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Date: Tue, 10 Mar 2020 22:10:15 +0100
From: Andreas Dröscher <musl@...free.ch>
To: musl@...ts.openwall.com
Subject: mips32 little endian -ENOSYS is not -(-ENOSYS)

Hi

I'm building a new toolchain for a very old hardware with a very old Linux 
Kernel (2.6.20). The CPU is a Alchemy (now AMD) AU1100 (production was 
discontinued).

Obviously the Kernel lacks a lot of the modern system calls. I however expect 
the general system call interface to be consistent. Moreover, musl has fallbacks 
for many system-calls in place, kudos! However, the fallback is never triggered. 
I will present the issue on one example (epoll):

excerpt from src/linux/epoll.c:
int epoll_create1(int flags)
{
int r = __syscall(SYS_epoll_create1, flags);
#ifdef SYS_epoll_create
if (r==-ENOSYS && !flags) r = __syscall(SYS_epoll_create, 1);
#endif
return __syscall_ret(r);
}

If r is -89 (negative ENOSYS) the fallback is triggered else the result is 
returned as it is. However, in my case __syscall returnes 89 (positive ENOSYS).
I've tracked the return into the kernel and there the negative value is 
returned. The Kernel additionally sets r7 to 1.

excerpt from arch/mips/syscall_arch.h:
static inline long __syscall1(long n, long a)
{
register long r4 __asm__("$4") = a;
register long r7 __asm__("$7");
register long r2 __asm__("$2") = n;
__asm__ __volatile__ (
"syscall"
: "+r"(r2), "=r"(r7)
: "r"(r4)
: SYSCALL_CLOBBERLIST, "$8", "$9", "$10");
return r7 ? -r2 : r2;
}

I assume the "bug" is triggered by __syscall1 If r7 is set it will change the 
sign of r2. I can patch that by replacing:
return r7 ? -r2 : r2;
with
return (r7 && r2 > 0) ? -r2 : r2;

However I've no idea if I'm triggering any side effects or if I selected the 
wrong implementation for my architecture.

Any suggestions are appreciated.
~Andreas

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