[sanitizer] Add a function to gather random bytes

Summary:
AFAICT compiler-rt doesn't have a function that would return 'good' random
bytes to seed a PRNG. Currently, the `SizeClassAllocator64` uses addresses
returned by `mmap` to seed its PRNG, which is not ideal, and  
`SizeClassAllocator32` doesn't benefit from the entropy offered by its 64-bit
counterpart address space, so right now it has nothing. This function aims at
solving this, allowing to implement good 32-bit chunk randomization. Scudo also
has a function that does this for Cookie purposes, which would go away in a
later CL once this lands.

This function will try the `getrandom` syscall if available, and fallback to
`/dev/urandom` if not.

Unfortunately, I do not have a way to implement and test a Mac and Windows
version, so those are unimplemented as of now. Note that `kRandomShuffleChunks`
is only used on Linux for now.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: zturner, rnk, llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D34412

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@305922 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/sanitizer_common/sanitizer_mac.cc b/lib/sanitizer_common/sanitizer_mac.cc
index a788a0915..b48238106 100644
--- a/lib/sanitizer_common/sanitizer_mac.cc
+++ b/lib/sanitizer_common/sanitizer_mac.cc
@@ -923,6 +923,11 @@ void CheckNoDeepBind(const char *filename, int flag) {
   // Do nothing.
 }
 
+// FIXME: implement on this platform.
+bool GetRandom(void *buffer, uptr length) {
+  UNIMPLEMENTED();
+}
+
 }  // namespace __sanitizer
 
 #endif  // SANITIZER_MAC