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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    Rich Felker‏ @RichFelker 12 Oct 2016

    TIL someone added a Linux kernel "feature" to let any process in same cgroup steal contents of freed memory. https://lkml.org/lkml/2012/1/18/308 …

    12:57 PM - 12 Oct 2016
    • 24 Retweets
    • 40 Likes
    • Charles Goodier jakob Jed Davis 🏳️‍🌈 Jeremi M Gosney Daniel Dettlaff Eoin Brazil *mut "invisible chaotic self-modifying biomachine" Pangur Bán 0xAX
    4 replies 24 retweets 40 likes
      1. New conversation
      2. Rich Felker‏ @RichFelker 12 Oct 2016
        Replying to @RichFelker

        It only works with a malloc that uses MAP_UNINITIALIZED and MADV_DONTNEED (maybe jemalloc?), but presumably they use this...

        1 reply 0 retweets 0 likes
      3. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @RichFelker

        Only Facebook's internal jemalloc uses MAP_UNINITIALIZED.

        1 reply 0 retweets 0 likes
      4. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @RichFelker

        Note the patch is from Facebook. MADV_FREE landed upstream now though. Shouldn't have nearly as much need for this stuff now.

        1 reply 0 retweets 1 like
      5. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @RichFelker

        MADV_FREE doesn't drop any pages. It only marks the range as reclaimable. Then drops them when under memory pressure.

        1 reply 0 retweets 2 likes
      6. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @RichFelker

        It relies on dirty bit to know if the pages were written to. Means uninit data can be odd even without optimizing compilers.

        1 reply 0 retweets 0 likes
      7. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @RichFelker

        i.e. uninitialized malloc memory can start as non-zero and then become zero, if malloc marked that memory with MADV_FREE.

        0 replies 0 retweets 1 like
      8. End of conversation
      1. New conversation
      2. Էդգար/Эдгар/إدكار‏ @EdgarArout 12 Oct 2016
        Replying to @RichFelker

        what is a use case for this ?

        1 reply 0 retweets 0 likes
      3. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @EdgarArout @RichFelker

        Improving performance by reducing page zeroing. MADV_FREE should negate 99.9% of the desire to want these things.

        1 reply 0 retweets 3 likes
      4. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @EdgarArout @RichFelker

        A nice thing about MADV_FREE is that even if the memory is handed out again, unused ranges can still be dropped.

        1 reply 0 retweets 1 like
      5. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @EdgarArout @RichFelker

        It's pretty much designed for malloc. There's still a niche for a full range granularity equivalent with undo op.

        1 reply 0 retweets 1 like
      6. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @EdgarArout @RichFelker

        Android has something close to that via ashmem. Can unpin a range of ashmem pages, then try pin them back in memory.

        1 reply 0 retweets 1 like
      7. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @EdgarArout @RichFelker

        Chromium and Firefox have to use more memory or sacrifice performance on desktop Linux vs. Windows, Android, etc.

        0 replies 0 retweets 1 like
      8. End of conversation
      1. New conversation
      2. whitequark‏ @whitequark 12 Oct 2016
        Replying to @RichFelker @cynicalsecurity

        is that actually in the kernel? or just a random patch no one committed?

        1 reply 0 retweets 0 likes
      3. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @whitequark @RichFelker @cynicalsecurity

        MAP_UNINITIALIZED is in the kernel. This is likely something FB uses with internal jemalloc / HHVM.

        1 reply 0 retweets 0 likes
      4. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @whitequark and

        CopperheadOS Retweeted CopperheadOS

        See https://twitter.com/CopperheadOS/status/786296202438467585 … though. MADV_FREE is quite new so they probably didn't switch yet.

        CopperheadOS added,

        CopperheadOS @CopperheadOS
        Replying to @EdgarArout @RichFelker
        Improving performance by reducing page zeroing. MADV_FREE should negate 99.9% of the desire to want these things.
        1 reply 0 retweets 0 likes
      5. CopperheadOS‏ @CopperheadOS 12 Oct 2016
        Replying to @CopperheadOS @whitequark and

        See https://github.com/torvalds/linux/commit/854e9ed09dedf0c19ac8640e91bcc74bc3f9e5c9 …. Commit message mentions Facebook's MAP_UNINITIALIZED usage / exts.

        0 replies 0 retweets 3 likes
      6. End of conversation
      1. Shawn Webb‏ @lattera 12 Oct 2016
        Replying to @RichFelker

        and that feature still exists today ;)

        0 replies 0 retweets 0 likes
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