Ah, I think I understand what you're getting at. You'll want to read this: https://github.com/golang/go/blob/b44b360dd40a537e953225d70ca96361e2c57600/src/runtime/stack.go#L21 TL;DR the thing the header is checking against isn't the bottom of stack, it is a mark some constant above the bottom.
> what frame size is safe StackSmall, which is 128 bytes. On Thursday, April 17, 2025 at 4:48:18 PM UTC-7 Wojciech Kaczmarek wrote: > piątek, 18 kwietnia 2025 o 00:10:02 UTC+2 Keith Randall napisał(a): > > On Thursday, April 17, 2025 at 11:50:18 AM UTC-7 Wojciech Kaczmarek wrote: > > Hi Gophers, > > I am experimenting with assembler, ABI0 and stuff. > While I was learning the ABI0 frame structure from disassembled code, I > noticed that the stack check happens before subtracting from RSP (I'm on > arm64, but the same applies to SP on amd64, I mean the hardware register > ofc). > Then, what if the frame being allocated is big enough to cause trouble? > > > Do you mean, what if the stack check fails? It calls into > runtime.morestack. morestack will allocate a new, larger stack and copy the > existing contents over. > > > I wasn't quite asking about check fail scenario. The stack check is done > before frame is even considered, and later the frame size is subtracted > from RSP. But then there is no checking again. > > > Also, after the CALL runtime.morestack*, what is the safe amount to > subtract from RSP? > > > There's no specific amount. Probably the frame that didn't fit at the > start will fit after calling morestack. But not necessarily, if the frame > is really large it will call into morestack again. (That's why after > calling morestack we jump to the top of the function and redo the stack > check.) > > > Hmm, as above - it seems that g.stackguard0 is compared to the RSP before > frame gets allocated on the stack, so the question remains - what frame > size is safe? > (I am also thinking about some form of limited "alloca", to have a NOFRAME > function and make frame manually in the asm - the more it seems important > to know what limit on modifying RSP is safe here). > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/b9302710-7325-4a95-b814-519bf0c25407n%40googlegroups.com.
