Messages in this thread Patch in this message | | | Date | Thu, 20 Nov 2014 14:52:34 +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 02:13 PM, AKASHI Takahiro wrote: > 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
I needed to correct the code here: w8 should be w26, thinking of compat syscalls.
> + b.ne 1f // system call > + mov x0, #-ENOSYS > + str x0, [sp, #S_X0] > +1:
and this part might better be generalized like the following:
__sys_trace: cmp w26, w25 // cannot use x26 and x25 here b.hs 1f // scno > sc_nr || scno < 0 b 2f 1: mov x0, #-ENOSYS str x0, [sp, #S_X0] 2:
If you will be comfortable, I will submit a new patch soon.
-Takahiro AKASHI
> 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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