[compiler-rt] [profile] Implement a non-mmap path when reading profile files from a non-local filesystem (PR #131177)

[Hubert Tong via llvm-commits]

================
@@ -258,6 +267,120 @@ COMPILER_RT_VISIBILITY FILE *lprofOpenFileEx(const char *ProfileName) {
   return f;
 }
 
+// Return 1 (true) if the file descriptor Fd represents a file that is on a
+// local filesystem, otherwise return 0.
+static int is_local_filesystem(int Fd) {
+#if defined(_AIX)
+  struct statfs Vfs;
+  if (fstatfs(Fd, &Vfs) != 0) {
+    PROF_ERR("%s: fstatfs(%d) failed: %s\n", __func__, Fd, strerror(errno));
+    return 0;
+  }
+
+  int Ret;
+  size_t BufSize = 2048u;
+  char *Buf;
+  int Tries = 3;
+  while (Tries--) {
+    Buf = malloc(BufSize);
+    Ret = mntctl(MCTL_QUERY, BufSize, Buf);
+    if (Ret != 0)
+      break;
+    BufSize = *(unsigned int *)Buf;
+    free(Buf);
+  }
+
+  if (Ret != -1) {
+    // Look for the correct vmount entry.
+    char *CurObjPtr = Buf;
+    while (Ret--) {
+      struct vmount *Vp = (struct vmount *)CurObjPtr;
+      _Static_assert(sizeof(Vfs.f_fsid) == sizeof(Vp->vmt_fsid),
+                     "fsid length mismatch");
+      if (memcmp(&Vfs.f_fsid, &Vp->vmt_fsid, sizeof Vfs.f_fsid) == 0) {
+        int Answer = (Vp->vmt_flags & MNT_REMOTE) == 0;
+        free(Buf);
+        return Answer;
+      }
+      CurObjPtr += Vp->vmt_length;
+    }
+  }
+
+  free(Buf);
+  // There was an error in mntctl or vmount entry not found; "remote" is the
+  // conservative answer.
+#endif
+  return 0;
+}
+
+static int isMmapSafe(int Fd) {
+  if (getenv("LLVM_NO_MMAP")) // For testing purposes.
+    return 0;
+  (void)&is_local_filesystem; // a fake reference to satisfy -Wunused-function
+#ifdef _AIX
+  return is_local_filesystem(Fd);
+#else
+  return 1;
+#endif
+}
+
+COMPILER_RT_VISIBILITY void lprofGetFileContentBuffer(FILE *F, uint64_t Length,
+                                                      ManagedMemory *Buf) {
+  Buf->Status = MM_INVALID;
+  if (isMmapSafe(fileno(F))) {
+    Buf->Addr =
+        mmap(NULL, Length, PROT_READ, MAP_SHARED | MAP_FILE, fileno(F), 0);
+    if (Buf->Addr == MAP_FAILED)
+      PROF_ERR("%s: mmap failed: %s\n", __func__, strerror(errno))
+    else
+      Buf->Status = MM_MMAP;
+    return;
+  }
+
+  if (getenv("LLVM_PROFILE_VERBOSE"))
+    PROF_NOTE("%s\n", "Could not use mmap; using fread instead.");
+
+  void *Buffer = malloc(Length);
+  if (!Buffer) {
+    PROF_ERR("%s: malloc failed: %s\n", __func__, strerror(errno));
+    return;
+  }
+  if (ftell(F) != 0) {
+    PROF_ERR("%s: expecting ftell to return zero\n", __func__);
+    free(Buffer);
+    return;
+  }
+
+  // Read the entire file into memory.
+  size_t BytesRead = fread(Buffer, 1, Length, F);
+  if (BytesRead != (size_t)Length) {
+    PROF_ERR("%s: fread failed%s\n", __func__,
+             feof(F) ? ": end of file reached" : "");
+    free(Buffer);
+    return;
+  }
+
+  // Reading was successful, record the result in the Buf parameter.
+  Buf->Addr = Buffer;
+  Buf->Status = MM_MALLOC;
+}
+
+void lprofReleaseBuffer(ManagedMemory *Buf, size_t Length) {
+  switch (Buf->Status) {
+  case MM_MALLOC:
+    free(Buf->Addr);
+    break;
+  case MM_MMAP:
+    munmap(Buf->Addr, Length);
----------------
hubert-reinterpretcast wrote:

Uses of `munmap` being replaced in the PR cast to `void` (presumably to prevent a possibly platform-specific warning that the return value is unused):
```suggestion
    (void)munmap(Buf->Addr, Length);
```

https://github.com/llvm/llvm-project/pull/131177