Dangling pointer detector.

Dangling pointers are not a problem unless they are dereferenced and used. However, they are a source of UaF bugs and highly discouraged unless you are 100% confident that they are never dereferenced after the pointed-to objects are freed.

See also the guide: [how to fix dangling pointers. docs/dangling_ptr.md

Behind build flags, Chrome implements a dangling pointer detector. It causes Chrome to crash, whenever a raw_ptr becomes dangling:

raw_ptr<T> ptr_never_dangling;

On the other hand, we cannot simply ban all the usage of dangling pointers because there are valid use cases. The DisableDanglingPtrDetection option can be used to annotate “intentional-and-safe” dangling pointers. It is meant to be used as a last resort, only if there is no better way to re-architecture the code.

raw_ptr<T, DisableDanglingPtrDetection> ptr_may_dangle;

The DanglingUntriaged has been used to annotate pre-existing dangling pointers in Chrome:

raw_ptr<T, DanglingUntriaged> ptr_dangling_mysteriously;

Contrary to DisableDanglingPtrDetection, we don't know yet why it dangles. It is meant to be either refactored to avoid dangling, or turned into “DisableDanglingPtrDetection” with a comment explaining what happens.

How to check for dangling pointers?

It is gated behind both build and runtime flags:

Build flags

gn args ./out/dangling/

use_goma = true
is_debug = false  # Important! (*)
dcheck_always_on = true
enable_backup_ref_ptr_support = true  # true by default on most platforms
enable_dangling_raw_ptr_checks = true

(*) We want to emphasize that is_debug = false is important. It is a common mistake to set it to true, which in turn turns on component builds, which disables PartitionAlloc-Everywhere. enable_backup_ref_ptr_support = true can't be used without PartitionAlloc-Everywhere, and is silently set to false.

Runtime flags

./out/dangling/content_shell \
   --enable-features=PartitionAllocBackupRefPtr,PartitionAllocDanglingPtr

By default, Chrome will crash on the first dangling raw_ptr detected.

Runtime flags options:

Crash (default)

--enable-features=PartitionAllocBackupRefPtr,PartitionAllocDanglingPtr:mode/crash

Record a list of signatures

Example usage:

./out/dangling/content_shell \
   --enable-features=PartitionAllocBackupRefPtr,PartitionAllocDanglingPtr:mode/log_signature \
   |& tee output

The logs can be filtered and transformed into a tab separated table:

cat output \
 | grep "DanglingSignature" \
 | cut -f2,3 \
 | sort \
 | uniq -c \
 | sed -E 's/^ *//; s/ /\t/' \
 | sort -rn

This is used to list issues and track progresses.