using ptrace to cancel a syscall on sparc

[Mike Frysinger]

On 21 Dec 2015 02:31, Dmitry V. Levin wrote:
> On Sun, Dec 20, 2015 at 12:47:54AM -0500, Mike Frysinger wrote:
> > i've been playing with ptrace on sparc and trying to use it to watch and
> > cancel specific syscalls.  i have this working for other arches already.
> [...]
> > i'm having trouble with canceling of the syscall itself.  seems like
> > no matter what i stuff into o0, the kernel executes the unlink.  i've
> > tried tracing arch/sparc/kernel/syscalls.S and kernel/head_64.S, the
> > the entry is linux_sparc_syscall32 which calls linux_syscall_trace32,
> > but it seems like the o0 stuff doesn't seem to work for me.  my sparc
> > asm foo isn't strong enough to figure out what's going wrong :/.
> 
> Yes, sparc is odd in this respect: whatever you write to u_regs[] on
> entering syscall, it doesn't affect syscall number or syscall arguments.

looks like the bug is in arch/sparc/kernel/syscalls.S:linux_syscall_trace32
(and linux_syscall_trace).  they don't reload the args from the pt_regs
struct after calling syscall_trace_enter.  i put in a small hack:
linux_syscall_trace32:
	call	syscall_trace_enter
	 add	%sp, PTREGS_OFF, %o0
	brnz,pn	%o0, 3f
	 mov	-ENOSYS, %o0
+
+	ldx	[%sp + PTREGS_OFF + PT_V9_G1], %g1
+	cmp	%g1, NR_syscalls
=	bgeu,pn	%xcc, 3f
+	 mov	-ENOSYS, %o0
+
	srl	%i0, 0, %o0
	srl	%i4, 0, %o4
...

it's enough for my use case (cancel the call), but it's not entirely correct.
i think it needs to re-initialize %l7 with the final syscall pointer via the
syscall table, and it needs to reload PT_V9_I{0..5}.  i have no idea which
regs need stuffing though, especially in light of the %l7 optimization.  and
i'm not familiar at all with the apparent parallelism via IEU0/IEU1 groups.
so i won't bother with trying to write a full patch.  hopefully sparc guys
will notice & post a fix ;).

i'm attaching my simple test in case it helps.  just do:
  $ gcc ptrace-test.c && ./a.out
the logging output should indicate when things are passing.
-mike
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