Suppressing SIGBUS signals

Posted Jun 27, 2021 17:17 UTC (Sun) by NYKevin (subscriber, #129325)
In reply to: Suppressing SIGBUS signals by roc
Parent article: Suppressing SIGBUS signals

I don't think it is semantically possible to avoid making a copy. The length of the mmap is equal to the length on disk, and because Linux went with the no-mandatory-file-locking model, that length can change at any time, regardless of whether you're using Rust, C, or a magnetic needle and a steady hand. If you want to end up with a "safe" object, you need it to not get suddenly truncated/zero-filled while you're halfway through processing it.

Making a copy also resolves issues of the form "What if someone decides to scribble all over the file without changing its length, while I'm halfway through processing it?" More generally, this falls into the "validate, then process" model of doing things - you can't validate something if it can change out from under you!

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Suppressing SIGBUS signals

Posted Jun 28, 2021 2:50 UTC (Mon) by ilammy (subscriber, #145312) [Link] (2 responses)

I wonder if some sort of copy-on-write private file mappings could help with that by avoiding copying the entire range (since mapped files tend to be huge). Like, you map a file a get a snapshot of its contents. Your process writing to that memory copies a page just for you and never syncs that page with the actual file. If any other process touches the file in any way via non-cow mapping or normal file ops, then the original data is copied and other process cow-mappings are dissociated from the file.

Suppressing SIGBUS signals

Posted Jun 30, 2021 11:57 UTC (Wed) by hmh (subscriber, #3838) [Link] (1 responses)

That looks like it would have been a much more generally useful (and expensive, complicated, etc) feature to implement than suppressing sigbus or zero-extending...

Mmap snapshot (maybe with a read-only result if that would be much easier or cheaper to implement and still cover the use cases).

But until someone offers to do that work...

Suppressing SIGBUS signals

Posted Jul 1, 2021 2:16 UTC (Thu) by ilammy (subscriber, #145312) [Link]

After doing some research, this idea is implemented in some systems as a MAP_COPY flag for mmap() [1]. That's basically the semantics of MAP_PRIVATE mapping, but with a “snapshot” guarantee that the data won't change, making your copy genuinely private, but avoiding the actual copy if the data does not change.

While thinking of how this could be implemented, I realized that it could be quite expensive, complicated, and full of “spooky action at a distance”. If some process grabs or released a MAP_COPY mapping of a file, then all existing processes must be made aware of it (e.g., by turning all mappings for everyone RO and catching page faults). Any change by the other process forces said process to expend some kernel time doing the copy of the page for the benefit of some other process, which is not particularly fair.

Turns out, adding MAP_COPY into Linux was discussed several times [2][3], but it's still considered a pretty stupid idea.

[1]: https://www.gnu.org/software/hurd/glibc/mmap.html
[2]: https://yarchive.net/comp/linux/map_copy.html
[3]: https://www.spinics.net/lists/linux-mm/msg119339.html