fork fails after nmap with hint address in an unmapped memory region

[Corinna Vinschen]

On Dec  9 23:40, Brian Inglis wrote:
> On 2017-12-09 15:51, Jon Turney wrote:
> > On 09/12/2017 18:29, Brian Inglis wrote:
> >> On 2017-12-09 08:53, Stéphane Mbape via cygwin wrote:
> >>> Le 09/12/2017 à 16:48, Andrey Repin a écrit :
> >>>>> While embeding luajit in a c  program, I found myself unable to fork
> >>>>> processes.
> >>>>> Investigations prove that it was related to nmap.
> >>>>> To be accurate, calling nmap with hint address in a unmapped memory
> >>>>> region will cause all forks to fail with
> >>>>> "fixup_mmaps_after_fork: ReadProcessMemory failed for MAP_PRIVATE
> >>>>> address 0x6FFFFFE0000, Win32 error 299"
> >>>>> There is a sample code below.
> >>>> You forgot to mention Cygwin version you're using, and please provide the
> >>>> sample as an attach to save people the copy-pasting issues.
> >>> Cygwin version: 2.9.0
> >>> OS: Windows 10
> >>> Arch: 64bit
> >>> The sample was also attached.
> >>
> >> Confirmed reproducible; addr2line does not give anything useful from the
> >> stackdump, but included raw stackdump below, in case it gives hints.
> > 
> > If you're going to use addr2line to interpret a .stackdump file, I'm afraid
> > you're just supposed to know that the cygwin1.dll has an ImageBase of
> > 0x0000000180040000 on x86_64
> 
> Who came up with that magic number 6G256K and why - would a round 7/8G not do?

Long discussion on cygwin-developers way back when we developed the 64
bit version.  Basically:

* Leave lower 32 bit area of 44 bit address space free for Windows
* Leave upper 32 bit area of 44 bit address space free for Windows

The rest just came about:

  0x1:00000000 - 0x1:80000000	Stacks
  0x1:80000000 - 0x2:00000000	Cygwin DLL plus helper space
  0x2:00000000 - 0x4:00000000	Rebased DLLs
  0x4:00000000 - 0x6:00000000	Non-rebased DLLs
  0x6:00000000			Start address heap
  ...
0x700:00000000			Start address of mmap top down allocations

So heap and mmap are growing from different directions.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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