Linus Torvalds Tactfully Discusses Value of getrandom() Upgrade for Linux vDSO

Linux's vDSO (or virtual dynamic shared object) is 'a small shared library that the kernel automatically maps into the address space of all user-space applications,' according to its man page. 'There are some system calls the kernel provides that user-space code ends up using frequently, to the point that such calls can dominate overall performance... due both to the frequency of the call as well as the context-switch overhead that results from exiting user space and entering the kernel.'



But Linus Torvalds had a lot to say about a proposed getrandom() upgrade, reports Phoronix:

This getrandom() work in the vDSO has been through 20+ rounds of review over the past 2+ years, but... Torvalds took some time out of his U.S. Independence Day to argue the merits of the patches on the Linux kernel mailing list. Torvalds kicked things off by writing:

Nobody has explained to me what has changed since your last vdso getrandom, and I'm not planning on pulling it unless that fundamental flaw is fixed. Why is this _so_ critical that it needs a vdso? Why isn't user space just doing it itself? What's so magical about this all?

This all seems entirely pointless to me still, because it's optimizing something that nobody seems to care about, adding new VM infrastructure, new magic system calls, yadda yadda. I was very sceptical last time, and absolutely _nothing_ has changed. Not a peep on why it's now suddenly so hugely important again. We don't add stuff 'just because we can'. We need to have a damn good reason for it. And I still don't see the reason, and I haven't seen anybody even trying to explain the reason.

And then he responded to himself, adding:

In other words, I want to see actual *users* piping up and saying 'this is a problem, here's my real load that spends 10% of time on getrandom(), and this fixes it'. I'm not AT ALL interested in microbenchmarks or theoretical 'if users need high-performance random numbers'. I need a real actual live user that says 'I can't just use rdrand and my own chacha mixing on top' and explains why having a SSE2 chachacha in kernel code exposed as a vdso is so critical, and a magical buffer maintained by the kernel.'

Torvalds also added in a third message:

One final note: the reason I'm so negative about this all is that the random number subsystem has such an absolutely _horrendous_ history of two main conflicting issues: people wanting reasonable usable random numbers on one side, and then the people that discuss what the word 'entropy' means on the other side. And honestly, I don't want the kernel stuck even *more* in the middle of that morass....

Torvalds made additional comments. ('This smells. It's BS...') Advocating for the change was WiredGuard developer Jason Donenfeld, and more communication happened (and continues to happen... 40 messages and counting).

At one point the discussion evolved to Torvalds saying 'Bah. I guess I'll have to walk through the patch series once again. I'm still not thrilled about it. But I'll give it another go...'

Read more of this story at Slashdot.