Date: Thu, 01 May 2008 11:57:42 +0200 From: Pieter de Boer <pieter@thedarkside.nl> To: Roman Divacky <rdivacky@freebsd.org> Cc: freebsd-emulation@freebsd.org Subject: Re: Linux compat ioctl return values Message-ID: <48199416.5000407@thedarkside.nl> In-Reply-To: <20080501081811.GB54624@freebsd.org> References: <481897AB.7070003@thedarkside.nl> <20080501081811.GB54624@freebsd.org>
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Roman Divacky wrote: >> I've been working on a kernel driver that creates a device. This device >> in turn is opened and ioctl'd from a Linux executable. I've registered a >> handler for these ioctl's and my ioctl handler is succesfully executed. >> >> My ioctl-handler returns a large positive value, but the userland >> application retrieves the value 1, EPERM. If I return 42, the userland >> application retrieves 42, but 260 is retrieved as 1. It appears there's a >> threshold somewhere above which the return value is set to 1, but I >> haven't been able to find out where in the code this is done. The Linux >> executable actually expects the value I return, and doesn't work when >> EPERM is found instead. >> >> So, the question is: does anyone know where such a threshold may >> reside and how to work around it? > > this is done in (for i386) sys/i386/i386/trap.c around line 1050. > > in short, we define in the sysvec structure sv_errtbl and if returned > error > the size of the table we just return -1. error table for > linux is roughly to 70. thats why you are getting -1 (1 after translation) > > you might extend the errno table (i386/linux/linux_sysvec.c for i386, line 126) The issue appears to be a bit more involved. It seems that in Linux, when the ioctl() syscall returns a negative value 'error', 'errno' is set to '-error' and the return value of the ioctl() library call is -1. All positive values are simply passed through: when the ioctl() syscall returns 35235, the ioctl() library call also returns 35235. This seems to be a difference in semantics between FreeBSD and Linux; FreeBSD is a bit more conservative. As the trap code in sys/i386/i386/trap.c is used for both FreeBSD and Linux executables, I wonder how to differentiate between both in trap.c. To see if I can at least make my Linux executable work for now, I'm going to test the following patch (to trap.c): - error = -1; /* XXX */ + /* Do nothing */ I suppose a patch that differentiates between Linux and FreeBSD syscalls is needed here, but how this could be done, dunno. -- Pieter
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