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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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    1. 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 …

      4 replies 24 retweets 40 likes
      Rich Felker‏ @RichFelker 12 Oct 2016

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

      12:58 PM - 12 Oct 2016
      1 reply 0 retweets 0 likes
        1. New conversation
        2. CopperheadOS‏ @CopperheadOS 12 Oct 2016
          Replying to @RichFelker

          Only Facebook's internal jemalloc uses MAP_UNINITIALIZED.

          1 reply 0 retweets 0 likes
        3. 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
        4. 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
        5. 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
        6. 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
        7. End of conversation

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