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Re: [PATCH] MIPS: fix forced successful syscalls

To: Tanguy Bouzeloc <tanguy.bouzeloc@efixo.com>
Subject: Re: [PATCH] MIPS: fix forced successful syscalls
From: Ralf Baechle <ralf@linux-mips.org>
Date: Tue, 8 Oct 2013 09:42:15 +0200
Cc: "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>
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On Thu, Oct 03, 2013 at 04:53:31PM +0200, Tanguy Bouzeloc wrote:

> On 10/02/2013 11:19 AM, Ralf Baechle wrote:
> >To my personal embarassment I have to admit that I knew about this since the
> >day the syscall wrapper was written - but was considering it an acceptable
> >bug ...
> >
> >Where it really bits is sigreturn and similar which use the following
> >stunt:
> >
> >         /*
> >          * Don't let your children do this ...
> >          */
> >         __asm__ __volatile__(
> >                 "move\t$29, %0\n\t"
> >                 "j\tsyscall_exit"
> >                 :/* no outputs */
> >                 :"r" (&regs));
> >         /* Unreached */
> >
> >to keep the syscall return path from tampering with the return value.
> >
> >The scall*.S part of your patch is clearing TIF_NOERROR using a non-atomic
> >LW/SW sequence.  This needs to be done atomically or the thread's flags
> >variable might get corrupted.  This is complicated by MIPS I, R5900 and
> >afair some older oddball not-quite MIPS II CPUs lacking LL/SC rsp. LLD/SCD.
> >
> >   Ralf
> >
> 
> I discover the issue when changing the HZ of the kernel to 100HZ, in
> this case the jiffies returned to the userland are the same as the
> kernel ticks and it'll wrap after 5 minutes of uptime. With kernel
> HZ at 250 or 1000H it'll make happen the ticks wrap after 230~260j.
> 
> Unfortunately programs relying on ticks (they shouldn't but that
> happens) have unpredictable behavior for 11.3s before the wrap.
> 
> I can update the patch in order to access atomically the thread
> flags, the point is ... it'll make the kernel incompatible with old
> hardware.

The price to pay: an ifdef ...

#include <asm/sgidefs.H>
...
#if (_MIPS_ISA > _MIPS_ISA_MIPS1)
... LL/SC code goes here
#else
... LL/SC-less code goes here
#endif

But I'm wondering if we can move this hopefully relativly rare special
case into do_notify_resume() and handle it in plain C there.

  Ralf

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