> On 10. Jun 2020, at 20:30, Damjan Jovanovic <damjan....@gmail.com> wrote:
>
> MAP_FIXED is generally bad news, as it overwrites any prior mappings within
> the range of addresses being mapped to.
>
> They should use MAP_FIXED | MAP_EXCL instead, which will fail if any mappings
> already exist in the range, and then maybe retry with another range if it
> fails. Linux and NetBSD have MAP_TRYFIXED instead, which does the retrying
> internally. Or at the very least, run mincore() on every page in the range to
> verify that nothing is mapped before using mmap() with MAP_FIXED.
It is used in syzkaller. Some go code generates C include files... So right now
I might want
to stick with a value.
>
> If there is no other way but to use a single hardcoded value, check
> /proc/<pid>/map for a number of different processes, 32 and 64 bit, and find
> an address range that isn't used often.
Thanks for the hint. I tried to find one. Let's see how good this guess is.
Best regards
Michael
>
> Damjan
>
>
> On Wed, Jun 10, 2020 at 7:40 PM Michael Tuexen <tue...@freebsd.org> wrote:
> > On 10. Jun 2020, at 18:59, Mark Johnston <ma...@freebsd.org> wrote:
> >
> > On Wed, Jun 10, 2020 at 06:41:50PM +0200, Michael Tuexen wrote:
> >> Dear all,
> >>
> >> consider the following program test.c:
> >>
> >> #include <sys/mman.h>
> >> #include <stdio.h>
> >>
> >> int
> >> main(void)
> >> {
> >> void *p;
> >>
> >> p = mmap((void *)0x20000000, 0x1000000, PROT_READ | PROT_WRITE |
> >> PROT_EXEC, MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> >> printf("p= %p\n", p);
> >> return (0);
> >> }
> >>
> >> On i386 the following happens:
> >> * when compiling it with cc and running it, it crashes.
> >> * when compiling it with gcc it runs fine.
> >>
> >> On amd64 the following happens:
> >> * when compiling it with cc -m64 it runs fine.
> >> * when compiling it with cc -m32 is crashes.
> >> * when compiling it with gcc -m64 it runs fine.
> >> * when compiling it with gcc -m32 it runs fine.
> >>
> >> So why does the above program crash when compiled for 32-bit when using
> >> clang, but runs fine when compiled with gcc.
> >
> > The difference is between ld.bfd and ld.lld, which emit executables with
> > different entry point addresses. cc -m32 -fuse-ld=bfd gives an
> > executable that does not crash.
> >
> > When linked with lld, libc and ld-elf get mapped into the region
> > [0x20000000,0x21000000], so the program crashes when the libc.so mapping
> > is overwritten with that created by the mmap() call and the program
> > calls printf().
> >
> >> I'm testing this on 32-bit and 64-bit head systems. gcc is from ports.
> >>
> >> The reason I'm looking into it is that I want to get syzkaller working on
> >> 32-bit with clang.
> >
> > Do you know why SYZ_DATA_OFFSET is hard-coded the way it is? It looks
> > like it works more or less by accident, but at a glance I don't see why
> > it has to be a fixed mapping.
> I don't know, it comes from:
> https://github.com/google/syzkaller/blob/master/sys/targets/targets.go#L450
>
> Do you have a value which can be used on FreeBSD? Then we can just change
> it...
>
> Best regards
> Michael
>
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