[PATCH 1/2] powerpc: properly negate error in syscall_set_return_value() in sc case

[Dmitry V. Levin]

According to the Power Architecture Linux system call ABI documented in
[1], when the syscall is made with the sc instruction, both a value and an
error condition are returned, where r3 register contains the return value,
and cr0.SO bit specifies the error condition.  When cr0.SO is clear, the
syscall succeeded and r3 is the return value.  When cr0.SO is set, the
syscall failed and r3 is the error value.  This syscall return semantics
was implemented from the very beginning of Power Architecture on Linux,
and syscall tracers and debuggers like strace that read or modify syscall
return information also rely on this ABI.

r3 and cr0.SO are exposed directly via struct pt_regs where gpr[3] and
(ccr & 0x10000000) correspond to r3 and cr0.SO, respectively.
For example, here is an excerpt from check_syscall_restart() that assigns
these members of struct pt_regs:
        regs->result = -EINTR;
        regs->gpr[3] = EINTR;
        regs->ccr |= 0x10000000;
In this example, the semantics of negative ERRORCODE that's being used
virtually everywhere in generic kernel code is translated to powerpc sc
syscall return ABI which uses positive ERRORCODE and cr0.SO bit.

Also, r3 and cr0.SO are exposed indirectly via helpers.
For example, here is an excerpt from syscall_get_error():
        /*
         * If the system call failed,
         * regs->gpr[3] contains a positive ERRORCODE.
         */
        return (regs->ccr & 0x10000000UL) ? -regs->gpr[3] : 0;
and here is another example, from regs_return_value():
        if (is_syscall_success(regs))
                return regs->gpr[3];
        else
                return -regs->gpr[3];
In these examples, the powerpc sc syscall return ABI which uses positive
ERRORCODE and cr0.SO bit is translated to the semantics of negative
ERRORCODE that's being used virtually everywhere in generic kernel code.

Up to a certain point in time the kernel managed to implement the powerpc
sc syscall return ABI in all cases where struct pt_regs was exposed to user
space.

The situation changed when SECCOMP_RET_TRACE support was introduced.
At this point the -ERRORCODE semantics that was used under the hood to
implement seccomp on powerpc became exposed to user space.  The tracer
handling PTRACE_EVENT_SECCOMP is not just able to observe -ENOSYS in gpr[3]
- this is relatively harmless as at this stage there is no syscall return
yet so the powerpc sc syscall return ABI does not apply.  What's important
is that the tracer can change the syscall number to -1 thus making the
syscall fail, and at this point the tracer is also able to specify the
error value.  This has to be done in accordance with the syscall return
ABI, however, the current implementation of do_seccomp() supports both the
generic kernel -ERRORCODE return value ABI and the powerpc sc syscall
return ABI, thanks to syscall_exit_prepare() that converts the former to
the latter.  Consequently, seccomp_bpf selftest passes both with and
without this change.

Now comes the moment when PTRACE_SET_SYSCALL_INFO is going to be
introduced.  PTRACE_SET_SYSCALL_INFO is a generic ptrace API that
complements PTRACE_GET_SYSCALL_INFO by letting the ptracer modify
the details of the system calls the tracee is blocked in.

One of the helpers that have to be used to implement
PTRACE_SET_SYSCALL_INFO is syscall_set_return_value().
This helper complements other two helpers, syscall_get_error() and
syscall_get_return_value(), that are currently used to implement
PTRACE_GET_SYSCALL_INFO on syscall return.  When syscall_set_return_value()
is used to set an error code, the caller specifies it as a negative value
in -ERRORCODE format.

Unfortunately, this does not work well on powerpc since commit 1b1a3702a65c
("powerpc: Don't negate error in syscall_set_return_value()") because
syscall_set_return_value() does not follow the powerpc sc syscall return
ABI:
	/*
	 * 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;
	}

The reason why this syscall_set_return_value() implementation was able to
get away with violating the powerpc sc syscall return ABI is the following:
Up to now, syscall_set_return_value() on powerpc could be called only from
do_syscall_trace_enter() via do_seccomp(), there was no way it could be
called from do_syscall_trace_leave() which is the point where tracers on
syscall return are activated and the powerpc sc syscall return ABI has
to be respected.

Introduction of PTRACE_SET_SYSCALL_INFO necessitates a change of
syscall_set_return_value() to comply with the powerpc sc syscall return
ABI.  Without the change, the upcoming ptrace/set_syscall_info selftest
fails with the following diagnostics:

  # set_syscall_info.c:119:set_syscall_info:Expected exp_exit->rval (-38) == info->exit.rval (38)
  # set_syscall_info.c:120:set_syscall_info:wait #4: PTRACE_GET_SYSCALL_INFO #2: exit stop mismatch

Note that since backwards compatibility with the current implementation has
to be provided, the kernel has to continue supporting simultaneously both
the generic kernel -ERRORCODE return value ABI and the powerpc sc syscall
return ABI at least for PTRACE_EVENT_SECCOMP tracers.  Consequently, since
the point of __secure_computing() invocation and up to the point of
conversion in syscall_exit_prepare(), gpr[3] may be set according to either
of these two ABIs.  An attempt to address code inconsistencies in syscall
error return handling that were introduced as a side effect of the dual
ABI support follows in a separate patch.

Link: https://www.kernel.org/doc/html/latest/arch/powerpc/syscall64-abi.html#return-value [1]
Signed-off-by: Dmitry V. Levin <ldv at strace.io>
Reviewed-by: Alexey Gladkov <legion at kernel.org>
---
 arch/powerpc/include/asm/syscall.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/syscall.h b/arch/powerpc/include/asm/syscall.h
index 3dd36c5e334a..422d7735ace6 100644
--- a/arch/powerpc/include/asm/syscall.h
+++ b/arch/powerpc/include/asm/syscall.h
@@ -82,7 +82,11 @@ static inline void syscall_set_return_value(struct task_struct *task,
 		 */
 		if (error) {
 			regs->ccr |= 0x10000000L;
-			regs->gpr[3] = error;
+			/*
+			 * In case of an error regs->gpr[3] contains
+			 * a positive ERRORCODE.
+			 */
+			regs->gpr[3] = -error;
 		} else {
 			regs->ccr &= ~0x10000000L;
 			regs->gpr[3] = val;
-- 
ldv