Re: [PATCH 3/6] linux: Implement mremap in C

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella via Libc-alpha:

> Variadic function calls in syscalls.list does not work for all ABIs
> (for instance where the argument are passed on the stack instead of
> registers) and might have underlying issues depending of the variadic
> type (for instance if a 64-bit argument is used).
>
> Checked on x86_64-linux-gnu.

Where exactly does this break?

If the syscall generator cannot handle pointer arguments, we should fix
that.  As far as I can tell, we currently assume that the kernel can
handle sign-extended pointers.  I don't see any special code for
processing a/p/s pointers, for example.  That impacts far more than just
mremap.  These pointers are generally not marked as U, for which we have
special processing.

Conditional varargs processing for system calls is very bad.  It causes
an ongoing maintenance headache, and weird application issues.  For
example, Linux 5.7 added MREMAP_DONTUNMAP, and it can use the address
argument just like MREMAP_FIXED.  But the current mremap implementation
always passes NULL as a system call argument.

Then there's the issue where varargs functions have more stringent ABI
requirements than non-varargs functions, as we recently saw with prctl.

I think we should just revert this change, to be honest.

Thanks,
Florian