[compiler-rt] c82f3ca - [scudo] Add StackDepot lock to enable/disable. (#79670)

[via llvm-commits]

Author: Evgenii Stepanov
Date: 2024-01-29T14:22:24-08:00
New Revision: c82f3caf5683a443692162d9fbebc31019af7ce8

URL: https://github.com/llvm/llvm-project/commit/c82f3caf5683a443692162d9fbebc31019af7ce8
DIFF: https://github.com/llvm/llvm-project/commit/c82f3caf5683a443692162d9fbebc31019af7ce8.diff

LOG: [scudo] Add StackDepot lock to enable/disable. (#79670)

Scudo grabs all allocator locks in a pthread_atfork before the fork, and releases them after. This allows malloc to be used in a fork child of a multithreaded process, which is expressly forbidden by the standard, but very widely used. For example, Android's init uses std::string after fork when spawning services in android::init::EnterNamespaces and other places.

Any lock that is necessary to serve an allocator call must be handled this way. Otherwise there is a possibility that the lock is held during the call to fork, which results in it being held forever in the child process, and the next operation that needs it deadlocks.

Added: 
    

Modified: 
    compiler-rt/lib/scudo/standalone/combined.h
    compiler-rt/lib/scudo/standalone/stack_depot.h

Removed: 
    


################################################################################
diff  --git a/compiler-rt/lib/scudo/standalone/combined.h b/compiler-rt/lib/scudo/standalone/combined.h
index 4624f83d142a0..80774073522a7 100644
--- a/compiler-rt/lib/scudo/standalone/combined.h
+++ b/compiler-rt/lib/scudo/standalone/combined.h
@@ -691,10 +691,12 @@ class Allocator {
     Quarantine.disable();
     Primary.disable();
     Secondary.disable();
+    Depot.disable();
   }
 
   void enable() NO_THREAD_SAFETY_ANALYSIS {
     initThreadMaybe();
+    Depot.enable();
     Secondary.enable();
     Primary.enable();
     Quarantine.enable();

diff  --git a/compiler-rt/lib/scudo/standalone/stack_depot.h b/compiler-rt/lib/scudo/standalone/stack_depot.h
index 12c35eb2a4f33..e887d1b43a7cf 100644
--- a/compiler-rt/lib/scudo/standalone/stack_depot.h
+++ b/compiler-rt/lib/scudo/standalone/stack_depot.h
@@ -136,6 +136,13 @@ class StackDepot {
   u64 operator[](uptr RingPos) const {
     return atomic_load_relaxed(&Ring[RingPos & RingMask]);
   }
+
+  // This is done for the purpose of fork safety in multithreaded programs and
+  // does not fully disable StackDepot. In particular, find() still works and
+  // only insert() is blocked.
+  void disable() NO_THREAD_SAFETY_ANALYSIS { RingEndMu.lock(); }
+
+  void enable() NO_THREAD_SAFETY_ANALYSIS { RingEndMu.unlock(); }
 };
 
 } // namespace scudo