Re: [PATCH] Add build time guard to detect off_t mismatch

On Thu, May 7, 2026 at 4:22 PM Jan Kiszka <[email protected]> wrote:
> OK, given that Xenomai intercepts the mmap path, we could argue that
> this is also stable relevant.
>
> Can you confirm that this issue is truly gone in next, whatever mode you
> choose on both sides?

Still had no chance to build or run tests with Xenomai next on my ARM boards.
But I had a look at the code.

I think it's still broken, at least for the following scenario:
- 32 bit application built with  _FILE_OFFSET_BITS=64
- 32 bit libcobalt without
- application calls mmap() using __RT(mmap), so __cobalt_mmap() in libcobalt
- off_t differs in size ass seen by caller and callee

On x86 it works by chance because callee reads only the first 4 bytes
of the 8 byte off_t.
All parameters are on the stack and have 4 byte alignment.

On ARM EABI the story is different.
The first four parameters are in registers r0 to r3, all others on the stack.
5th parameter, fd, is a 4 byte integer, so the caller needs to add 4
bytes padding
to make sure the 6th parameter, off_t, is on an 8 byte alignment
boundary since off_t
is from the callers point of view 8 bytes.
The callee expects off_t being 4 bytes, knows nothing about the
padding and reads
the uninitialized padding bytes as offset.

I don't see how this case is avoided in next.

-- 
Thanks,
//richard