Re: Linux x86-32 addr space split

From: Casper H.S. Dik (Casper.Dik_at_Sun.COM)
Date: 02/02/05

  • Next message: Josef Moellers: "Re: Linux x86-32 addr space split" 
    Date: 02 Feb 2005 11:19:54 GMT

    Josef Moellers <josef.moellers@fujitsu-siemens.com> writes:

    >The advantage is that the kernel will have the current process' address=20
    >space as part of its own address space. In a large number of cases, this =

    >makes expensice copies between separate address spaces (user<->kernel)=20
    >unnecessary.

    The copies are still necessary, they're just cheaper because you read/write
    to the same address space.

    >Note To stay portable, one should nonetheless use the appropriate=20
    >functions/macros when copying between user and kernel space, even if=20
    >they will then just to a copy.

    That's not just about portability; referencing user data from the
    kernel directly may cause a page fault which won't generally be handled
    by the kernel (typically, it won't generate a page fault because system
    calls are typically handed data which has just been touched).

    When Solaris for UltraSPARC was done, the kernel and userland got
    a separate address space as the CPU supports that cheaply. This
    flushed out a number of those bugs. If you tried hard, you could
    make the kernel panic abusing those bugs before, but nobody noticed
    (or they were written of as transient failures)

    Casper

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