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    SubjectRe: [uml-devel] [RFC] PATCH 3/4 - Time virtualization : PTRACE_SYSCALL_MASK
    On Wed, Apr 26, 2006 at 05:47:54PM +0200, Blaisorblade wrote:
    > If we can do without MASK_STRICT_VERIFY, that works fully, and
    > anyway it's simpler - however, say, when running strace -e read,tee
    > (sys_tee will soon be added, it seems) this call would fail, while it
    > would be desirable to have it work as strace -e read.
    >
    > MASK_STRICT_VERIFY isn't necessarily the best solution, but if
    > userspace must search the maximum allowed syscall by multiple
    > attempts, we've still a bad API.
    >
    > Probably, a better option (_instead_ of MASK_STRICT_VERIFY) would be
    > to return somewhere an "extended error code" saying which is the
    > last allowed syscall or (better) which is the first syscall which
    > failed. I.e. if there is strace -e read,splice,tee and nor splice nor
    > tee are supported, then this value would be __NR_splice and strace (or
    > any app) could then decide what to do.

    Why not just zero out the bits that the kernel knows about? Then, if
    we return -EINVAL, the process just looks at the remaining bits that
    are set to see what system calls the kernel didn't know about.

    Jeff
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