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Date: Tue, 10 Mar 2020 22:18:08 -0400
From: Rich Felker <dalias@...c.org>
To: Andreas Dröscher <musl@...free.ch>
Cc: musl@...ts.openwall.com
Subject: Re: mips32 little endian -ENOSYS is not -(-ENOSYS)

On Wed, Mar 11, 2020 at 03:08:22AM +0100, Andreas Dröscher wrote:
> >>Sorry for not including that excerpt in the first place:
> >>
> >>illegal_syscall:
> >>	li	v0, -ENOSYS			# error
> >>	sw	v0, PT_R2(sp)
> >>	li	t0, 1				# set error flag
> >>	sw	t0, PT_R7(sp)
> >>	j	o32_syscall_exit
> >>	END(handle_sys)
> >>
> >>Source: https://github.com/torvalds/linux/blob/62d0cfcb27cf755cebdc93ca95dabc83608007cd/arch/mips/kernel/scall32-o32.S#L186
> >
> >OK, this was fixed by commit bda8229bdd087167f463ad5e74299987924f8137
> >in 2008. But it looks like there's still another path, called
> >"einval" from before commit fb498e2570eedc6c9c3d165e370624dfc3aed97b,
> >that returns -ENOSYS. All of this is awful, and I think your fix is
> >probably the right thing to do.
> 
> Thank you very much for your review.
> 
> Just as a side note. I’ve just figured out that there is a second
> issue with old kernels.
> 
> The current implementation of __syscall5, __syscall6 and __syscall7
> (those use caller saved registers) violate the calling conventions
> of MIPS32 Linux Kernels prior 2.6.35. Those were assuming that the
> instruction immediately preceding the SYSCALL instruction was an
> instruction for loading the syscall number.
> 
> I’ll will try to rearrange the stack pushes to accommodate this
> requirement and report back if I manage to come up with something
> presentable.

Uhg, so commit 604f8d3d8b08ee4f548de193050ef93a7753c2e0 was probably
wrong and there was a reason for the nonsensical code it removed:
making old broken kernels happy. I'm not sure if you can just revert
it or need to make new changes.

Do you know if this "rule" applies to n32/n64 too or just o32?

Rich

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