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    SubjectRe: [PATCH 03/10] ptrace: implement PTRACE_SEIZE
    On Monday 16 May 2011 20:17, Tejun Heo wrote:
    > PTRACE_ATTACH implicitly issues SIGSTOP on attach which has side
    > effects on tracee signal and job control states. This patch
    > implements a new ptrace request PTRACE_SEIZE which attaches and traps
    > tracee without affecting its signal and job control states.
    > The usage is the same with PTRACE_ATTACH but it takes PTRACE_SEIZE_*
    > flags in @data. Currently, the only defined flag is
    > PTRACE_SEIZE_DEVEL which is a temporary flag to enable PTRACE_SEIZE.
    > PTRACE_SEIZE will change ptrace behaviors outside of attach itself.
    > The changes will be implemented gradually and the DEVEL flag is to
    > prevent programs which expect full SEIZE behavior from using it before
    > all the behavior modifications are complete while allowing unit
    > testing. The flag will be removed once SEIZE behaviors are completely
    > implemented.
    > After PTRACE_SEIZE, tracee will trap. Which trap will happen isn't
    > fixed. If other trap conditions exist (e.g. signal delivery), they
    > might be taken; otherwise, a trap with exit_code == (SIGTRAP |
    > PTRACE_EVENT_STOP << 8) is taken. If seized, this trap is also used
    > for group stop traps instead of exit_code == 0 with NULL GETSIGINFO.
    > * PTRACE_SEIZE doesn't affect signal or group stop state.
    > * After PTRACE_SEIZE, one trap will happen which might be a
    > * If PTRACE_SEIZE'd, group stop also uses PTRACE_EVENT_STOP trap which
    > uses exit_code of (SIGTRAP | PTRACE_EVENT_STOP << 8) instead of the
    > stopping signal number and returns usual trap siginfo on
    > PTRACE_GETSIGINFO instead of NULL.
    > Note that there currently is no way to find out the stopping signal
    > number while seized. This will be improved by future patches.
    > Seizing sets PT_SEIZED in ->ptrace of the tracee. This flag will be
    > used to determine whether new SEIZE behaviors should be enabled.

    This makes PTRACE_EVENT_STOP similar to other PTRACE_EVENTs.
    The only difference is that it can't be set by PTRACE_SETOPTIONS
    as other events do, but activated implicitly by PTRACE_SEIZE.

    This made me thinking.

    How about making API even more similar to existing one?

    Create PTRACE_O_TRACESTOP, make it settable by PTRACE_SETOPTIONS too.

    Make PTRACE_SEIZE take the mask of PTRACE_O_xyz flags
    as data argument.
    If PTRACE_O_TRACESTOP is set, it works as you described above.
    If PTRACE_O_TRACESTOP is not set, then it works as good old PTRACE_ATTACH.
    In both cases, immediately at attach it sets opts a-la PTRACE_SETOPTIONS.

    We can even avoid introducing PTRACE_SEIZE at all, because
    currently PTRACE_ATTACH ignores its data argument.

    I know, I know, "this changes API", but did we ever promise
    that PTRACE_ATTACH with nonzero data arg is a valid usage?
    Also, I perused first 10 pages of google code search results
    and I see that everybody passes 0 or NULL.


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