[sanitizer] Large range support in IsAccessibleMemoryRange

[vitalybuka]

The comment stated that it's slow, but likely it's a deadlock,
as write can be blocked.

Also we can't be sure that page_size * 10 is appropriate size.

Still most likely this is NFC, as the max size we use is 32,
and should fit in any buffer.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Vitaly Buka (vitalybuka)

Changes

The comment stated that it's slow, but likely it's a deadlock,
as write can be blocked.

Also we can't be sure that page_size * 10 is appropriate size.

Still most likely this is NFC, as the max size we use is 32,
and should fit in any buffer.

---

Full diff: https://github.com/llvm/llvm-project/pull/112665.diff

2 Files Affected:

• (modified) compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp (+34-18)
• (modified) compiler-rt/lib/sanitizer_common/tests/sanitizer_posix_test.cpp (+11)

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
index 9ffb36f812c45d..0f8fdd4487efb6 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
@@ -288,26 +288,42 @@ bool SignalContext::IsStackOverflow() const {
 
 #endif  // SANITIZER_GO
 
+static void SetNonBlock(int fd) {
+  int res = fcntl(fd, F_GETFL, 0);
+  CHECK(!internal_iserror(res, nullptr));
+
+  res |= O_NONBLOCK;
+  res = fcntl(fd, F_SETFL, res);
+  CHECK(!internal_iserror(res, nullptr));
+}
+
 bool IsAccessibleMemoryRange(uptr beg, uptr size) {
-  uptr page_size = GetPageSizeCached();
-  // Checking too large memory ranges is slow.
-  CHECK_LT(size, page_size * 10);
-  int sock_pair[2];
-  if (pipe(sock_pair))
-    return false;
-  uptr bytes_written =
-      internal_write(sock_pair[1], reinterpret_cast<void *>(beg), size);
-  int write_errno;
-  bool result;
-  if (internal_iserror(bytes_written, &write_errno)) {
-    CHECK_EQ(EFAULT, write_errno);
-    result = false;
-  } else {
-    result = (bytes_written == size);
+  for (uptr to_write = size; to_write;) {
+    // `read` from `sock_pair[0]` into a dummy buffer to free up the pipe buffer
+    // for more `write` is slower than just recreating a pipe.
+    int sock_pair[2];
+    if (pipe(sock_pair))
+      return false;
+
+    auto cleanup = at_scope_exit([&]() {
+      internal_close(sock_pair[0]);
+      internal_close(sock_pair[1]);
+    });
+
+    SetNonBlock(sock_pair[1]);
+
+    int write_errno;
+    uptr bytes_written =
+        internal_write(sock_pair[1], reinterpret_cast<char *>(beg), to_write);
+    if (internal_iserror(bytes_written, &write_errno)) {
+      CHECK_EQ(EFAULT, write_errno);
+      return false;
+    }
+    beg += bytes_written;
+    to_write -= bytes_written;
   }
-  internal_close(sock_pair[0]);
-  internal_close(sock_pair[1]);
-  return result;
+
+  return true;
 }
 
 void PlatformPrepareForSandboxing(void *args) {
diff --git a/compiler-rt/lib/sanitizer_common/tests/sanitizer_posix_test.cpp b/compiler-rt/lib/sanitizer_common/tests/sanitizer_posix_test.cpp
index bed19d15a8ec77..9feb22221f005e 100644
--- a/compiler-rt/lib/sanitizer_common/tests/sanitizer_posix_test.cpp
+++ b/compiler-rt/lib/sanitizer_common/tests/sanitizer_posix_test.cpp
@@ -82,6 +82,17 @@ TEST(SanitizerCommon, IsAccessibleMemoryRange) {
   munmap((void *)mem, 3 * page_size);
 }
 
+TEST(SanitizerCommon, IsAccessibleMemoryRangeLarge) {
+  const int size = GetPageSize() * 10000;
+
+  uptr mem = (uptr)mmap(0, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON,
+                        -1, 0);
+
+  EXPECT_TRUE(IsAccessibleMemoryRange(mem, size));
+
+  munmap((void *)mem, size);
+}
+
 }  // namespace __sanitizer
 
 #endif  // SANITIZER_POSIX

[fmayer]

FYI, If you want to optimize this on Linux you could probably also use sendfile or splice there. Wrong CL :)

[fmayer]

Thanks