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    SubjectRe: [PATCH v9 2/6] arm64: ptrace: allow tracer to skip a system call
    On 11/26/2014 10:02 PM, Will Deacon wrote:
    > On Wed, Nov 26, 2014 at 04:49:47AM +0000, AKASHI Takahiro wrote:
    >> If tracer modifies a syscall number to -1, this traced system call should
    >> be skipped with a return value specified in x0.
    >> This patch implements this semantics.
    >>
    >> Please note:
    >> * syscall entry tracing and syscall exit tracing (ftrace tracepoint and
    >> audit) are always executed, if enabled, even when skipping a system call
    >> (that is, -1).
    >> In this way, we can avoid a potential bug where audit_syscall_entry()
    >> might be called without audit_syscall_exit() at the previous system call
    >> being called, that would cause OOPs in audit_syscall_entry().
    >>
    >> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
    >> ---
    >> arch/arm64/kernel/entry.S | 10 +++++++++-
    >> 1 file changed, 9 insertions(+), 1 deletion(-)
    >>
    >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
    >> index 726b910..946ec52 100644
    >> --- a/arch/arm64/kernel/entry.S
    >> +++ b/arch/arm64/kernel/entry.S
    >> @@ -161,6 +161,7 @@
    >> */
    >> sc_nr .req x25 // number of system calls
    >> scno .req x26 // syscall number
    >> +scno_w .req w26 // syscall number (lower 32 bits)
    >> stbl .req x27 // syscall table pointer
    >> tsk .req x28 // current thread_info
    >>
    >> @@ -668,8 +669,14 @@ ENDPROC(el0_svc)
    >> * switches, and waiting for our parent to respond.
    >> */
    >> __sys_trace:
    >> - mov x0, sp
    >> + cmp scno_w, #-1 // set default errno for
    >
    > I hate that we have to use scno_w, but the only alternative I can think of
    > is using w8 directly, which isn't any better and doesn't work for compat.
    > Ho-hum, I guess we'll stick with what you have.

    The possible approaches might be:
    * use 32-bit registers for scno & sc_nr and use "sxtw scno, w8," or
    * use an extra reg like
    __sys_trace:
    mov x0, #0xffffffff
    cmp scno, x0
    b.ne 1f

    >> + b.ne 1f // user-issued syscall(-1)
    >> + mov x0, #-ENOSYS
    >> + str x0, [sp]
    >
    > Can you use #S_X0 here for clarity, please?

    Okey.

    >> +1: mov x0, sp
    >> bl syscall_trace_enter
    >> + cmp w0, #-1 // skip the syscall?
    >> + b.eq __sys_trace_return_skipped
    >> adr lr, __sys_trace_return // return address
    >> uxtw scno, w0 // syscall number (possibly new)
    >> mov x1, sp // pointer to regs
    >> @@ -684,6 +691,7 @@ __sys_trace:
    >>
    >> __sys_trace_return:
    >> str x0, [sp] // save returned x0
    >
    > and update this guy too.

    Sure.

    -Takahiro AKASHI

    > Will
    > --
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