Subject: Re: [PATCH v2 1/7] powerpc: properly negate error in syscall_set_return_value()

From: "Dmitry V. Levin" <[email-masked]>
Date: Mon, 20 Jan 2025 19:12:49 +0200

On Mon, Jan 20, 2025 at 02:51:38PM +0100, Christophe Leroy wrote:
> Le 14/01/2025 à 18:04, Dmitry V. Levin a écrit :
> > On Mon, Jan 13, 2025 at 06:34:44PM +0100, Christophe Leroy wrote:
> >> Le 13/01/2025 à 18:10, Dmitry V. Levin a écrit :
> >>> Bring syscall_set_return_value() in sync with syscall_get_error(),
> >>> and let upcoming ptrace/set_syscall_info selftest pass on powerpc.
> >>>
> >>> This reverts commit 1b1a3702a65c ("powerpc: Don't negate error in
> >>> syscall_set_return_value()").
> >>
> >> There is a clear detailed explanation in that commit of why it needs to
> >> be done.
> >>
> >> If you think that commit is wrong you have to explain why with at least
> >> the same level of details.
> > 
> > OK, please have a look whether this explanation is clear and detailed enough:
> > 
> > =======
> > powerpc: properly negate error in syscall_set_return_value()
> > 
> > When syscall_set_return_value() is used to set an error code, the caller
> > specifies it as a negative value in -ERRORCODE form.
> > 
> > In !trap_is_scv case the error code is traditionally stored as follows:
> > gpr[3] contains a positive ERRORCODE, and ccr has 0x10000000 flag set.
> > Here are a few examples to illustrate this convention.  The first one
> > is from syscall_get_error():
> >          /*
> >           * If the system call failed,
> >           * regs->gpr[3] contains a positive ERRORCODE.
> >           */
> >          return (regs->ccr & 0x10000000UL) ? -regs->gpr[3] : 0;
> > 
> > The second example is from regs_return_value():
> >          if (is_syscall_success(regs))
> >                  return regs->gpr[3];
> >          else
> >                  return -regs->gpr[3];
> > 
> > The third example is from check_syscall_restart():
> >          regs->result = -EINTR;
> >          regs->gpr[3] = EINTR;
> >          regs->ccr |= 0x10000000;
> > 
> > Compared with these examples, the failure of syscall_set_return_value()
> > to assign a positive ERRORCODE into regs->gpr[3] is clearly visible:
> > 	/*
> > 	 * In the general case it's not obvious that we must deal with
> > 	 * CCR here, as the syscall exit path will also do that for us.
> > 	 * However there are some places, eg. the signal code, which
> > 	 * check ccr to decide if the value in r3 is actually an error.
> > 	 */
> > 	if (error) {
> > 		regs->ccr |= 0x10000000L;
> > 		regs->gpr[3] = error;
> > 	} else {
> > 		regs->ccr &= ~0x10000000L;
> > 		regs->gpr[3] = val;
> > 	}
> > 
> > This fix brings syscall_set_return_value() in sync with syscall_get_error()
> > and lets upcoming ptrace/set_syscall_info selftest pass on powerpc.
> > 
> > Fixes: 1b1a3702a65c ("powerpc: Don't negate error in syscall_set_return_value()").
> > =======
> > 
> > 
> 
> I think there is still something going wrong.
> 
> do_seccomp() sets regs->gpr[3] = -ENOSYS; by default.
> 
> Then it calls __secure_computing() which returns what __seccomp_filter() 
> returns.
> 
> In case of error, __seccomp_filter() calls syscall_set_return_value() 
> with a negative value then returns -1
> 
> do_seccomp() is called by do_syscall_trace_enter() which returns -1 when 
> do_seccomp() doesn't return 0.
> 
> do_syscall_trace_enter() is called by system_call_exception() and 
> returns -1, so syscall_exception() returns regs->gpr[3]
> 
> In entry_32.S, transfer_to_syscall, syscall_exit_prepare() is then 
> called with the return of syscall_exception() as first parameter, which 
> leads to:
> 
> 	if (unlikely(r3 >= (unsigned long)-MAX_ERRNO) && is_not_scv) {
> 		if (likely(!(ti_flags & (_TIF_NOERROR | _TIF_RESTOREALL)))) {
> 			r3 = -r3;
> 			regs->ccr |= 0x10000000; /* Set SO bit in CR */
> 		}
> 	}

Note the "unlikely" keyword here reminding us once more that in !scv case
regs->gpr[3] does not normally have -ERRORCODE form.

> By chance, because you have already changed the sign of gpr[3], the 
> above test fails and nothing is done to r3, and because you have also 
> already set regs->ccr it works.
> 
> But all this looks inconsistent with the fact that do_seccomp sets 
> -ENOSYS as default value
> 
> Also, when do_seccomp() returns 0, do_syscall_trace_enter() check the 
> syscall number and when it is wrong it goes to skip: which sets 
> regs->gpr[3] = -ENOSYS;

It looks like do_seccomp() and do_syscall_trace_enter() get away by sheer
luck, implicitly relying on syscall_exit_prepare() transparently fixing
regs->gpr[3] for them.

> So really I think it is not in line with your changes to set positive 
> value in gpr[3].
> 
> Maybe your change is still correct but it needs to be handled completely 
> in that case.

By the way, is there any reasons why do_seccomp() and
do_syscall_trace_enter() don't use syscall_set_return_value() yet?

-- 
ldv