mmap with MAP_PRIVATE is useless (was Re: Bug#569505: git-core: 'git add' corrupts repository if the working directory is modified as it runs)

[Paolo Bonzini <bonzini@gnu.org>]

> I tested this with a number of kernel versions from 2.6.27 to 2.6.31.
> All of them reproduced this problem.  I checked this because strace
> shows 'git add' doing a mmap(..., MAP_PRIVATE, ...) of its input file,
> so I was wondering if there might have been a recent change in mmap()
> behavior in either git or the kernel.

 From mmap(2): "it is unspecified whether changes made to the file after 
the mmap() call are visible in the mapped region".

You may think that doing a dummy "*p = *p" every 4096 bytes `fixes' it 
(because it causes copy-on-write for every page) but even that does not 
work because you can get a SIGBUS if the file is truncated while you 
have it mapped privately (e.g. by fopen ("file", "w") or open with O_TRUNC).

Testcase:

#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>

int
main ()
{
   system ("echo foo > file.test");
   int f = open ("file.test", O_RDONLY);
   char *p = mmap (NULL, 4096, PROT_READ, MAP_PRIVATE, f, 0);
   close (f);
   printf ("%s", p);       // prints "foo\n"

   f = open ("file.test", O_RDWR | O_CREAT | O_TRUNC, 0666);
   write (f, "bar\n", 4);  // comment out and next printf SIGSEGVs
   printf ("%s", p);       // prints "bar\n"
}

This means that MAP_PRIVATE is utterly useless.

Paolo