Re: Making a syscall before ld.so self-relocation on MIPS

[Joseph Myers <joseph@codesourcery.com>]

On Wed, 17 May 2023, Florian Weimer via Libc-alpha wrote:

> * Joseph Myers:
> 
> > On Wed, 17 May 2023, Florian Weimer via Libc-alpha wrote:
> >
> >> I'm reviewing HIDDEN_VAR_NEEDS_DYNAMIC_RELOC if they support inline
> >> system calls (without function calls), so that I can make a system call
> >> (mmap actually) before initial self-relocation of the dynamic loader.
> >> 
> >> So far, it looks good in theory, except on MIPS.  First there is MIPS16:
> >> 
> >> /* There's no MIPS16 syscall instruction, so we go through out-of-line
> >>    standard MIPS wrappers.  These do use inline snippets below though,
> >>    through INTERNAL_SYSCALL_MIPS16.  Spilling the syscall number to
> >>    memory gives the best code in that case, avoiding the need to save
> >>    and restore a static register.  */
> >> 
> >> Is MIPS16 the default in GCC?  We don't build for explicitly in
> >> scripts/build-many-glibcs.py.
> >
> > No, it's not the default; you need to use the -mips16 option.
> 
> And apparently this is expected to work for building glibc?  We have
> some support code for it.

Yes, it's supposed to work for building glibc (without actually being a 
different ABI).  I haven't tested it for several years.

> I guess I could override that for elf/rtld.c with -mno-mips16.  Would it
> be okay to do that, and rely on GCC generating a direct bal instruction
> for invoking the system call helper?

I think using -mno-mips16 for elf/rtld.c is OK.

-- 
Joseph S. Myers
joseph@codesourcery.com