util: add optional -fbounds-safety annotation for readFromFd

Makefile

@@ -43,6 +43,12 @@ ifdef DEBUG
 	CXXFLAGS += -g -ggdb -gdwarf-4
 endif
 
+# Optional Clang -fbounds-safety (OFF by default). Requires a toolchain with
+# ptrcheck.h / -fbounds-safety. Default builds leave NSJAIL_* macros inert.
+ifdef ENABLE_FBOUNDS_SAFETY
+	CXXFLAGS += -DNSJAIL_SUPPORT_FBOUNDS_SAFETY -fbounds-safety
+endif
+
 BIN = nsjail
 LIBS = kafel/libkafel.a
 TEST_BINS = tests/nstun_buffer_budget_test tests/nstun_policy_test tests/nstun_ip_test

bounds_safety.h

@@ -0,0 +1,44 @@
+/*
+ * bounds_safety.h — portability macros for optional -fbounds-safety
+ *
+ * Copyright 2026 Jeff Bindel <jeff@incrediblybased.co>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * When NSJAIL_SUPPORT_FBOUNDS_SAFETY is defined (typically via
+ * -DNSJAIL_SUPPORT_FBOUNDS_SAFETY and a Clang toolchain that implements
+ * -fbounds-safety), these macros expand to Clang bounds annotations.
+ * Otherwise they expand to nothing so default builds are unchanged.
+ *
+ * Pattern matches libwebp / libpng / lz4 inert-macro -fbounds-safety
+ * adoption: annotations are inert unless explicitly enabled.
+ */
+#ifndef NSJAIL_BOUNDS_SAFETY_H_
+#define NSJAIL_BOUNDS_SAFETY_H_
+
+#ifdef NSJAIL_SUPPORT_FBOUNDS_SAFETY
+
+#include <ptrcheck.h>
+/* Non-ABI-breaking sized-by annotations for buffer pointer parameters.
+ * Prefer NSJAIL_SIZED_BY when the companion argument is a byte capacity.
+ * Use *_OR_NULL when the pointer may be NULL while the size is zero.
+ */
+#define NSJAIL_SIZED_BY(n) __sized_by(n)
+#define NSJAIL_SIZED_BY_OR_NULL(n) __sized_by_or_null(n)
+#define NSJAIL_COUNTED_BY(n) __counted_by(n)
+#define NSJAIL_COUNTED_BY_OR_NULL(n) __counted_by_or_null(n)
+
+#else /* !NSJAIL_SUPPORT_FBOUNDS_SAFETY */
+
+#define NSJAIL_SIZED_BY(n)
+#define NSJAIL_SIZED_BY_OR_NULL(n)
+#define NSJAIL_COUNTED_BY(n)
+#define NSJAIL_COUNTED_BY_OR_NULL(n)
+
+#endif /* NSJAIL_SUPPORT_FBOUNDS_SAFETY */
+
+#endif /* NSJAIL_BOUNDS_SAFETY_H_ */

util.h

@@ -34,6 +34,7 @@
 #include <string>
 #include <vector>
 
+#include "bounds_safety.h"
 #include "nsjail.h"
 
 #define RETURN_ON_FAILURE(expr)                                                                    \
@@ -47,7 +48,7 @@
 
 namespace util {
 
-ssize_t readFromFd(int fd, void* buf, size_t len);
+ssize_t readFromFd(int fd, void* NSJAIL_SIZED_BY(len) buf, size_t len);
 ssize_t readFromFile(const char* fname, void* buf, size_t len);
 bool readFromFileToStr(const char* fname, std::string* str);
 bool writeToFd(int fd, const void* buf, size_t len);