[ARC] segmentation fault

[zevv]

The snippet below crashes with a SIGSEGV on arc:

nim c -d:useMalloc --os:any --gc:arc -r test.nim

type
  Thing = object
    s1: string
    s2: string
  
var box: seq[Thing]

box.add Thing(s1: "")

Does not crash when changing the type to:

type
  Thing = object
    s1: string
    s2: int

but does crash when making it

type
  Thing = object
    s1: string
    s2: int
    s3: int

[zevv]

Preliminary conclusion: with arc Nim tries to zero out the Thing(s1:, "") instance after adding it it to the box, but the C codegen emits the Thing as a NIM_CONST. In some cases - not sure how or when yet (object size?) - this ends up in a read-only memory page which causes the segmentation fault on memset.

That's what you get from writing over a const.

Interestingly: why does this not happen more often?

I guess the exact behaviour depends on gcc version, operating system, but at least @disruptek was able to reproduce with the above example. Writing to a const is UB anyway.

gcc version 9.2.1 20200104 (Debian 9.2.1-22) 
Linux platdoos 5.4.0-1-amd64 #1 SMP Debian 5.4.6-1 (2019-12-27) x86_64 GNU/Linux