Re: [PATCH v3 1/3] powerpc: Don't force ENOSYS as error on syscall fail

In certain scenarios - e.g. seccomp filtering with ERRNO as default action -
the system call fails for other reasons than the syscall not being available.
The seccomp filter can be configured to store a user-defined error code on
return from a blacklisted syscall. Don't always set ENOSYS on
do_syscall_trace_enter failure.

Delegate setting ENOSYS in case of failure, where appropriate, to
do_syscall_trace_enter.

v3:
- keep setting ENOSYS in the syscall entry assembly for scenarios without
   syscall tracing

v2:
- move setting ENOSYS as errno from the syscall entry assembly to
   do_syscall_trace_enter, only in the specific case

Signed-off-by: Bogdan Purcareata <redacted>
---
  arch/powerpc/kernel/entry_32.S | 7 ++++++-
  arch/powerpc/kernel/entry_64.S | 5 +++--
  arch/powerpc/kernel/ptrace.c   | 4 +++-
  3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 46fc0f4..b2f88cd 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S

@@ -333,12 +333,12 @@ _GLOBAL(DoSyscall)
  	lwz	r11,TI_FLAGS(r10)
  	andi.	r11,r11,_TIF_SYSCALL_DOTRACE
  	bne-	syscall_dotrace
-syscall_dotrace_cont:
  	cmplwi	0,r0,NR_syscalls
  	lis	r10,sys_call_table@h
  	ori	r10,r10,sys_call_table@l
  	slwi	r0,r0,2
  	bge-	66f
+syscall_dotrace_cont:
  	lwzx	r10,r10,r0	/* Fetch system call handler [ptr] */
  	mtlr	r10
  	addi	r9,r1,STACK_FRAME_OVERHEAD

@@ -457,6 +457,11 @@ syscall_dotrace:
  	lwz	r7,GPR7(r1)
  	lwz	r8,GPR8(r1)
  	REST_NVGPRS(r1)
+	cmplwi	0,r0,NR_syscalls
+	lis	r10,sys_call_table@h
+	ori	r10,r10,sys_call_table@l
+	slwi	r0,r0,2
+	bge-	ret_from_syscall
  	b	syscall_dotrace_cont

  syscall_exit_work:

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index d180caf2..0d22fa8 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S

@@ -144,7 +144,6 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR)
  	ld	r10,TI_FLAGS(r11)
  	andi.	r11,r10,_TIF_SYSCALL_DOTRACE
  	bne	syscall_dotrace
-.Lsyscall_dotrace_cont:
  	cmpldi	0,r0,NR_syscalls
  	bge-	syscall_enosys

@@ -253,7 +252,9 @@ syscall_dotrace:
  	addi	r9,r1,STACK_FRAME_OVERHEAD
  	CURRENT_THREAD_INFO(r10, r1)
  	ld	r10,TI_FLAGS(r10)
-	b	.Lsyscall_dotrace_cont
+	cmpldi	0,r0,NR_syscalls
+	bge-	syscall_exit

+	b	system_call

  syscall_enosys:
  	li	r3,-ENOSYS

diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
index f21897b..2edae06 100644
--- a/arch/powerpc/kernel/ptrace.c
+++ b/arch/powerpc/kernel/ptrace.c

@@ -1775,13 +1775,15 @@ long do_syscall_trace_enter(struct pt_regs *regs)
  	secure_computing_strict(regs->gpr[0]);

  	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
-	    tracehook_report_syscall_entry(regs))
+	    tracehook_report_syscall_entry(regs)) {
  		/*
  		 * Tracing decided this syscall should not happen.
  		 * We'll return a bogus call number to get an ENOSYS
  		 * error, but leave the original number in regs->gpr[0].
  		 */
  		ret = -1L;
+		syscall_set_return_value(current, regs, ENOSYS, 0);
+	}

  	if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
  		trace_sys_enter(regs, regs->gpr[0]);