[PATCH v2] m68k: Fix seccomp filtering on ColdFire and 68000

[Christopher Lusk <clusk@northecho.dev>]

m68k selects HAVE_ARCH_SECCOMP_FILTER for all configurations, but the
ColdFire and 68000 syscall entry paths only branch to
syscall_trace_enter() for TIF_SYSCALL_TRACE. TIF_SECCOMP alone falls
through to syscall dispatch, leaving installed filters ineffective.

Test the combined TIF_SYSCALL_TRACE and TIF_SECCOMP mask in both paths
and route either flag through the existing slow path. Leave the classic
MMU syscall entry implementation unchanged.

Validated the combined test under QEMU mcf5208evb (ColdFire): filters
denying getpid, openat, unlinkat, and reboot each returned -EPERM. A
classic-MMU control (q800) denied the filtered getpid syscall both before
and after this change. The 68000 path shares the same source-level
omission and receives the identical masked test but was not separately
emulated. Compile-tested W=1 with CONFIG_SECCOMP_FILTER=y on both
m5208evb_defconfig and a custom no-MMU UCSIMM configuration; the latter
built arch/m68k/68000/entry.o and vmlinux.

Fixes: 6baaade15594 ("m68k: Add kernel seccomp support")
Cc: stable@vger.kernel.org # v6.3+
Suggested-by: Andreas Schwab <schwab@linux-m68k.org>
Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Christopher Lusk <clusk@northecho.dev>
---

Notes:
    Changes in v2:
    - Combine the TRACE/SECCOMP tests into a single masked branch per Andreas
      Schwab; scoped to the ColdFire/68000 paths, classic MMU unchanged.
    
    No in-tree m68k defconfig selects the pure-68000/DragonBall path.
    Compile-tested W=1 with a custom no-MMU UCSIMM configuration and
    CONFIG_SECCOMP_FILTER=y; arch/m68k/68000/entry.o and vmlinux both built
    successfully. The combined gate uses the same flags byte as the classic-MMU
    entry path.

 arch/m68k/68000/entry.S    | 3 ++-
 arch/m68k/coldfire/entry.S | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/m68k/68000/entry.S b/arch/m68k/68000/entry.S
index c257cc415..0d3cbeded 100644
--- a/arch/m68k/68000/entry.S
+++ b/arch/m68k/68000/entry.S
@@ -79,7 +79,8 @@ ENTRY(system_call)
 
 	/* Doing a trace ? */
 	getthreadinfo
-	btst	#(TIF_SYSCALL_TRACE%8),%a2@(TINFO_FLAGS+(31-TIF_SYSCALL_TRACE)/8)
+	moveb	%a2@(TINFO_FLAGS+2),%d1
+	andl	#((_TIF_SYSCALL_TRACE + _TIF_SECCOMP) >> 8),%d1
 	jne	do_trace
 	cmpl	#NR_syscalls,%d0
 	jcc	badsys
diff --git a/arch/m68k/coldfire/entry.S b/arch/m68k/coldfire/entry.S
index 4ea08336e..d658abdc5 100644
--- a/arch/m68k/coldfire/entry.S
+++ b/arch/m68k/coldfire/entry.S
@@ -72,7 +72,8 @@ ENTRY(system_call)
 	movel	%d2,%a0
 	movel	%a0@,%a1		/* save top of frame */
 	movel	%sp,%a1@(TASK_THREAD+THREAD_ESP0)
-	btst	#(TIF_SYSCALL_TRACE%8),%a0@(TINFO_FLAGS+(31-TIF_SYSCALL_TRACE)/8)
+	moveb	%a0@(TINFO_FLAGS+2),%d2
+	andl	#((_TIF_SYSCALL_TRACE + _TIF_SECCOMP) >> 8),%d2
 	bnes	1f
 
 	movel	%d3,%a0
-- 
2.55.0