[libc-commits] [libc] [libc][fcntl] Implement posix_fadvise function. (PR #220790)

[Pavel Labath via libc-commits]

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@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Linux implementation of posix_fadvise.
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/fcntl/posix_fadvise.h"
+
+#include "hdr/stdint_proxy.h"
+#include "hdr/types/off_t.h"
+#include "src/__support/CPP/bit.h"
+#include "src/__support/OSUtil/syscall.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+
+#include <sys/syscall.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, posix_fadvise,
+                   (int fd, off_t offset, off_t len, int advice)) {
+  int ret;
+  if constexpr (sizeof(long) == sizeof(uint32_t) &&
+                sizeof(off_t) == sizeof(uint64_t)) {
+    uint64_t offset_bits = cpp::bit_cast<uint64_t>(offset);
+    long offset_low = static_cast<long>(offset_bits & UINT32_MAX);
+    long offset_high = static_cast<long>(offset_bits >> 32);
+
+#if defined(SYS_fadvise64_64) || defined(SYS_arm_fadvise64_64)
+    uint64_t len_bits = cpp::bit_cast<uint64_t>(len);
+    long len_low = static_cast<long>(len_bits & UINT32_MAX);
+    long len_high = static_cast<long>(len_bits >> 32);
+#endif
+
+#if defined(SYS_fadvise64_64)
+    ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_fadvise64_64, fd, offset_low,
+                                            offset_high, len_low, len_high,
+                                            advice);
+#elif defined(SYS_arm_fadvise64_64)
+    ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_arm_fadvise64_64, fd, advice,
+                                            offset_low, offset_high, len_low,
+                                            len_high);
+#elif defined(SYS_fadvise64)
+    ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_fadvise64, fd, offset_low,
+                                            offset_high,
+                                            static_cast<size_t>(len), advice);
----------------
labath wrote:

This should check that high bits of `len` are zero before casting, so that we don't silently turn user's `1<<32 + 47` into a `47`. See e.g. src/__support/OSUtil/linux/syscall_wrappers/setitimer.h.

https://github.com/llvm/llvm-project/pull/220790