| Message ID | 546D7860.2010300@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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 >>
On Thu, Nov 20, 2014 at 05:13:04AM +0000, 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: > >> 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? > > 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).) But it's exactly these drawbacks that I'm objected to. syscall(-1) shouldn't be treated any differently to syscall(42) with respect to restarting, exactly like x86. Will
On 11/21/2014 04:17 AM, Will Deacon wrote: > On Thu, Nov 20, 2014 at 05:13:04AM +0000, 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: >>>> 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? >> >> 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).) > > But it's exactly these drawbacks that I'm objected to. syscall(-1) shouldn't > be treated any differently to syscall(42) with respect to restarting, > exactly like x86. Can you elaborate a bit more as to "restarting?" We can't make any assumption about the number of arguments taken by *invalid* syscall(-1) and so changing a value in x0 (or any other registers) doesn't make any difference. () -Takahiro AKASHI > Will >
On Tue, Nov 25, 2014 at 07:42:10AM +0000, AKASHI Takahiro wrote: > On 11/21/2014 04:17 AM, Will Deacon wrote: > > On Thu, Nov 20, 2014 at 05:13:04AM +0000, 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: > >>>> 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? > >> > >> 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).) > > > > But it's exactly these drawbacks that I'm objected to. syscall(-1) shouldn't > > be treated any differently to syscall(42) with respect to restarting, > > exactly like x86. > > Can you elaborate a bit more as to "restarting?" Sorry, I meant skipping. There was another thread about syscall restarting at the same time I wrote that, so my mind was elsewhere! > We can't make any assumption about the number of arguments taken by *invalid* syscall(-1) > and so changing a value in x0 (or any other registers) doesn't make any difference. > () Ok, that's a fair point. Will
On Thu, Nov 20, 2014 at 05:52:34AM +0000, AKASHI Takahiro wrote: > On 11/20/2014 02:13 PM, AKASHI Takahiro wrote: > > On 11/20/2014 04:06 AM, Will Deacon wrote: > >> 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. Yes, please send a new series including this change. Will
On Thu, Nov 20, 2014 at 02:13:04PM +0900, AKASHI Takahiro wrote: > On 11/20/2014 04:06 AM, Will Deacon wrote: > >What does x86 do? > > On x86, syscall(-1) returns -ENOSYS if not traced, and we can change a return > value if traced. ... which is used for UML (user mode Linux). UML works by spawning processes under the host kernel, which run with syscall tracing enabled, with the UML kernel as the tracer. The UML kernel tracer receives the syscall trace event when the child tries to execute a syscall, decodes the syscall, executes syscall in the UML kernel, and then cancels the syscall in the host kernel, setting the return code appropriately.
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