[Ltrace-devel] displaying arguments and build vs. host fiasco

[Petr Machata]

Hi list,

I had a chance to look to the remaining ppc64/ppc32 test case failure in 
ltrace.  I believe this is a general problem, when you take exactly this 
example to x86_64/i386 it breaks, too.

Here's the reproducer program, ww.c:

struct ww { int a; int *pa; };

#ifdef LIB
int ble (void *x) { return 1; }
#else
int ble (void *x);
#endif

#ifndef LIB
int main () {
   struct ww x;
   x.a = 3;
   x.pa = &x.a;
   return ble (&x);
}
#endif

And corresponding config file, ww.conf:
void ble(struct(int,int*)*)

Use normal steps to compile ltrace in 64-bit mode, then compile the test 
case like this:

$ gcc -DLIB -fpic -shared ~/ww.c -o libww.so
$ gcc -ULIB ~/ww.c ./libww.so -o ww
$ gcc -DLIB -fpic -shared -m32 ~/ww.c -o libww32.so
$ gcc -ULIB -m32 ~/ww.c ./libww32.so -o ww32

And run (output trimmed):

# ./ltrace -F ~/ww.conf ./ww
ble({ 3, 3 }) = <void>

# ./ltrace -F ~/ww.conf ./ww32
ble({ 3, -2311104 }) = <void>

The value -2311104 is some random bit of the process address space, it 
might end up being "?" if the region it tries to read is inaccessible. 
The problem here is that 64-bit ltrace thinks the pointer alignment is 8 
bytes, but it's tracing 32-bit inferior that thinks it should rather be 
4.  In this particular case I can go to read_config_file.c and hard-code 
pointer alignment to 4 bytes, which fixes it.  At least on ppc, on x8664 
this didn't work for me for some reason, it must be more or differently 
buggy.

So,
- front end needs to leave these sorts of decisions, like pointer 
alignment, to back end.  The simple trick with alignof won't work in 
general.
- front end needs to know it may need to precompute 32-bit layouts even 
on 64-bit.
- which means that i386 and x86_64 can no longer be totally separate 
backends, at least not the way they are now, or at least x86_64 has to 
include i386.  I'm thinking it shouldn't be terribly hard to merge them, 
ppc and s390 have always been like that AFAIK.

Well, tell me your thoughts.
PM