runtime: use getrandom(2) for readRandom

[apocelipes]

Proposal Details

Currently when we set the random seed on Linux, the runtime's fallback path needs to open, read and then close "/dev/urandom". This requires 3 system calls. A single getrandom does the same thing and is simpler than using open() -> read() -> closefd().

#67001 Raised the minimum supported Linux kernel version to 3.17+, so that "getrandom(2)" can be used in the runtime package and we can do all the things in one system call now.

We can wrap getrandom with internal/runtime/syscall.Syscall6 so that the helper function readRandom will no longer need to open and close "/dev/urandom".

Since Go 1.24 will only support Linux kernel 3.17+ (or 3.10+ with getrandom patches), I think a fallback is unnecessary (and a fallback can easily be added if someone needs it).

[gabyhelp]

Related Issues

• proposal: runtime: add option to specify seed for `globalRand` #67690

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

I don't think a proposal is necessary as there are no user facing changes?
Crashing on unavailability seems ok given #66821

[randall77]

On linux at least, the runtime does not normally call getrandom. We use the auxv random data at startup.
The readRandom path is kind of a fallback.
(I'm not sure precisely when linux gives us the auxv. But I imagine most modern linuxes do - that path has been in the codebase for a while.)

[apocelipes]

On linux at least, the runtime does not normally call getrandom. We use the auxv random data at startup. The readRandom path is kind of a fallback. (I'm not sure precisely when linux gives us the auxv. But I imagine most modern linuxes do - that path has been in the codebase for a while.)

Yes, but I think it's also worth to simplify the fallback path. All this changes will be at most a middle size CL.

[randall77]

Yeah, no problem, I was not arguing against the proposed change. Simplification is good. Just mentioning it probably won't affect any behavior.

[randall77]

Given #67001 is accepted, I don't think this needs to be a proposal. Taking out of the proposal process.