[compiler-rt] [scudo] Only read urandom if getrandom syscall isn't available. (PR #161889)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Christopher Ferris (cferris1000)

<details>
<summary>Changes</summary>

If the getrandom system call is available, but the call returns an error, it could mean that the system doesn't have enough randomness to respond yet. Trying to read /dev/urandom will likely block and cause initialization to be stalled. Therefore, return false in this case and use the backup random data.

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Full diff: https://github.com/llvm/llvm-project/pull/161889.diff


1 Files Affected:

- (modified) compiler-rt/lib/scudo/standalone/linux.cpp (+6) 


``````````diff
diff --git a/compiler-rt/lib/scudo/standalone/linux.cpp b/compiler-rt/lib/scudo/standalone/linux.cpp
index 6cc8e0c786e06..57171edac1e9e 100644
--- a/compiler-rt/lib/scudo/standalone/linux.cpp
+++ b/compiler-rt/lib/scudo/standalone/linux.cpp
@@ -192,6 +192,12 @@ bool getRandom(void *Buffer, uptr Length, UNUSED bool Blocking) {
       syscall(SYS_getrandom, Buffer, Length, Blocking ? 0 : GRND_NONBLOCK);
   if (ReadBytes == static_cast<ssize_t>(Length))
     return true;
+  // If this system call is not implemented in the kernel, then we will try
+  // and use /dev/urandom. Otherwise, if the syscall fails, return false
+  // assuming that trying to read /dev/urandom will cause a delay waiting for
+  // the random data to be usable.
+  if (errno != ENOSYS)
+    return false;
 #endif // defined(SYS_getrandom)
   // Up to 256 bytes, a read off /dev/urandom will not be interrupted.
   // Blocking is moot here, O_NONBLOCK has no effect when opening /dev/urandom.

``````````

</details>


https://github.com/llvm/llvm-project/pull/161889