bug#47125: 28.0.50; pdumper assumes compile time page size remains valid

[Daniel Colascione]

On 3/13/21 9:37 PM, Eli Zaretskii wrote:

> > From: Pip Cet <pipcet@gmail.com>
> > Date: Sat, 13 Mar 2021 21:38:16 +0000
> >
> > I'm running Debian GNU/Linux (the Linux part is not provided by
> > Debian) on an Apple M1-based machine. This currently involves running
> > a kernel compiled with a 16 KB page size (the only fully functional
> > kernel is currently available as a binary as recompilation of the
> > alleged source fails to produce a fully working kernel).
> >
> > The Debian-packaged Emacs version does not start. Compiling from
> > scratch works fine.
> >
> > After some investigation, this is because pdumper assumes that an
> > address aligned according to the page size at build time is
> > sufficiently aligned for mmap to work with the MAP_FIXED flag, when it
> > comes to loading the dump. That's not true because the Debian Emacs
> > was apparently built with a 4 KB page size, so it will not run on a
> > system with a 16 KB page size.
> >
> > I've confirmed that I get the same error on current master if I modify
> > getpagesize to return 4096 rather than the correct value.
> >
> > I think it would be best to handle this case gracefully, and I thought
> > pdumper already did that, but it appears to simply fail.
> >
> > There are good reasons for increasing the page size, so this is likely
> > to happen more often and on other architectures with varying page
> > sizes.
> CC'ing Daniel, in case he has comments and/or suggestions.

We should modify this function to do what the doc comment says then. :-) 
I'm not sure if there's any reliable way to know what the worst case 
allocation granularity actually is: a quick grep through /usr/include 
didn't turn up anything. Maybe we should just use a minimum of 16kB 
here? It's not as if we'd be wasting a ton of RAM doing so.

/* Worst-case allocation granularity on any system that might load
   this dump.  */
static int
dump_get_page_size (void)
{
#if defined (WINDOWSNT) || defined (CYGWIN)
  return 64 * 1024;  /* Worst-case allocation granularity.  */
#else
  return getpagesize ();
#endif
}