Messages in this thread Patch in this message | | | Date | Thu, 20 Nov 2014 14:13:04 +0900 | From | AKASHI Takahiro <> | Subject | Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call |
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On 11/20/2014 04:06 AM, Will Deacon wrote: > On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote: >> On 11/18/2014 11:04 PM, Will Deacon wrote: >>> On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote: >>>> >>>> + if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) { >>>> + /* >>>> + * user-issued syscall(-1): >>>> + * RESTRICTION: We always return ENOSYS whatever value is >>>> + * stored in x0 (a return value) at this point. >>>> + * Normally, with ptrace off, syscall(-1) returns -ENOSYS. >>>> + * With ptrace on, however, if a tracer didn't pay any >>>> + * attention to user-issued syscall(-1) and just let it go >>>> + * without a hack here, it would return a value in x0 as in >>>> + * other system call cases. This means that this system call >>>> + * might succeed and see any bogus return value. >>>> + * This should be definitely avoided. >>>> + */ >>>> + regs->regs[0] = -ENOSYS; >>>> + } >>> >>> I'm still really uncomfortable with this, and it doesn't seem to match what >>> arch/arm/ does either. >> >> Yeah, I know but >> as I mentioned before, syscall(-1) will be signaled on arm, and so we don't >> have to care about a return value :) > > What does x86 do?
On x86, syscall(-1) returns -ENOSYS if not traced, and we can change a return value if traced.
>>> Doesn't it also prevent a tracer from skipping syscall(-1)? >> >> Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly >> replaced with -1 by a tracer, and, in this sense, it is *skipped*. > > Ok, but now userspace sees -ENOSYS for a skipped system call in that case, > whereas it would usually see whatever the trace put in x0, right?
Yes. If you don't really like this behavior, how about this patch instead of my [2/6] patch?
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index 726b910..1ef57d0 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -668,8 +668,15 @@ ENDPROC(el0_svc) * switches, and waiting for our parent to respond. */ __sys_trace: + cmp w8, #-1 // default errno for invalid + b.ne 1f // system call + mov x0, #-ENOSYS + str x0, [sp, #S_X0] +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 +__sys_trace_return_skipped: mov x0, sp bl syscall_trace_exit b ret_to_user With this change, I believe, syscall(-1) returns -ENOSYS by default whether traced or not, and still you can change a return value when tracing. (But a drawback here is that a tracer will see -ENOSYS in x0 even at syscall entry for syscall(-1).)
-Takahiro AKASHI
> Will >
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