[libc-commits] [libc] [libc] Add tmpnam implementation (PR #204901)

[via libc-commits]

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+//===----------------------------------------------------------------------===//
+//
+// 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
+/// Declaration of tmpnam, a POSIX function that generate a string that is a
+/// valid pathname that does not name an existing file.
+/// See:
+/// https://pubs.opengroup.org/onlinepubs/9799919799/functions/tmpnam.html
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/stdio/tmpnam.h"
+#include "hdr/errno_macros.h"
+#include "hdr/stdio_macros.h"
+#include "hdr/unistd_macros.h"
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/CPP/string_view.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/access.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/getrandom.h"
+#include "src/__support/macros/config.h"
+#include "src/string/memory_utils/inline_memcpy.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+static char tmpbuf[L_tmpnam];
+static cpp::Atomic<size_t> tmpnam_budget = TMP_MAX;
+
+// Partially thread-safe:
+// - When null is handed it is not thread-safe.
+// - We do some work to ensure that cases where we need to use tmpnam_budget
+//   that we lock around it.
+LLVM_LIBC_FUNCTION(char *, tmpnam, (char *s)) {
+  if (s == nullptr)
+    s = tmpbuf;
+
+  // here if the s is null then use tmpbuf and if sizeof
+  // POSIX portable filename character set, sorted by ASCII value.
+  // See
+  // https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap03.html#tag_03_265
+  const char charset[] = "-.0123456789"
+                         "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                         "_"
+                         "abcdefghijklmnopqrstuvwxyz";
+
+  // We want to construct: P_tmpdir / <14 random chars> \0
+  // P_tmpdir is "/tmp" (length 4).
+  // Total length must be L_tmpnam (20).
+  // /tmp/ is 5 chars.
+  // Random suffix is 14 chars.
+  // Null terminator is 1 char.
+  // Total: 5 + 14 + 1 = 20.
+  constexpr cpp::string_view PREFIX = P_tmpdir "/";
+  static_assert(PREFIX.size() + 14 + 1 == L_tmpnam, "L_tmpnam mismatch");
+  inline_memcpy(s, PREFIX.data(), PREFIX.size());
+  constexpr size_t PREFIX_SIZE = PREFIX.size();
+  constexpr size_t SUFFIX_SIZE = 14;
+
+  bool is_unique = false;
+  while (!is_unique) {
+    size_t curr_budget = tmpnam_budget.load(cpp::MemoryOrder::RELAXED);
+
+    do {
+      if (curr_budget == 0)
+        break;
+    } while (
+        !tmpnam_budget.compare_exchange_strong(curr_budget, curr_budget - 1));
+
+    if (curr_budget == 0)
+      break;
+
+    uint8_t rand_bytes[L_tmpnam];
+    auto ret = linux_syscalls::getrandom(rand_bytes, SUFFIX_SIZE, 0);
+    if (!ret.has_value()) {
+      /* return nullptr when getrandom fails but consume tmpnam budget */
+      return nullptr;
+    }
+
+    for (size_t i = 0; i < SUFFIX_SIZE; i++) {
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shubhe25p wrote:

Wow that's interesting, thanks for highlighting the issue and performance fix looks great as well. Do we need to write that we no longer use the full POSIX portable filename set somewhere in there?

https://github.com/llvm/llvm-project/pull/204901