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Re: System call select on R4600

To: Macleod <macleod@mail2000.com.tw>
Subject: Re: System call select on R4600
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 25 Aug 2004 10:00:45 +0200
Cc: linux-mips@linux-mips.org
In-reply-to: <1093400284.64232.macleod@mail2000.com.tw>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <1093400284.64232.macleod@mail2000.com.tw>
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On Wed, Aug 25, 2004 at 10:18:04AM +0800, Macleod wrote:

>  After trace system, found this problem is from scall_o32.S
>  line 161 in 2.4.26 kernel.
> 
>  bltz t0, bad_stack   # -> sp is bad
> 
>  If stack address larger than 0x7fffffff, branch will take, 
>  and that's why I got "-4142" errno on select system call
>  even parameters in stack are correct. I tried to remove this
>  line and seems "select" works fine.

Oh yes, I forgot on this one :-)  It's intentional; making all syscalls
work from kernel mode would add some overhead.  Possible solutions:

 - move to 64-bit kernels; the 64-bit syscall interface happens to support
   upto 8 arguments for syscalls from kernel mode.
 - move your module or parts of it to userspace.  Something that's using
   select suspiciously looks like something that shouldn't be in the kernel.
 - use the proper kernel APIs.  Kernel programming isn't user space
   programming.

  Ralf

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