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    SubjectRe: 32-bit syscalls from 64-bit process on x86-64?
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    On Sat, 2004-12-04 at 15:40 +0100, Petr Vandrovec wrote:
    > On Fri, Dec 03, 2004 at 03:16:29PM -0800, Jeremy Fitzhardinge wrote:
    > > On Fri, 2004-12-03 at 07:15 +0100, Andi Kleen wrote:
    > > > int 0x80 from 64bit will call 32bit syscalls.
    > >
    > > Hm, that's not what I found. If I run int 0x80 with [re]ax=1, I get
    > > exit with a 32-bit process and write with a 64-bit one.
    >
    > Seems to work for me. See second example below. If it won't work, you
    > can always use reverse procedure to one I used in syscall.c - only
    > problem is that you'll have to allocate some memory below 4GB to
    > hold code & stack during 32bit syscall.
    >
    > > > But some things
    > > > will not work properly, e.g. signals because they test the 32bit
    > > > flag in the task_struct. So you would still get 64bit signal frames.
    > >
    > > OK, I can deal with that, since signals are never directly delivered to
    > > the client.
    > >
    > > > There are some other such tests, but they probably wont affect you.
    > > >
    > > > There were some plans at one point to allow to toggle the flag
    > > > using personality or prctl, but so far it hasn't been implemented.
    > > > There is also no way to do 64bit calls from a 32bit executable.
    > >
    > > What about my idea of using distinct ranges of syscall numbers to
    > > disambiguate them?
    >
    > What about this? This does 64bit syscalls from 32bit executable.
    >
    > Developed/tested on 2.6.10-rc2-mm2. You may want to replace $0x33 with
    > some other value on 2.4.x kernels, but otherwise it should work fine for
    > you. And well, you may want to do some additional coding so you can
    > actually pass & retrieve 64bit values to/from syscalls.
    >
    > When it works correctly, program should print something and exit with code
    > 111. If it sigsegvs or exits with 222, something went wrong.

    Both of these work for me on a stock FC3 kernel (2.6.9-1.681_FC3). Not
    sure what I was doing wrong...

    Is 32-from-64 syscalls considered to be a documented API - something
    which I can rely on?

    The 64-from-32 switch is also very interesting; I hadn't realized you
    could switch modes that easily within a process.

    J

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