I'm developing software for an embedded mips based platform using uClibc ntpl-svn 00891d9cf07efd992023f255164bba93d657ece4 compiled with gcc-4.4.3 (although gcc versions doesn't seem to matter). Reading the debug output of the pppd i found some nonsense error messages. Adding some extra lines to the code, i noticed errno was always holding the value of the requested filedescriptor using normal read/write operations. In order to isolate the bug i wrote the following testframe:

int main(int argh, char **argv){
    unsigned char buf[32];
    int ter, sz;

    while (1) {
	puts("trying to read from non existing descriptor.");
	sz = read(666, buf, 16);
	ter = errno;
	puts("done.");
	if (sz < 0) {
		puts("read failed.");
		printf("errno holds a value of %d.\n", ter);
	} else
		puts("erg... read succeded?");
	puts("---");
    }
    return 0;
}

It showed up that errno really was set to 666 instead of EBADF (Bad file descriptor).

I already added the patch labeled 'MIPS: restore INLINE_SYSCALL macro' hoping this would fix the issue, but it didn't. I have been digging through the change logs and I think this hasn't been fixed yet.