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    Subject[patch] arch-specific cond_syscall usage issues
    Experimenting with trying to use cond_syscall for a few arch-specific
    syscalls, I discovered that it can't actually be used outside the file
    in which sys_ni_syscall is declared because the assembler doesn't feel
    obliged to output the symbol in that case:

    weak.c:

    #define cond_syscall(x) asm(".weak\t" #x "\n\t.set\t" #x ",sys_ni_syscall");
    cond_syscall(sys_foo);

    $ nm weak.o
    U sys_ni_syscall

    One arch (PPC) is apparently trying to use cond_syscall this way
    anyway, though it's probably never been actually tested as the above
    test was done on a PPC.

    After trying a bunch of tricks to get it to work nicely, I decided
    there are basically two alternatives: make weak versions of
    sys_ni_syscall wherever they're wanted or put the arch-specific
    cond_syscalls in kernel/sys.c where sys_ni_syscall is defined.

    The former approach is a bit crufty and doesn't actually do the right
    thing in practice as you'll get multiple copies of sys_ni_syscall in
    your final image.

    The latter introduces some slight arch-pollution in sys.c, but as
    arch-specific cond_syscalls aren't all that frequent, it should be
    pretty minor. So here's a patch to move the current offender to sys.c:

    tiny-mpm/arch/ppc/kernel/syscalls.c | 2 --
    tiny-mpm/kernel/sys.c | 3 +++
    2 files changed, 3 insertions(+), 2 deletions(-)

    diff -puN arch/ppc/kernel/syscalls.c~ppc_cond_syscall arch/ppc/kernel/syscalls.c
    --- tiny/arch/ppc/kernel/syscalls.c~ppc_cond_syscall 2004-01-09 21:15:02.000000000 -0600
    +++ tiny-mpm/arch/ppc/kernel/syscalls.c 2004-01-09 21:15:08.000000000 -0600
    @@ -271,5 +271,3 @@ long ppc_fadvise64_64(int fd, int advice
    {
    return sys_fadvise64_64(fd, offset, len, advice);
    }
    -
    -cond_syscall(sys_pciconfig_iobase);
    diff -puN kernel/sys.c~ppc_cond_syscall kernel/sys.c
    --- tiny/kernel/sys.c~ppc_cond_syscall 2004-01-09 21:15:02.000000000 -0600
    +++ tiny-mpm/kernel/sys.c 2004-01-09 21:15:02.000000000 -0600
    @@ -252,6 +252,9 @@ cond_syscall(sys_epoll_wait)
    cond_syscall(sys_pciconfig_read)
    cond_syscall(sys_pciconfig_write)

    +/* arch-specific weak syscall entries */
    +cond_syscall(sys_pciconfig_iobase)
    +
    static int set_one_prio(struct task_struct *p, int niceval, int error)
    {
    int no_nice;
    _

    --
    Matt Mackall : http://www.selenic.com : Linux development and consulting
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