After enabling GENERIC_ENTRY on PowerPC, seccomp filters using
SCMP_ACT_ERRNO without an explicit errnoRet value return ENOSYS
(Function not implemented) instead of the expected EPERM (Operation
not permitted).

The issue occurs in system_call_exception() when syscall_enter_from_user_mode()
returns -1 to indicate the syscall should be skipped (e.g., blocked by seccomp).
The current code treats this -1 as a syscall number and compares it against
NR_syscalls. Since -1 is greater than NR_syscalls,
the code incorrectly returns -ENOSYS, overwriting the errno that seccomp
already set via syscall_set_return_value().

The generic entry code in syscall_trace_enter() calls __secure_computing(),
which sets the appropriate errno in regs->gpr[3] and returns -1 to signal
that the syscall should be skipped. However, the PowerPC syscall handler
was not checking for this -1 return value before validating the syscall
number.

Fix this by explicitly checking if syscall_enter_from_user_mode() returns
-1 and returning the value already set in regs->gpr[3] (the errno from
seccomp) before performing the syscall number validation.

Also Move the syscall_enter_from_user_mode() call and the seccomp/ptrace
skip check to after the NR_syscalls bounds check.

When syscall -1 was passed, the r0 == -1L check would trigger before
the NR_syscalls check, causing syscall_get_error() to return 0 instead
of -ENOSYS. This resulted in a silent success (ret=0, errno=0) instead
of the expected ENOSYS error.

By moving syscall_enter_from_user_mode() after the bounds check, an
initial syscall number of -1 is correctly rejected with -ENOSYS first.
The seccomp/ptrace skip path still works correctly for valid syscall
numbers that get overridden to -1 by seccomp or ptrace.

This aligns PowerPC's behavior with other architectures using GENERIC_ENTRY
and restores correct seccomp errno handling.

Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
Reported-by: Michal Suchánek <redacted>
Closes: https://lore.kernel.org/all/ajpp-_XnbF3UTM_E@kunlun.suse.cz/ (local)
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <redacted>
---

v1 -> v2:
  - Fix issues in the previous fix (Michal)
v1: https://lore.kernel.org/all/20260624171520.772408-1-mkchauras@gmail.com (local)

  arch/powerpc/kernel/syscall.c | 7 ++++++-
  1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index a9da2af6efa8..36d73933a311 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c

@@ -20,7 +20,6 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
  	syscall_fn f;
  
  	add_random_kstack_offset();
-	r0 = syscall_enter_from_user_mode(regs, r0);
  
  	if (unlikely(r0 >= NR_syscalls)) {
  		if (unlikely(trap_is_unsupported_scv(regs))) {

@@ -31,6 +30,12 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
  		return -ENOSYS;
  	}
  
+	r0 = syscall_enter_from_user_mode(regs, r0);
+