Re: Possible shared mapping bug in 2.4.23 (at least MIPS/Sparc)

From: Peter Horton (pdh_at_colonel-panic.org)
Date: 12/14/03

  • Next message: Marco Roeland: "Re: In fs/proc/array.c error in function proc_pid_stat" 
    Date:	Sun, 14 Dec 2003 10:38:03 +0000
To: Linus Torvalds <torvalds@osdl.org>

    On Sat, Dec 13, 2003 at 05:41:16PM -0800, Linus Torvalds wrote:
    >
    > On Sat, 13 Dec 2003, Jamie Lokier wrote:
    > >
    > > Peter Horton wrote:
    > > > A quick look at sparc and sparc64 seem to show the same problem.
    > >
    > > D-cache incoherence with unsuitably aligned multiple MAP_FIXED
    > > mappings is also observed on SH4, SH5, PA-RISC 1.1d. The kernel may
    > > have the same behaviour on those platforms: allowing a mapping that
    > > should not be allowed.
    >
    > Why?
    >
    > If the user asks for it, it's the users own damn fault. Nobody guarantees
    > cache coherency to users who require fixed addresses.
    >
    > Just document it as a bug in the user program if this causes problems.
    > Don't blame the kernel - the kernel is only doing what the user asked it
    > to do.
    >

    I've seen code written for X86 use MAP_FIXED to create self wrapping
    ring buffers. Surely it's better to fail the mmap() on other archs
    rather than for the code to fail in unexpected ways?

    It's a bug either way ... either the test should be fixed up or it
    should be removed from arch_get_unmapped_area() to save confusion.

    P.
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