Claiming Bitcoin's Bug Bounty | Hacker News Reader

Claiming Bitcoin's Bug Bounty

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> If a mapping to be removed was private, any modifications made in this address range shall be discarded.

Changes to private mappings aren't supposed to be reflected in the underlying file, so this statement is a bit of a truism.

Changes to shared mappings, on the other hand, are supposed to be reflected in the file, and that's what this article is about.

Can you link to the man page you are reading, because this is not in the Debian page for mmap(2), which includes munmap.

Additionally, the statement you quote explicitly relates to private mappings. LevelDB uses shared mappings.

Deleted comment

It uses shared writable mappings to reflect writes in the file on disk, and the core purpose of msync is to synchronize the file with the memory map.

Your random quotes from man pages are out of context (and keep changing as you edit your posts).

Man, it's annoying when someone deletes comments, especially right through a thread!

The Darwin man page for munmap has more detail: https://developer.apple.com/library/mac/documentation/Darwin... See DIRTY PAGE HANDLING.

  If the mapping maps data from a file (MAP_SHARED), then the memory will eventually be written back to disk if it's dirty.  This will happen automatically at some point in the future (implementation dependent).

The XNU source may be useful: http://fxr.watson.org/fxr/ident?v=xnu-2050.18.24&im=excerpts...

Does this mean it's a bug in Darwin?

Has it been verified that the bug is fixed as in no further database corruption? I'm confused what you mean by this "sample" you got from Gavin.

Gavin posted a link to some corrupt LevelDB instances here: https://github.com/bitcoin/bitcoin/issues/2770 He also sent the same link to me via email.

I took a look at them, and figured out what in the write path could have gone wrong to create the bad behavior. It turned out to be in db/log_writer.cc and deps.

I cannot promise there are no other bugs within Bitcoin or LevelDB, but this resolves one issue. I'm interested in seeing it tested in the wide area, just to make sure.

Of course, if there are other bugs, I'm just as happy to debug and squash them.

I assume he meant one of the log/database files that contains the corruption.

That's what I initially thought, but then I missed the part where they've confirmed that this fixes the bug. Has this been tested?

People were reporting it only happens on some installs and takes up to five days to present itself. Considering that this was only released today it seems too premature to confirm that.

Whenever I fix a bug I wait for confirmation that the bug has actually been fixed before I submit the patch. Sometimes, especially when fixing race conditions that occur unpredictably, it takes a while to confirm that the bug has been fixed. Not that I'm skeptical - it seems like they're pretty confident that they've fixed it.

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