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

Dmitry V. Levin
2025-01-23T18:28:22Z

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 */
> }
> }
>
> 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;
>
> 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.

Indeed, there is an inconsistency in !trap_is_scv case.

In some places such as syscall_get_error() and regs_return_value() the
semantics is as I described earlier: gpr[3] contains a positive ERRORCODE
and ccr has 0x10000000 flag set. This semantics is a part of the ABI and
therefore cannot be changed.

In some other places like do_seccomp() and do_syscall_trace_enter() the
semantics is similar to the trap_is_scv case: gpr[3] contains a negative
ERRORCODE and ccr is unchanged. In addition, system_call_exception()
returns the system call function return value when it is executed, and
gpr[3] otherwise. The value returned by system_call_exception() is passed
on to syscall_exit_prepare() which performs the conversion you mentioned.

What's remarkable is that in those places that are a part of the ABI the
traditional semantics is kept, while in other places the implementation
follows the trap_is_scv-like semantics, while traditional semantics is
also supported there.

The only case where I see some intersection is do_seccomp() where the
tracer would be able to see -ENOSYS in gpr[3]. However, the seccomp stop
is not the place where the tracer *reads* the system call exit status,
so whatever was written in gpr[3] before __secure_computing() is not
really relevant, consequently, selftests/seccomp/seccomp_bpf passes with
this patch applied as well as without it.

After looking at system_call_exception() I doubt this inconsistency can be
easily avoided, so I don't see how this patch could be enhanced further,
and what else could I do with the patch besides dropping it and letting
!trap_is_scv case be unsupported by PTRACE_SET_SYSCALL_INFO API, which
would be unfortunate.

--
ldv