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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. majek04‏ @majek04 Feb 28

      Today I learned - getpid() in glibc used to cache the result (pid). This was removed in recent glibc because it was "not reliable". What? How can getpid value change over time???? https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal …

      5 replies 10 retweets 28 likes
      Rich Felker‏ @RichFelker Feb 28
      Replying to @majek04 @jessfraz

      fork. The fork function itself updated the cached value but vfork, clone, manual fork syscall, etc. could all be problematic.

      3:40 PM - 28 Feb 2018
      • 1 Retweet
      • 7 Likes
      • Wilfried Goesgens Mischan Toosarani-Hausberger jessie frazelle 👩🏼‍🚀 Jed Davis 🏳️‍🌈 Jonathan Cast Dougall the network is just an abstraction layer
      3 replies 1 retweet 7 likes
        1. New conversation
        2. the network is just an abstraction layer‏ @__dotblake Feb 28
          Replying to @RichFelker @majek04 @jessfraz

          thanks Rich. what's .@musllibc's behavior in this regard? http://www.etalabs.net/compare_libcs.html …

          1 reply 0 retweets 1 like
        3. Rich Felker‏ @RichFelker Feb 28
          Replying to @__dotblake @majek04 and

          We don't cache pid and mask all signals between getpid & use of the result. But tid is cached in tcb because otherwise locks are hideously slow.

          1 reply 1 retweet 2 likes
        4. Rich Felker‏ @RichFelker Feb 28
          Replying to @RichFelker @__dotblake and

          Difference is that most functions that use tid aren't AS-safe, meaning use in contexts where weird forkings could have invalidated them is already UB.

          1 reply 0 retweets 2 likes
        5. Rich Felker‏ @RichFelker Feb 28
          Replying to @RichFelker @__dotblake and

          For the few that are AS-safe (raise comes to mind) we mask signals and gettid.

          0 replies 0 retweets 2 likes
        6. End of conversation
        1. New conversation
        2. Jed Davis  🏳️‍🌈‏ @xlerb Feb 28
          Replying to @RichFelker @majek04 @jessfraz

          glibc's clone() function does update the pid/tid caches. This is why Chromium ≥43 and Firefox ≥60 (I hope) use it for sandboxing, even though it insists on calling a function on a new stack & needs a longjmp trick to recover the underlying syscall's fork()-like behavior.

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Feb 28
          Replying to @xlerb @majek04 @jessfraz

          I don't see why they care about the new function (vs returns-twice) aspect - if you're sandboxing without calling execve after forking, you're not really sandboxing because you leaked all the parent's data to the child...

          1 reply 0 retweets 0 likes
        4. Jed Davis  🏳️‍🌈‏ @xlerb Feb 28
          Replying to @RichFelker @majek04 @jessfraz

          Content/renderer processes are exec'ed, but there's also the CLONE_FS'ed chroot helper. I suppose that could all be redone with callbacks, but the longjmp hack also works. My point was mostly just that, if not for pid/tid caching, we'd just syscall(__NR_clone, ...).

          0 replies 0 retweets 0 likes
        5. End of conversation
        1. Mako‏ @makomk Mar 1
          Replying to @RichFelker @majek04 @jessfraz

          Yup. I encountered some hilarity with this when running code under QEMU's user-mode emulation and forking the emulator process; if I remember correctly the emulated glibc didn't update its cached value and got very confused.

          0 replies 0 retweets 0 likes
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