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.
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