whence;
+    flk64.l_start = flk->l_start;
+    flk64.l_len = flk->l_len;
+    flk64.l_pid = flk->l_pid;
+    // create a syscall
+    int retVal = LIBC_NAMESPACE::syscall_impl<int>(SYS_fcntl, fd, cmd, &flk64);
+    // On failure, return
+    if (retVal == -1)
+      return -1;
+    // Check for overflow, i.e. the offsets are not the same when cast
+    // to off_t from off64_t.
+    if (static_cast<off_t>(flk64.l_len) != flk64.l_len ||
+        static_cast<off_t>(flk64.l_start) != flk64.l_start) {
+      libc_errno = EOVERFLOW;
+      return -1;
+    }
+    // Now copy back into flk, in case flk64 got modified
+    flk->l_type = flk64.l_type;
+    flk->l_whence = flk64.l_whence;
+    flk->l_start = flk64.l_start;
+    flk->l_len = flk64.l_len;
+    flk->l_pid = flk64.l_pid;
+    return retVal;
+  }
+  case F_GETOWN: {
+    struct f_owner_ex fex;
+    int retVal =
+        LIBC_NAMESPACE::syscall_impl<int>(SYS_fcntl, fd, F_GETOWN_EX, &fex);
+    if (retVal == -EINVAL)
+      return LIBC_NAMESPACE::syscall_impl<int>(SYS_fcntl, fd, cmd,
+                                               reinterpret_cast<void *>(arg));
+    if (static_cast<unsigned long>(retVal) <= -4096UL)
----------------
simonzgx wrote:

> I'm confused by this logic. If the return result is negative but malformed, should we return errors instead?

Frankly, I'm also puzzled by this code. I found the same code in [GLIBC](https://github.com/bminor/glibc/blob/master/sysdeps/unix/sysv/linux/fcntl.c#L92), yet I'm also uncertain as to why the GLIBC's authors chose this way. Perhaps we could set it aside for now, and I'll open a new issue to address it after I delve into it.

https://github.com/llvm/llvm-project/pull/94186