[libc] Make the bump pointer explicitly return null on buffer oveerrun

[jhuber6]

Summary

We use a simple bump ptr in the libc tests. If we run out of data we
can currently return other static memory and have weird failure cases.
We should fail more explicitly here by returning a null pointer instead.

[sivachandra]

libc/test/IntegrationTest/test.cpp

60

s/uint64_t/ptrdiff_t ?
s/memory_size/MEMORY_SIZE

60–61

s/16384/MEMORY_SIZE

69

s/uint64_t/ptrdiff_t?

libc/test/UnitTest/HermeticTestUtils.cpp

32–33

Ditto.

[jhuber6]

libc/test/IntegrationTest/test.cpp

69

The size is unsigned so we'll get warnings about the comparison. Since we know ptr >= memory we know that this will never be negative.

libc/test/IntegrationTest/test.cpp

 
 // Integration tests cannot use the SCUDO standalone allocator as SCUDO pulls
 // various other parts of the libc. Since SCUDO development does not use
 // LLVM libc build rules, it is very hard to keep track or pull all that SCUDO
 // requires. Hence, as a work around for this problem, we use a simple allocator
 // which just hands out continuous blocks from a statically allocated chunk of
 // memory.
 
-static uint8_t memory[16384];
+static constexpr uint64_t MEMORY_SIZE = 16384;
+static uint8_t memory[MEMORY_SIZE];
 static uint8_t *ptr = memory;
 
 extern "C" {
 
 void *malloc(size_t s) {
   void *mem = ptr;
   ptr += s;
-  return mem;
+  return static_cast<uint64_t>(ptr - memory) >= MEMORY_SIZE ? nullptr : mem;
 }
 
 void free(void *) {}
 
 void *realloc(void *ptr, size_t s) {
   free(ptr);
   return malloc(s);
 }
 
 // Integration tests are linked with -nostdlib. BFD linker expects
 // __dso_handle when -nostdlib is used.
 void *__dso_handle = nullptr;
 } // extern "C"

libc/test/UnitTest/HermeticTestUtils.cpp

 namespace {
 
 // Integration tests cannot use the SCUDO standalone allocator as SCUDO pulls
 // various other parts of the libc. Since SCUDO development does not use
 // LLVM libc build rules, it is very hard to keep track or pull all that SCUDO
 // requires. Hence, as a work around for this problem, we use a simple allocator
 // which just hands out continuous blocks from a statically allocated chunk of
 // memory.
-static uint8_t memory[16384];
+static constexpr uint64_t MEMORY_SIZE = 16384;
+static uint8_t memory[MEMORY_SIZE];
 static uint8_t *ptr = memory;
 
 } // anonymous namespace
 
 extern "C" {
 
 // Hermetic tests rely on the following memory functions. This is because the
 // compiler code generation can emit calls to them. We want to map the external

 
 constexpr uint64_t ALIGNMENT = alignof(uintptr_t);
 
 void *malloc(size_t s) {
   // Keep the bump pointer aligned on an eight byte boundary.
   s = ((s + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;
   void *mem = ptr;
   ptr += s;
-  return mem;
+  return static_cast<uint64_t>(ptr - memory) >= MEMORY_SIZE ? nullptr : mem;
 }
 
 void free(void *) {}
 
 void *realloc(void *ptr, size_t s) {
   free(ptr);
   return malloc(s);
 }