runtime: systemstack should align stack on 16-bytes

Question

runtime: systemstack should align stack on 16-bytes

4ad opened this issue 10 years ago · 13 comments

All stacks must be 16-byte aligned, but newsysmon gets called with SP%16=8.

Answer 1 · 2015-02-04T23:43:50.000Z

:crying:

On Thu, Feb 5, 2015 at 9:07 AM, Aram Hăvărneanu notifications@github.com
wrote:

All stacks must be 16-byte aligned, but newsysmon gets called with SP%16=8.

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Reply to this email directly or view it on GitHub
https://github.com/4ad/go/issues/100.

Answer 2 · 2015-02-04T23:44:49.000Z

Doesn't 6g have this restriction, amd64 stacks and function calls should be
16 byte aligned, yes ?

On Thu, Feb 5, 2015 at 10:43 AM, Dave Cheney dave@cheney.net wrote:

:crying:

On Thu, Feb 5, 2015 at 9:07 AM, Aram Hăvărneanu notifications@github.com
wrote:

All stacks must be 16-byte aligned, but newsysmon gets called with
SP%16=8.

—
Reply to this email directly or view it on GitHub
https://github.com/4ad/go/issues/100.

Answer 3 · 2015-02-05T00:10:04.000Z

AMD64 ABI has this requirement, but we pretty much disregard it. In places where this truly matter we align the stack pointer before calling external code that expect proper SP alignment.

Answer 4 · 2015-02-05T09:01:56.000Z

Liblink properly aligns everything if we do the right thing (e.g. have proper framesizes), it fails only for $0 functions which modify the stack pointer themselves.

It's trivial to fix all these places, though I am not sure how that impacts stack traces. Can gotraceback deal with this?

Answer 5 · 2015-02-07T21:43:17.000Z

I don't think gotraceback can deal with manually aligned stacks. That's why we only align the stack before calling external code (e.g. windows "syscalls").

Answer 6 · 2015-02-07T21:55:37.000Z

This might be a problem then. I don't think the fix is too hard though.

Answer 7 · 2015-02-07T22:06:55.000Z

I mean, if liblink aligns the stack frame by adjusting frame size, then there is no problems for gotraceback. (however, adjusting the frame size might have other issues for hand written assembly functions) but if we adjust SP manully by SUB and AND, then gotraceback can't handle this case. We control the Go ABI, do we need to maintain proper stack alignment internally? I think the only case where this matters is before we call foreign functions (e.g. syscall and cgo). Linux syscall shouldn't care about user stack so we only need to maintain correct stack alignment for cgocalls.

Answer 8 · 2015-02-07T22:07:53.000Z

Yes, you get a SIGBUS if the stack is not aligned.

Answer 9 · 2015-02-07T22:08:48.000Z

(Not only stack, but any kind of indirect reference through SP too, e.g. 16(SP) crashes if SP%16=8).

Answer 10 · 2015-02-07T22:33:33.000Z

OK, so we need to make cmd/7g adjust the frame size 8 mod 16 and also make liblink give an error if total frame size is not aligned to 16 bytes?

Answer 11 · 2015-02-07T22:38:54.000Z

Not really. I think everything is mostly correct now, apart from assembly functions that modify the stack pointer manually in an incorrect way. The way liblink works now is that if you ask for a non-aligned frame size, it will align it for you by making it bigger. In other words, it will always generate correct code.

I think this is good and better than if we just require the user of liblink (assemblers, compiler) to specify correct framesize, because the passed framesize, or the framesize set in assembly always matches what the human user would expect, so it's easy to verify, and also makes go vet just work without significant changes.

Answer 12 · 2015-02-17T05:54:11.000Z

I think this has been fully fixed, no?

Answer 13 · 2015-02-17T10:20:18.000Z

I think so, yes.