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

[Florian Weimer <fweimer@redhat.com>]

* 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.

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?

Thanks,
Florian