https://mjtsai.com/blog/2022/09/20/zeroing-freed-memory/

Tuesday, September 20, 2022 [Tweets] [Favorites]

Zeroing Freed Memory

Apple:

    The system memory allocator free operation zeroes out all
    deallocated blocks in iOS 16.1 beta or later. Invalid accesses to
    free memory might result in new crashes or corruption, including
    NULL-pointer dereferences and non-zero memory being returned from
    calloc.

There's a corresponding change in macOS Ventura.

David Smith:

    malloc folks spent a while optimizing to compensate [for the
    performance regression]

mvb:

    There's also a performance improvement, because it makes the
    memory compressor work better

Marco Arment:

    I love this, frankly, but I bet it's going to uncover a good deal
    of bugs and crashes that worked until now because of luck.

Hopefully you aren't relying on any abandoned software.

Update (2022-09-22): See also: Hacker News.

David Anson:

    Any reason Apple couldn't restrict the new memory zeroing
    behavior to newly-compiled binaries to avoid the risk of breaking
    legacy software?

I was wondering about that, too. Usually, though not always,
potentially breaking changes like this are called out in the release
notes as taking effect when you compile with a new SDK. That was not
the case here, so I assume there's no linked-on-or-after check.

iOS iOS 16 Mac macOS 13 Ventura Memory Management Optimization
Programming Security

2 Comments

2 Comments

Sebby
September 20, 2022 7:56 PM

Good!

Marcos
September 20, 2022 8:39 PM

> Hopefully you aren't relying on any abandoned software.

Can't they make this change only for software linked against the
latest SDK?

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