Re: gcc versus clang issue for 32-bit binaries

> On 10. Jun 2020, at 18:59, Mark Johnston <markj_at_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.
It looks like 0x10000000 works fine on my 32-bit VM.
I added you as a reviewer on https://github.com/google/syzkaller/pull/1809

Best regards
Michael