Messages in this thread Patch in this message | | | Date | Thu, 27 Nov 2014 10:07:22 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v9 2/6] arm64: ptrace: allow tracer to skip a system call |
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On Thu, Nov 27, 2014 at 06:46:29AM +0000, AKASHI Takahiro wrote: > 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
or even better: mov w0, #-1
> cmp scno, x0 > b.ne 1f
Good thinking, I prefer that.
Will
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