Re: [PATCH v6 5/6] tracing: Show inode and device major:minor in deferred user space stacktrace

[Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>]

On August 29, 2025 2:13:33 PM GMT-03:00, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Fri, 29 Aug 2025 at 10:02, Steven Rostedt <rostedt@goodmis.org> wrote:
>>
>> Note, the ring buffer can be mapped to user space. So anything written into
>> the buffer is already exposed.
>
>Oh, good point. Yeah, that means that you have to do the hashing
>immediately. Too bad. Because while 'vma->vm_file' is basically free
>(since you have to access the vma for other reasons anyway), a good
>hash isn't.


Can't we continue with that idea by using some VMA sequential number that don't expose anything critical to user space but allows us to match stack entries to mmap records?

Deferring the heavily lift to when needed is great.

- Arnaldo 

>
>siphash is good and fast for being what it is, but it's not completely
>free. It's something like 50 shift/xor pairs, and it obviously needs
>to also access that secret hash value that is likely behind a cache
>miss..
>
>Still, I suspect it's the best we've got.
>
>(If hashing is noticeable, it *might* be worth it to use
>'siphash_1u32()' and only hash 32 bits of the pointers. That makes the
>hashing slightly cheaper, and since the low bits of the pointer will
>be zero anyway due to alignment, and the high bits don't have a lot of
>information in them either, it doesn't actually remove much
>information. You might get collissions if the two pointers are exactly
>32 GB apart or whatever, but that sounds really really unlucky)
>
>                Linus

- Arnaldo