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marcan42's profile
Hector Martin
Hector Martin
Hector Martin
@marcan42

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Hector Martin

@marcan42

If it ain't broke, I'll fix it! I'm porting Linux to Apple Silicon Macs at @AsahiLinux. http://patreon.com/marcan  | http://github.com/sponsors/marcan 

Tokyo, Japan
marcan.st
Joined May 2009

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    1. Hector Martin‏ @marcan42 7 Feb 2019
      • Report Tweet
      • Report NetzDG Violation

      Today in POSIX surprises: `ln -f` is not *quite* atomic due to the combined semantics of link() and rename(); it racily uses a temp file and if the stars align might unlink() an identially-named temp file created by another process.

      6 replies 59 retweets 193 likes
    2. Donald “chronos” King  🏳️‍🌈‏ @dlkingauthor 7 Feb 2019
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      Replying to @marcan42

      I've been repeatedly surprised by the number of colleagues who believe that link(2) and/or symlink(2) will atomically overwrite an existing file, without needing to unlink(2) the target and thus open the attendant race condition.

      1 reply 0 retweets 1 like
    3. Hector Martin‏ @marcan42 7 Feb 2019
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      Replying to @dlkingauthor

      Well you can rename() over the target, and that works perfectly well for symlinks. The problem is that rename() has a stupidly defined corner case where it returns *successfully* and yet *does nothing* when both sides are the same inode. And that opens up another race.

      2 replies 1 retweet 6 likes
    4. Jann Horn‏ @tehjh 8 Feb 2019
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      Replying to @marcan42 @dlkingauthor

      oh, yuck. on modern linux, you could probably work around that by using renameat2() with RENAME_EXCHANGE and then always unlinking the source afterwards?

      1 reply 0 retweets 3 likes
    5. Hector Martin‏ @marcan42 8 Feb 2019
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      Replying to @tehjh @dlkingauthor

      Ah, I was looking for something like that. Yeah, that ought to work. That might actually solve the (unrelated) use case I was trying to solve when investigating this... hmm.

      1 reply 0 retweets 1 like
    6. Hector Martin‏ @marcan42 8 Feb 2019
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      Replying to @marcan42 @tehjh @dlkingauthor

      (I need to atomically exchange two hard links to the same inode or atomically overwrite a hard link with an identical hard link... if that sounds pointless, that's because it is in POSIX terms... but it isn't on CephFS due to an implementation detail!)

      1 reply 0 retweets 2 likes
    7. Jann Horn‏ @tehjh 8 Feb 2019
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      Replying to @marcan42 @dlkingauthor

      nope, vfs_rename() bails out immediately if source==target

      1 reply 0 retweets 0 likes
      Hector Martin‏ @marcan42 8 Feb 2019
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      Replying to @tehjh @dlkingauthor

      Meh. Wonder if libcephfs does the same check of if it'll fly there. My current best plan is link(src, tmp); symlink(tmp, tmp2); rename(tmp2, src); stat(other_hardlink); rename(tmp, src);. Involves a temporary symlink but should keep src accessible while doing what I want.

      8:22 AM - 8 Feb 2019
      • 1 Retweet
      • 1 Like
      • ⚖/MortoOnTech☮
      0 replies 1 retweet 1 like

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